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Android ListView工作原理完全解析,带你从源码的角度彻底理解

程序员文章站 2022-04-13 23:24:24
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在Android所有常用的原生控件当中,用法最复杂的应该就是ListView了,它专门用于处理那种内容元素很多,手机屏幕无法展示出所有内容的情况。ListView可以使用列表的形式来展示内容,超出屏幕部分的内容只需要通过手指滑动就可以移动到屏幕内了。


另外ListView还有一个非常神奇的功能,我相信大家应该都体验过,即使在ListView中加载非常非常多的数据,比如达到成百上千条甚至更多,ListView都不会发生OOM或者崩溃,而且随着我们手指滑动来浏览更多数据时,程序所占用的内存竟然都不会跟着增长。那么ListView是怎么实现这么神奇的功能的呢?当初我就抱着学习的心态花了很长时间把ListView的源码通读了一遍,基本了解了它的工作原理,在感叹Google大神能够写出如此精妙代码的同时我也有所敬畏,因为ListView的代码量比较大,复杂度也很高,很难用文字表达清楚,于是我就放弃了把它写成一篇博客的想法。那么现在回想起来这件事我已经肠子都悔青了,因为没过几个月时间我就把当初梳理清晰的源码又忘的一干二净。于是现在我又重新定下心来再次把ListView的源码重读了一遍,那么这次我一定要把它写成一篇博客,分享给大家的同时也当成我自己的笔记吧。


首先我们先来看一下ListView的继承结构,如下图所示:

Android ListView工作原理完全解析,带你从源码的角度彻底理解

可以看到,ListView的继承结构还是相当复杂的,它是直接继承自的AbsListView,而AbsListView有两个子实现类,一个是ListView,另一个就是GridView,因此我们从这一点就可以猜出来,ListView和GridView在工作原理和实现上都是有很多共同点的。然后AbsListView又继承自AdapterView,AdapterView继承自ViewGroup,后面就是我们所熟知的了。先把ListView的继承结构了解一下,待会儿有助于我们更加清晰地分析代码。



Adapter的作用


Adapter相信大家都不会陌生,我们平时使用ListView的时候一定都会用到它。那么话说回来大家有没有仔细想过,为什么需要Adapter这个东西呢?总感觉正因为有了Adapter,ListView的使用变得要比其它控件复杂得多。那么这里我们就先来学习一下Adapter到底起到了什么样的一个作用。


其实说到底,控件就是为了交互和展示数据用的,只不过ListView更加特殊,它是为了展示很多很多数据用的,但是ListView只承担交互和展示工作而已,至于这些数据来自哪里,ListView是不关心的。因此,我们能设想到的最基本的ListView工作模式就是要有一个ListView控件和一个数据源。


不过如果真的让ListView和数据源直接打交道的话,那ListView所要做的适配工作就非常繁杂了。因为数据源这个概念太模糊了,我们只知道它包含了很多数据而已,至于这个数据源到底是什么样类型,并没有严格的定义,有可能是数组,也有可能是集合,甚至有可能是数据库表中查询出来的游标。所以说如果ListView真的去为每一种数据源都进行适配操作的话,一是扩展性会比较差,内置了几种适配就只有几种适配,不能动态进行添加。二是超出了它本身应该负责的工作范围,不再是仅仅承担交互和展示工作就可以了,这样ListView就会变得比较臃肿。


那么显然Android开发团队是不会允许这种事情发生的,于是就有了Adapter这样一个机制的出现。顾名思义,Adapter是适配器的意思,它在ListView和数据源之间起到了一个桥梁的作用,ListView并不会直接和数据源打交道,而是会借助Adapter这个桥梁来去访问真正的数据源,与之前不同的是,Adapter的接口都是统一的,因此ListView不用再去担心任何适配方面的问题。而Adapter又是一个接口(interface),它可以去实现各种各样的子类,每个子类都能通过自己的逻辑来去完成特定的功能,以及与特定数据源的适配操作,比如说ArrayAdapter可以用于数组和List类型的数据源适配,SimpleCursorAdapter可以用于游标类型的数据源适配,这样就非常巧妙地把数据源适配困难的问题解决掉了,并且还拥有相当不错的扩展性。简单的原理示意图如下所示:

Android ListView工作原理完全解析,带你从源码的角度彻底理解

当然Adapter的作用不仅仅只有数据源适配这一点,还有一个非常非常重要的方法也需要我们在Adapter当中去重写,就是getView()方法,这个在下面的文章中还会详细讲到。


RecycleBin机制

那么在开始分析ListView的源码之前,还有一个东西是我们提前需要了解的,就是RecycleBin机制,这个机制也是ListView能够实现成百上千条数据都不会OOM最重要的一个原因。其实RecycleBin的代码并不多,只有300行左右,它是写在AbsListView中的一个内部类,所以所有继承自AbsListView的子类,也就是ListView和GridView,都可以使用这个机制。那我们来看一下RecycleBin中的主要代码,如下所示:

/** 
 * The RecycleBin facilitates reuse of views across layouts. The RecycleBin 
 * has two levels of storage: ActiveViews and ScrapViews. ActiveViews are 
 * those views which were onscreen at the start of a layout. By 
 * construction, they are displaying current information. At the end of 
 * layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews 
 * are old views that could potentially be used by the adapter to avoid 
 * allocating views unnecessarily. 
 *  
 * @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener) 
 * @see android.widget.AbsListView.RecyclerListener 
 */  
class RecycleBin {  
    private RecyclerListener mRecyclerListener;  
  
    /** 
     * The position of the first view stored in mActiveViews. 
     */  
    private int mFirstActivePosition;  
  
    /** 
     * Views that were on screen at the start of layout. This array is 
     * populated at the start of layout, and at the end of layout all view 
     * in mActiveViews are moved to mScrapViews. Views in mActiveViews 
     * represent a contiguous range of Views, with position of the first 
     * view store in mFirstActivePosition. 
     */  
    private View[] mActiveViews = new View[0];  
  
    /** 
     * Unsorted views that can be used by the adapter as a convert view. 
     */  
    private ArrayList<View>[] mScrapViews;  
  
    private int mViewTypeCount;  
  
    private ArrayList<View> mCurrentScrap;  
  
    /** 
     * Fill ActiveViews with all of the children of the AbsListView. 
     *  
     * @param childCount 
     *            The minimum number of views mActiveViews should hold 
     * @param firstActivePosition 
     *            The position of the first view that will be stored in 
     *            mActiveViews 
     */  
    void fillActiveViews(int childCount, int firstActivePosition) {  
        if (mActiveViews.length < childCount) {  
            mActiveViews = new View[childCount];  
        }  
        mFirstActivePosition = firstActivePosition;  
        final View[] activeViews = mActiveViews;  
        for (int i = 0; i < childCount; i++) {  
            View child = getChildAt(i);  
            AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();  
            // Don't put header or footer views into the scrap heap  
            if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {  
                // Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in  
                // active views.  
                // However, we will NOT place them into scrap views.  
                activeViews[i] = child;  
            }  
        }  
    }  
  
    /** 
     * Get the view corresponding to the specified position. The view will 
     * be removed from mActiveViews if it is found. 
     *  
     * @param position 
     *            The position to look up in mActiveViews 
     * @return The view if it is found, null otherwise 
     */  
    View getActiveView(int position) {  
        int index = position - mFirstActivePosition;  
        final View[] activeViews = mActiveViews;  
        if (index >= 0 && index < activeViews.length) {  
            final View match = activeViews[index];  
            activeViews[index] = null;  
            return match;  
        }  
        return null;  
    }  
  
    /** 
     * Put a view into the ScapViews list. These views are unordered. 
     *  
     * @param scrap 
     *            The view to add 
     */  
    void addScrapView(View scrap) {  
        AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();  
        if (lp == null) {  
            return;  
        }  
        // Don't put header or footer views or views that should be ignored  
        // into the scrap heap  
        int viewType = lp.viewType;  
        if (!shouldRecycleViewType(viewType)) {  
            if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {  
                removeDetachedView(scrap, false);  
            }  
            return;  
        }  
        if (mViewTypeCount == 1) {  
            dispatchFinishTemporaryDetach(scrap);  
            mCurrentScrap.add(scrap);  
        } else {  
            dispatchFinishTemporaryDetach(scrap);  
            mScrapViews[viewType].add(scrap);  
        }  
  
        if (mRecyclerListener != null) {  
            mRecyclerListener.onMovedToScrapHeap(scrap);  
        }  
    }  
  
    /** 
     * @return A view from the ScrapViews collection. These are unordered. 
     */  
    View getScrapView(int position) {  
        ArrayList<View> scrapViews;  
        if (mViewTypeCount == 1) {  
            scrapViews = mCurrentScrap;  
            int size = scrapViews.size();  
            if (size > 0) {  
                return scrapViews.remove(size - 1);  
            } else {  
                return null;  
            }  
        } else {  
            int whichScrap = mAdapter.getItemViewType(position);  
            if (whichScrap >= 0 && whichScrap < mScrapViews.length) {  
                scrapViews = mScrapViews[whichScrap];  
                int size = scrapViews.size();  
                if (size > 0) {  
                    return scrapViews.remove(size - 1);  
                }  
            }  
        }  
        return null;  
    }  
  
    public void setViewTypeCount(int viewTypeCount) {  
        if (viewTypeCount < 1) {  
            throw new IllegalArgumentException("Can't have a viewTypeCount < 1");  
        }  
        // noinspection unchecked  
        ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];  
        for (int i = 0; i < viewTypeCount; i++) {  
            scrapViews[i] = new ArrayList<View>();  
        }  
        mViewTypeCount = viewTypeCount;  
        mCurrentScrap = scrapViews[0];  
        mScrapViews = scrapViews;  
    }  
  
}

这里的RecycleBin代码并不全,我只是把最主要的几个方法提了出来。那么我们先来对这几个方法进行简单解读,这对后面分析ListView的工作原理将会有很大的帮助。

fillActiveViews() 这个方法接收两个参数,第一个参数表示要存储的view的数量,第二个参数表示ListView中第一个可见元素的position值。RecycleBin当中使用mActiveViews这个数组来存储View,调用这个方法后就会根据传入的参数来将ListView中的指定元素存储到mActiveViews数组当中。

getActiveView() 这个方法和fillActiveViews()是对应的,用于从mActiveViews数组当中获取数据。该方法接收一个position参数,表示元素在ListView当中的位置,方法内部会自动将position值转换成mActiveViews数组对应的下标值。需要注意的是,mActiveViews当中所存储的View,一旦被获取了之后就会从mActiveViews当中移除,下次获取同样位置的View将会返回null,也就是说mActiveViews不能被重复利用。

addScrapView() 用于将一个废弃的View进行缓存,该方法接收一个View参数,当有某个View确定要废弃掉的时候(比如滚动出了屏幕),就应该调用这个方法来对View进行缓存,RecycleBin当中使用mScrapViews和mCurrentScrap这两个List来存储废弃View。

getScrapView 用于从废弃缓存中取出一个View,这些废弃缓存中的View是没有顺序可言的,因此getScrapView()方法中的算法也非常简单,就是直接从mCurrentScrap当中获取尾部的一个scrap view进行返回。

setViewTypeCount() 我们都知道Adapter当中可以重写一个getViewTypeCount()来表示ListView中有几种类型的数据项,而setViewTypeCount()方法的作用就是为每种类型的数据项都单独启用一个RecycleBin缓存机制。实际上,getViewTypeCount()方法通常情况下使用的并不是很多,所以我们只要知道RecycleBin当中有这样一个功能就行了。


了解了RecycleBin中的主要方法以及它们的用处之后,下面就可以开始来分析ListView的工作原理了,这里我将还是按照以前分析源码的方式来进行,即跟着主线执行流程来逐步阅读并点到即止,不然的话要是把ListView所有的代码都贴出来,那么本篇文章将会很长很长了。


第一次Layout

View的执行流程无非就分为三步,onMeasure()用于测量View的大小,onLayout()用于确定View的布局,onDraw()用于将View绘制到界面上。而在ListView当中,onMeasure()并没有什么特殊的地方,因为它终归是一个View,占用的空间最多并且通常也就是整个屏幕。onDraw()在ListView当中也没有什么意义,因为ListView本身并不负责绘制,而是由ListView当中的子元素来进行绘制的。那么ListView大部分的神奇功能其实都是在onLayout()方法中进行的了,因此我们本篇文章也是主要分析的这个方法里的内容。


如果你到ListView源码中去找一找,你会发现ListView中是没有onLayout()这个方法的,这是因为这个方法是在ListView的父类AbsListView中实现的,代码如下所示:

/** 
 * Subclasses should NOT override this method but {@link #layoutChildren()} 
 * instead. 
 */  
@Override  
protected void onLayout(boolean changed, int l, int t, int r, int b) {  
    super.onLayout(changed, l, t, r, b);  
    mInLayout = true;  
    if (changed) {  
        int childCount = getChildCount();  
        for (int i = 0; i < childCount; i++) {  
            getChildAt(i).forceLayout();  
        }  
        mRecycler.markChildrenDirty();  
    }  
    layoutChildren();  
    mInLayout = false;  
}

可以看到,onLayout()方法中并没有做什么复杂的逻辑操作,主要就是一个判断,如果ListView的大小或者位置发生了变化,那么changed变量就会变成true,此时会要求所有的子布局都强制进行重绘。除此之外倒没有什么难理解的地方了,不过我们注意到,在第16行调用了layoutChildren()这个方法,从方法名上我们就可以猜出这个方法是用来进行子元素布局的,不过进入到这个方法当中你会发现这是个空方法,没有一行代码。这当然是可以理解的了,因为子元素的布局应该是由具体的实现类来负责完成的,而不是由父类完成。那么进入ListView的layoutChildren()方法,代码如下所示:

@Override  
protected void layoutChildren() {  
    final boolean blockLayoutRequests = mBlockLayoutRequests;  
    if (!blockLayoutRequests) {  
        mBlockLayoutRequests = true;  
    } else {  
        return;  
    }  
    try {  
        super.layoutChildren();  
        invalidate();  
        if (mAdapter == null) {  
            resetList();  
            invokeOnItemScrollListener();  
            return;  
        }  
        int childrenTop = mListPadding.top;  
        int childrenBottom = getBottom() - getTop() - mListPadding.bottom;  
        int childCount = getChildCount();  
        int index = 0;  
        int delta = 0;  
        View sel;  
        View oldSel = null;  
        View oldFirst = null;  
        View newSel = null;  
        View focusLayoutRestoreView = null;  
        // Remember stuff we will need down below  
        switch (mLayoutMode) {  
        case LAYOUT_SET_SELECTION:  
            index = mNextSelectedPosition - mFirstPosition;  
            if (index >= 0 && index < childCount) {  
                newSel = getChildAt(index);  
            }  
            break;  
        case LAYOUT_FORCE_TOP:  
        case LAYOUT_FORCE_BOTTOM:  
        case LAYOUT_SPECIFIC:  
        case LAYOUT_SYNC:  
            break;  
        case LAYOUT_MOVE_SELECTION:  
        default:  
            // Remember the previously selected view  
            index = mSelectedPosition - mFirstPosition;  
            if (index >= 0 && index < childCount) {  
                oldSel = getChildAt(index);  
            }  
            // Remember the previous first child  
            oldFirst = getChildAt(0);  
            if (mNextSelectedPosition >= 0) {  
                delta = mNextSelectedPosition - mSelectedPosition;  
            }  
            // Caution: newSel might be null  
            newSel = getChildAt(index + delta);  
        }  
        boolean dataChanged = mDataChanged;  
        if (dataChanged) {  
            handleDataChanged();  
        }  
        // Handle the empty set by removing all views that are visible  
        // and calling it a day  
        if (mItemCount == 0) {  
            resetList();  
            invokeOnItemScrollListener();  
            return;  
        } else if (mItemCount != mAdapter.getCount()) {  
            throw new IllegalStateException("The content of the adapter has changed but "  
                    + "ListView did not receive a notification. Make sure the content of "  
                    + "your adapter is not modified from a background thread, but only "  
                    + "from the UI thread. [in ListView(" + getId() + ", " + getClass()   
                    + ") with Adapter(" + mAdapter.getClass() + ")]");  
        }  
        setSelectedPositionInt(mNextSelectedPosition);  
        // Pull all children into the RecycleBin.  
        // These views will be reused if possible  
        final int firstPosition = mFirstPosition;  
        final RecycleBin recycleBin = mRecycler;  
        // reset the focus restoration  
        View focusLayoutRestoreDirectChild = null;  
        // Don't put header or footer views into the Recycler. Those are  
        // already cached in mHeaderViews;  
        if (dataChanged) {  
            for (int i = 0; i < childCount; i++) {  
                recycleBin.addScrapView(getChildAt(i));  
                if (ViewDebug.TRACE_RECYCLER) {  
                    ViewDebug.trace(getChildAt(i),  
                            ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);  
                }  
            }  
        } else {  
            recycleBin.fillActiveViews(childCount, firstPosition);  
        }  
        // take focus back to us temporarily to avoid the eventual  
        // call to clear focus when removing the focused child below  
        // from messing things up when ViewRoot assigns focus back  
        // to someone else  
        final View focusedChild = getFocusedChild();  
        if (focusedChild != null) {  
            // TODO: in some cases focusedChild.getParent() == null  
            // we can remember the focused view to restore after relayout if the  
            // data hasn't changed, or if the focused position is a header or footer  
            if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {  
                focusLayoutRestoreDirectChild = focusedChild;  
                // remember the specific view that had focus  
                focusLayoutRestoreView = findFocus();  
                if (focusLayoutRestoreView != null) {  
                    // tell it we are going to mess with it  
                    focusLayoutRestoreView.onStartTemporaryDetach();  
                }  
            }  
            requestFocus();  
        }  
        // Clear out old views  
        detachAllViewsFromParent();  
        switch (mLayoutMode) {  
        case LAYOUT_SET_SELECTION:  
            if (newSel != null) {  
                sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);  
            } else {  
                sel = fillFromMiddle(childrenTop, childrenBottom);  
            }  
            break;  
        case LAYOUT_SYNC:  
            sel = fillSpecific(mSyncPosition, mSpecificTop);  
            break;  
        case LAYOUT_FORCE_BOTTOM:  
            sel = fillUp(mItemCount - 1, childrenBottom);  
            adjustViewsUpOrDown();  
            break;  
        case LAYOUT_FORCE_TOP:  
            mFirstPosition = 0;  
            sel = fillFromTop(childrenTop);  
            adjustViewsUpOrDown();  
            break;  
        case LAYOUT_SPECIFIC:  
            sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);  
            break;  
        case LAYOUT_MOVE_SELECTION:  
            sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);  
            break;  
        default:  
            if (childCount == 0) {  
                if (!mStackFromBottom) {  
                    final int position = lookForSelectablePosition(0, true);  
                    setSelectedPositionInt(position);  
                    sel = fillFromTop(childrenTop);  
                } else {  
                    final int position = lookForSelectablePosition(mItemCount - 1, false);  
                    setSelectedPositionInt(position);  
                    sel = fillUp(mItemCount - 1, childrenBottom);  
                }  
            } else {  
                if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {  
                    sel = fillSpecific(mSelectedPosition,  
                            oldSel == null ? childrenTop : oldSel.getTop());  
                } else if (mFirstPosition < mItemCount) {  
                    sel = fillSpecific(mFirstPosition,  
                            oldFirst == null ? childrenTop : oldFirst.getTop());  
                } else {  
                    sel = fillSpecific(0, childrenTop);  
                }  
            }  
            break;  
        }  
        // Flush any cached views that did not get reused above  
        recycleBin.scrapActiveViews();  
        if (sel != null) {  
            // the current selected item should get focus if items  
            // are focusable  
            if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {  
                final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&  
                        focusLayoutRestoreView.requestFocus()) || sel.requestFocus();  
                if (!focusWasTaken) {  
                    // selected item didn't take focus, fine, but still want  
                    // to make sure something else outside of the selected view  
                    // has focus  
                    final View focused = getFocusedChild();  
                    if (focused != null) {  
                        focused.clearFocus();  
                    }  
                    positionSelector(sel);  
                } else {  
                    sel.setSelected(false);  
                    mSelectorRect.setEmpty();  
                }  
            } else {  
                positionSelector(sel);  
            }  
            mSelectedTop = sel.getTop();  
        } else {  
            if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {  
                View child = getChildAt(mMotionPosition - mFirstPosition);  
                if (child != null) positionSelector(child);  
            } else {  
                mSelectedTop = 0;  
                mSelectorRect.setEmpty();  
            }  
            // even if there is not selected position, we may need to restore  
            // focus (i.e. something focusable in touch mode)  
            if (hasFocus() && focusLayoutRestoreView != null) {  
                focusLayoutRestoreView.requestFocus();  
            }  
        }  
        // tell focus view we are done mucking with it, if it is still in  
        // our view hierarchy.  
        if (focusLayoutRestoreView != null  
                && focusLayoutRestoreView.getWindowToken() != null) {  
            focusLayoutRestoreView.onFinishTemporaryDetach();  
        }  
        mLayoutMode = LAYOUT_NORMAL;  
        mDataChanged = false;  
        mNeedSync = false;  
        setNextSelectedPositionInt(mSelectedPosition);  
        updateScrollIndicators();  
        if (mItemCount > 0) {  
            checkSelectionChanged();  
        }  
        invokeOnItemScrollListener();  
    } finally {  
        if (!blockLayoutRequests) {  
            mBlockLayoutRequests = false;  
        }  
    }  
}

这段代码比较长,我们挑重点的看。首先可以确定的是,ListView当中目前还没有任何子View,数据都还是由Adapter管理的,并没有展示到界面上,因此第19行getChildCount()方法得到的值肯定是0。接着在第81行会根据dataChanged这个布尔型的值来判断执行逻辑,dataChanged只有在数据源发生改变的情况下才会变成true,其它情况都是false,因此这里会进入到第90行的执行逻辑,调用RecycleBin的fillActiveViews()方法。按理来说,调用fillActiveViews()方法是为了将ListView的子View进行缓存的,可是目前ListView中还没有任何的子View,因此这一行暂时还起不了任何作用。


接下来在第114行会根据mLayoutMode的值来决定布局模式,默认情况下都是普通模式LAYOUT_NORMAL,因此会进入到第140行的default语句当中。而下面又会紧接着进行两次if判断,childCount目前是等于0的,并且默认的布局顺序是从上往下,因此会进入到第145行的fillFromTop()方法,我们跟进去瞧一瞧:

/** 
 * Fills the list from top to bottom, starting with mFirstPosition 
 * 
 * @param nextTop The location where the top of the first item should be 
 *        drawn 
 * 
 * @return The view that is currently selected 
 */  
private View fillFromTop(int nextTop) {  
    mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);  
    mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);  
    if (mFirstPosition < 0) {  
        mFirstPosition = 0;  
    }  
    return fillDown(mFirstPosition, nextTop);  
}

从这个方法的注释中可以看出,它所负责的主要任务就是从mFirstPosition开始,自顶至底去填充ListView。而这个方法本身并没有什么逻辑,就是判断了一下mFirstPosition值的合法性,然后调用fillDown()方法,那么我们就有理由可以猜测,填充ListView的操作是在fillDown()方法中完成的。进入fillDown()方法,代码如下所示:

/** 
 * Fills the list from pos down to the end of the list view. 
 * 
 * @param pos The first position to put in the list 
 * 
 * @param nextTop The location where the top of the item associated with pos 
 *        should be drawn 
 * 
 * @return The view that is currently selected, if it happens to be in the 
 *         range that we draw. 
 */  
private View fillDown(int pos, int nextTop) {  
    View selectedView = null;  
    int end = (getBottom() - getTop()) - mListPadding.bottom;  
    while (nextTop < end && pos < mItemCount) {  
        // is this the selected item?  
        boolean selected = pos == mSelectedPosition;  
        View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);  
        nextTop = child.getBottom() + mDividerHeight;  
        if (selected) {  
            selectedView = child;  
        }  
        pos++;  
    }  
    return selectedView;  
}

可以看到,这里使用了一个while循环来执行重复逻辑,一开始nextTop的值是第一个子元素顶部距离整个ListView顶部的像素值,pos则是刚刚传入的mFirstPosition的值,而end是ListView底部减去顶部所得的像素值,mItemCount则是Adapter中的元素数量。因此一开始的情况下nextTop必定是小于end值的,并且pos也是小于mItemCount值的。那么每执行一次while循环,pos的值都会加1,并且nextTop也会增加,当nextTop大于等于end时,也就是子元素已经超出当前屏幕了,或者pos大于等于mItemCount时,也就是所有Adapter中的元素都被遍历结束了,就会跳出while循环。


那么while循环当中又做了什么事情呢?值得让人留意的就是第18行调用的makeAndAddView()方法,进入到这个方法当中,代码如下所示:

/** 
 * Obtain the view and add it to our list of children. The view can be made 
 * fresh, converted from an unused view, or used as is if it was in the 
 * recycle bin. 
 * 
 * @param position Logical position in the list 
 * @param y Top or bottom edge of the view to add 
 * @param flow If flow is true, align top edge to y. If false, align bottom 
 *        edge to y. 
 * @param childrenLeft Left edge where children should be positioned 
 * @param selected Is this position selected? 
 * @return View that was added 
 */  
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,  
        boolean selected) {  
    View child;  
    if (!mDataChanged) {  
        // Try to use an exsiting view for this position  
        child = mRecycler.getActiveView(position);  
        if (child != null) {  
            // Found it -- we're using an existing child  
            // This just needs to be positioned  
            setupChild(child, position, y, flow, childrenLeft, selected, true);  
            return child;  
        }  
    }  
    // Make a new view for this position, or convert an unused view if possible  
    child = obtainView(position, mIsScrap);  
    // This needs to be positioned and measured  
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);  
    return child;  
}

这里在第19行尝试从RecycleBin当中快速获取一个active view,不过很遗憾的是目前RecycleBin当中还没有缓存任何的View,所以这里得到的值肯定是null。那么取得了null之后就会继续向下运行,到第28行会调用obtainView()方法来再次尝试获取一个View,这次的obtainView()方法是可以保证一定返回一个View的,于是下面立刻将获取到的View传入到了setupChild()方法当中。那么obtainView()内部到底是怎么工作的呢?我们先进入到这个方法里面看一下:

/** 
 * Get a view and have it show the data associated with the specified 
 * position. This is called when we have already discovered that the view is 
 * not available for reuse in the recycle bin. The only choices left are 
 * converting an old view or making a new one. 
 *  
 * @param position 
 *            The position to display 
 * @param isScrap 
 *            Array of at least 1 boolean, the first entry will become true 
 *            if the returned view was taken from the scrap heap, false if 
 *            otherwise. 
 *  
 * @return A view displaying the data associated with the specified position 
 */  
View obtainView(int position, boolean[] isScrap) {  
    isScrap[0] = false;  
    View scrapView;  
    scrapView = mRecycler.getScrapView(position);  
    View child;  
    if (scrapView != null) {  
        child = mAdapter.getView(position, scrapView, this);  
        if (child != scrapView) {  
            mRecycler.addScrapView(scrapView);  
            if (mCacheColorHint != 0) {  
                child.setDrawingCacheBackgroundColor(mCacheColorHint);  
            }  
        } else {  
            isScrap[0] = true;  
            dispatchFinishTemporaryDetach(child);  
        }  
    } else {  
        child = mAdapter.getView(position, null, this);  
        if (mCacheColorHint != 0) {  
            child.setDrawingCacheBackgroundColor(mCacheColorHint);  
        }  
    }  
    return child;  
}

obtainView()方法中的代码并不多,但却包含了非常非常重要的逻辑,不夸张的说,整个ListView中最重要的内容可能就在这个方法里了。那么我们还是按照执行流程来看,在第19行代码中调用了RecycleBin的getScrapView()方法来尝试获取一个废弃缓存中的View,同样的道理,这里肯定是获取不到的,getScrapView()方法会返回一个null。这时该怎么办呢?没有关系,代码会执行到第33行,调用mAdapter的getView()方法来去获取一个View。那么mAdapter是什么呢?当然就是当前ListView关联的适配器了。而getView()方法又是什么呢?还用说吗,这个就是我们平时使用ListView时最最经常重写的一个方法了,这里getView()方法中传入了三个参数,分别是position,null和this。


那么我们平时写ListView的Adapter时,getView()方法通常会怎么写呢?这里我举个简单的例子:

@Override  
public View getView(int position, View convertView, ViewGroup parent) {  
    Fruit fruit = getItem(position);  
    View view;  
    if (convertView == null) {  
        view = LayoutInflater.from(getContext()).inflate(resourceId, null);  
    } else {  
        view = convertView;  
    }  
    ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);  
    TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);  
    fruitImage.setImageResource(fruit.getImageId());  
    fruitName.setText(fruit.getName());  
    return view;  
}

getView()方法接受的三个参数,第一个参数position代表当前子元素的的位置,我们可以通过具体的位置来获取与其相关的数据。第二个参数convertView,刚才传入的是null,说明没有convertView可以利用,因此我们会调用LayoutInflater的inflate()方法来去加载一个布局。接下来会对这个view进行一些属性和值的设定,最后将view返回。


那么这个View也会作为obtainView()的结果进行返回,并最终传入到setupChild()方法当中。其实也就是说,第一次layout过程当中,所有的子View都是调用LayoutInflater的inflate()方法加载出来的,这样就会相对比较耗时,但是不用担心,后面就不会再有这种情况了,那么我们继续往下看:

/** 
 * Add a view as a child and make sure it is measured (if necessary) and 
 * positioned properly. 
 * 
 * @param child The view to add 
 * @param position The position of this child 
 * @param y The y position relative to which this view will be positioned 
 * @param flowDown If true, align top edge to y. If false, align bottom 
 *        edge to y. 
 * @param childrenLeft Left edge where children should be positioned 
 * @param selected Is this position selected? 
 * @param recycled Has this view been pulled from the recycle bin? If so it 
 *        does not need to be remeasured. 
 */  
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,  
        boolean selected, boolean recycled) {  
    final boolean isSelected = selected && shouldShowSelector();  
    final boolean updateChildSelected = isSelected != child.isSelected();  
    final int mode = mTouchMode;  
    final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&  
            mMotionPosition == position;  
    final boolean updateChildPressed = isPressed != child.isPressed();  
    final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();  
    // Respect layout params that are already in the view. Otherwise make some up...  
    // noinspection unchecked  
    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();  
    if (p == null) {  
        p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,  
                ViewGroup.LayoutParams.WRAP_CONTENT, 0);  
    }  
    p.viewType = mAdapter.getItemViewType(position);  
    if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&  
            p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {  
        attachViewToParent(child, flowDown ? -1 : 0, p);  
    } else {  
        p.forceAdd = false;  
        if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {  
            p.recycledHeaderFooter = true;  
        }  
        addViewInLayout(child, flowDown ? -1 : 0, p, true);  
    }  
    if (updateChildSelected) {  
        child.setSelected(isSelected);  
    }  
    if (updateChildPressed) {  
        child.setPressed(isPressed);  
    }  
    if (needToMeasure) {  
        int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,  
                mListPadding.left + mListPadding.right, p.width);  
        int lpHeight = p.height;  
        int childHeightSpec;  
        if (lpHeight > 0) {  
            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);  
        } else {  
            childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);  
        }  
        child.measure(childWidthSpec, childHeightSpec);  
    } else {  
        cleanupLayoutState(child);  
    }  
    final int w = child.getMeasuredWidth();  
    final int h = child.getMeasuredHeight();  
    final int childTop = flowDown ? y : y - h;  
    if (needToMeasure) {  
        final int childRight = childrenLeft + w;  
        final int childBottom = childTop + h;  
        child.layout(childrenLeft, childTop, childRight, childBottom);  
    } else {  
        child.offsetLeftAndRight(childrenLeft - child.getLeft());  
        child.offsetTopAndBottom(childTop - child.getTop());  
    }  
    if (mCachingStarted && !child.isDrawingCacheEnabled()) {  
        child.setDrawingCacheEnabled(true);  
    }  
}

setupChild()方法当中的代码虽然比较多,但是我们只看核心代码的话就非常简单了,刚才调用obtainView()方法获取到的子元素View,这里在第40行调用了addViewInLayout()方法将它添加到了ListView当中。那么根据fillDown()方法中的while循环,会让子元素View将整个ListView控件填满然后就跳出,也就是说即使我们的Adapter中有一千条数据,ListView也只会加载第一屏的数据,剩下的数据反正目前在屏幕上也看不到,所以不会去做多余的加载工作,这样就可以保证ListView中的内容能够迅速展示到屏幕上。


那么到此为止,第一次Layout过程结束。


第二次Layout


虽然我在源码中并没有找出具体的原因,但如果你自己做一下实验的话就会发现,即使是一个再简单的View,在展示到界面上之前都会经历至少两次onMeasure()和两次onLayout()的过程。其实这只是一个很小的细节,平时对我们影响并不大,因为不管是onMeasure()或者onLayout()几次,反正都是执行的相同的逻辑,我们并不需要进行过多关心。但是在ListView中情况就不一样了,因为这就意味着layoutChildren()过程会执行两次,而这个过程当中涉及到向ListView中添加子元素,如果相同的逻辑执行两遍的话,那么ListView中就会存在一份重复的数据了。因此ListView在layoutChildren()过程当中做了第二次Layout的逻辑处理,非常巧妙地解决了这个问题,下面我们就来分析一下第二次Layout的过程。


其实第二次Layout和第一次Layout的基本流程是差不多的,那么我们还是从layoutChildren()方法开始看起:

@Override  
protected void layoutChildren() {  
    final boolean blockLayoutRequests = mBlockLayoutRequests;  
    if (!blockLayoutRequests) {  
        mBlockLayoutRequests = true;  
    } else {  
        return;  
    }  
    try {  
        super.layoutChildren();  
        invalidate();  
        if (mAdapter == null) {  
            resetList();  
            invokeOnItemScrollListener();  
            return;  
        }  
        int childrenTop = mListPadding.top;  
        int childrenBottom = getBottom() - getTop() - mListPadding.bottom;  
        int childCount = getChildCount();  
        int index = 0;  
        int delta = 0;  
        View sel;  
        View oldSel = null;  
        View oldFirst = null;  
        View newSel = null;  
        View focusLayoutRestoreView = null;  
        // Remember stuff we will need down below  
        switch (mLayoutMode) {  
        case LAYOUT_SET_SELECTION:  
            index = mNextSelectedPosition - mFirstPosition;  
            if (index >= 0 && index < childCount) {  
                newSel = getChildAt(index);  
            }  
            break;  
        case LAYOUT_FORCE_TOP:  
        case LAYOUT_FORCE_BOTTOM:  
        case LAYOUT_SPECIFIC:  
        case LAYOUT_SYNC:  
            break;  
        case LAYOUT_MOVE_SELECTION:  
        default:  
            // Remember the previously selected view  
            index = mSelectedPosition - mFirstPosition;  
            if (index >= 0 && index < childCount) {  
                oldSel = getChildAt(index);  
            }  
            // Remember the previous first child  
            oldFirst = getChildAt(0);  
            if (mNextSelectedPosition >= 0) {  
                delta = mNextSelectedPosition - mSelectedPosition;  
            }  
            // Caution: newSel might be null  
            newSel = getChildAt(index + delta);  
        }  
        boolean dataChanged = mDataChanged;  
        if (dataChanged) {  
            handleDataChanged();  
        }  
        // Handle the empty set by removing all views that are visible  
        // and calling it a day  
        if (mItemCount == 0) {  
            resetList();  
            invokeOnItemScrollListener();  
            return;  
        } else if (mItemCount != mAdapter.getCount()) {  
            throw new IllegalStateException("The content of the adapter has changed but "  
                    + "ListView did not receive a notification. Make sure the content of "  
                    + "your adapter is not modified from a background thread, but only "  
                    + "from the UI thread. [in ListView(" + getId() + ", " + getClass()   
                    + ") with Adapter(" + mAdapter.getClass() + ")]");  
        }  
        setSelectedPositionInt(mNextSelectedPosition);  
        // Pull all children into the RecycleBin.  
        // These views will be reused if possible  
        final int firstPosition = mFirstPosition;  
        final RecycleBin recycleBin = mRecycler;  
        // reset the focus restoration  
        View focusLayoutRestoreDirectChild = null;  
        // Don't put header or footer views into the Recycler. Those are  
        // already cached in mHeaderViews;  
        if (dataChanged) {  
            for (int i = 0; i < childCount; i++) {  
                recycleBin.addScrapView(getChildAt(i));  
                if (ViewDebug.TRACE_RECYCLER) {  
                    ViewDebug.trace(getChildAt(i),  
                            ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);  
                }  
            }  
        } else {  
            recycleBin.fillActiveViews(childCount, firstPosition);  
        }  
        // take focus back to us temporarily to avoid the eventual  
        // call to clear focus when removing the focused child below  
        // from messing things up when ViewRoot assigns focus back  
        // to someone else  
        final View focusedChild = getFocusedChild();  
        if (focusedChild != null) {  
            // TODO: in some cases focusedChild.getParent() == null  
            // we can remember the focused view to restore after relayout if the  
            // data hasn't changed, or if the focused position is a header or footer  
            if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {  
                focusLayoutRestoreDirectChild = focusedChild;  
                // remember the specific view that had focus  
                focusLayoutRestoreView = findFocus();  
                if (focusLayoutRestoreView != null) {  
                    // tell it we are going to mess with it  
                    focusLayoutRestoreView.onStartTemporaryDetach();  
                }  
            }  
            requestFocus();  
        }  
        // Clear out old views  
        detachAllViewsFromParent();  
        switch (mLayoutMode) {  
        case LAYOUT_SET_SELECTION:  
            if (newSel != null) {  
                sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);  
            } else {  
                sel = fillFromMiddle(childrenTop, childrenBottom);  
            }  
            break;  
        case LAYOUT_SYNC:  
            sel = fillSpecific(mSyncPosition, mSpecificTop);  
            break;  
        case LAYOUT_FORCE_BOTTOM:  
            sel = fillUp(mItemCount - 1, childrenBottom);  
            adjustViewsUpOrDown();  
            break;  
        case LAYOUT_FORCE_TOP:  
            mFirstPosition = 0;  
            sel = fillFromTop(childrenTop);  
            adjustViewsUpOrDown();  
            break;  
        case LAYOUT_SPECIFIC:  
            sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);  
            break;  
        case LAYOUT_MOVE_SELECTION:  
            sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);  
            break;  
        default:  
            if (childCount == 0) {  
                if (!mStackFromBottom) {  
                    final int position = lookForSelectablePosition(0, true);  
                    setSelectedPositionInt(position);  
                    sel = fillFromTop(childrenTop);  
                } else {  
                    final int position = lookForSelectablePosition(mItemCount - 1, false);  
                    setSelectedPositionInt(position);  
                    sel = fillUp(mItemCount - 1, childrenBottom);  
                }  
            } else {  
                if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {  
                    sel = fillSpecific(mSelectedPosition,  
                            oldSel == null ? childrenTop : oldSel.getTop());  
                } else if (mFirstPosition < mItemCount) {  
                    sel = fillSpecific(mFirstPosition,  
                            oldFirst == null ? childrenTop : oldFirst.getTop());  
                } else {  
                    sel = fillSpecific(0, childrenTop);  
                }  
            }  
            break;  
        }  
        // Flush any cached views that did not get reused above  
        recycleBin.scrapActiveViews();  
        if (sel != null) {  
            // the current selected item should get focus if items  
            // are focusable  
            if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {  
                final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&  
                        focusLayoutRestoreView.requestFocus()) || sel.requestFocus();  
                if (!focusWasTaken) {  
                    // selected item didn't take focus, fine, but still want  
                    // to make sure something else outside of the selected view  
                    // has focus  
                    final View focused = getFocusedChild();  
                    if (focused != null) {  
                        focused.clearFocus();  
                    }  
                    positionSelector(sel);  
                } else {  
                    sel.setSelected(false);  
                    mSelectorRect.setEmpty();  
                }  
            } else {  
                positionSelector(sel);  
            }  
            mSelectedTop = sel.getTop();  
        } else {  
            if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {  
                View child = getChildAt(mMotionPosition - mFirstPosition);  
                if (child != null) positionSelector(child);  
            } else {  
                mSelectedTop = 0;  
                mSelectorRect.setEmpty();  
            }  
            // even if there is not selected position, we may need to restore  
            // focus (i.e. something focusable in touch mode)  
            if (hasFocus() && focusLayoutRestoreView != null) {  
                focusLayoutRestoreView.requestFocus();  
            }  
        }  
        // tell focus view we are done mucking with it, if it is still in  
        // our view hierarchy.  
        if (focusLayoutRestoreView != null  
                && focusLayoutRestoreView.getWindowToken() != null) {  
            focusLayoutRestoreView.onFinishTemporaryDetach();  
        }  
        mLayoutMode = LAYOUT_NORMAL;  
        mDataChanged = false;  
        mNeedSync = false;  
        setNextSelectedPositionInt(mSelectedPosition);  
        updateScrollIndicators();  
        if (mItemCount > 0) {  
            checkSelectionChanged();  
        }  
        invokeOnItemScrollListener();  
    } finally {  
        if (!blockLayoutRequests) {  
            mBlockLayoutRequests = false;  
        }  
    }  
}

同样还是在第19行,调用getChildCount()方法来获取子View的数量,只不过现在得到的值不会再是0了,而是ListView中一屏可以显示的子View数量,因为我们刚刚在第一次Layout过程当中向ListView添加了这么多的子View。下面在第90行调用了RecycleBin的fillActiveViews()方法,这次效果可就不一样了,因为目前ListView中已经有子View了,这样所有的子View都会被缓存到RecycleBin的mActiveViews数组当中,后面将会用到它们。


接下来将会是非常非常重要的一个操作,在第113行调用了detachAllViewsFromParent()方法。这个方法会将所有ListView当中的子View全部清除掉,从而保证第二次Layout过程不会产生一份重复的数据。那有的朋友可能会问了,这样把已经加载好的View又清除掉,待会还要再重新加载一遍,这不是严重影响效率吗?不用担心,还记得我们刚刚调用了RecycleBin的fillActiveViews()方法来缓存子View吗,待会儿将会直接使用这些缓存好的View来进行加载,而并不会重新执行一遍inflate过程,因此效率方面并不会有什么明显的影响。


那么我们接着看,在第141行的判断逻辑当中,由于不再等于0了,因此会进入到else语句当中。而else语句中又有三个逻辑判断,第一个逻辑判断不成立,因为默认情况下我们没有选中任何子元素,mSelectedPosition应该等于-1。第二个逻辑判断通常是成立的,因为mFirstPosition的值一开始是等于0的,只要adapter中的数据大于0条件就成立。那么进入到fillSpecific()方法当中,代码如下所示:

/** 
 * Put a specific item at a specific location on the screen and then build 
 * up and down from there. 
 * 
 * @param position The reference view to use as the starting point 
 * @param top Pixel offset from the top of this view to the top of the 
 *        reference view. 
 * 
 * @return The selected view, or null if the selected view is outside the 
 *         visible area. 
 */  
private View fillSpecific(int position, int top) {  
    boolean tempIsSelected = position == mSelectedPosition;  
    View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);  
    // Possibly changed again in fillUp if we add rows above this one.  
    mFirstPosition = position;  
    View above;  
    View below;  
    final int dividerHeight = mDividerHeight;  
    if (!mStackFromBottom) {  
        above = fillUp(position - 1, temp.getTop() - dividerHeight);  
        // This will correct for the top of the first view not touching the top of the list  
        adjustViewsUpOrDown();  
        below = fillDown(position + 1, temp.getBottom() + dividerHeight);  
        int childCount = getChildCount();  
        if (childCount > 0) {  
            correctTooHigh(childCount);  
        }  
    } else {  
        below = fillDown(position + 1, temp.getBottom() + dividerHeight);  
        // This will correct for the bottom of the last view not touching the bottom of the list  
        adjustViewsUpOrDown();  
        above = fillUp(position - 1, temp.getTop() - dividerHeight);  
        int childCount = getChildCount();  
        if (childCount > 0) {  
             correctTooLow(childCount);  
        }  
    }  
    if (tempIsSelected) {  
        return temp;  
    } else if (above != null) {  
        return above;  
    } else {  
        return below;  
    }  
}

fillSpecific()这算是一个新方法了,不过其实它和fillUp()、fillDown()方法功能也是差不多的,主要的区别在于,fillSpecific()方法会优先将指定位置的子View先加载到屏幕上,然后再加载该子View往上以及往下的其它子View。那么由于这里我们传入的position就是第一个子View的位置,于是fillSpecific()方法的作用就基本上和fillDown()方法是差不多的了,这里我们就不去关注太多它的细节,而是将精力放在makeAndAddView()方法上面。再次回到makeAndAddView()方法,代码如下所示:

/** 
 * Obtain the view and add it to our list of children. The view can be made 
 * fresh, converted from an unused view, or used as is if it was in the 
 * recycle bin. 
 * 
 * @param position Logical position in the list 
 * @param y Top or bottom edge of the view to add 
 * @param flow If flow is true, align top edge to y. If false, align bottom 
 *        edge to y. 
 * @param childrenLeft Left edge where children should be positioned 
 * @param selected Is this position selected? 
 * @return View that was added 
 */  
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,  
        boolean selected) {  
    View child;  
    if (!mDataChanged) {  
        // Try to use an exsiting view for this position  
        child = mRecycler.getActiveView(position);  
        if (child != null) {  
            // Found it -- we're using an existing child  
            // This just needs to be positioned  
            setupChild(child, position, y, flow, childrenLeft, selected, true);  
            return child;  
        }  
    }  
    // Make a new view for this position, or convert an unused view if possible  
    child = obtainView(position, mIsScrap);  
    // This needs to be positioned and measured  
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);  
    return child;  
}

仍然还是在第19行尝试从RecycleBin当中获取Active View,然而这次就一定可以获取到了,因为前面我们调用了RecycleBin的fillActiveViews()方法来缓存子View。那么既然如此,就不会再进入到第28行的obtainView()方法,而是会直接进入setupChild()方法当中,这样也省去了很多时间,因为如果在obtainView()方法中又要去infalte布局的话,那么ListView的初始加载效率就大大降低了。


注意在第23行,setupChild()方法的最后一个参数传入的是true,这个参数表明当前的View是之前被回收过的,那么我们再次回到setupChild()方法当中:

/** 
 * Add a view as a child and make sure it is measured (if necessary) and 
 * positioned properly. 
 * 
 * @param child The view to add 
 * @param position The position of this child 
 * @param y The y position relative to which this view will be positioned 
 * @param flowDown If true, align top edge to y. If false, align bottom 
 *        edge to y. 
 * @param childrenLeft Left edge where children should be positioned 
 * @param selected Is this position selected? 
 * @param recycled Has this view been pulled from the recycle bin? If so it 
 *        does not need to be remeasured. 
 */  
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,  
        boolean selected, boolean recycled) {  
    final boolean isSelected = selected && shouldShowSelector();  
    final boolean updateChildSelected = isSelected != child.isSelected();  
    final int mode = mTouchMode;  
    final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&  
            mMotionPosition == position;  
    final boolean updateChildPressed = isPressed != child.isPressed();  
    final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();  
    // Respect layout params that are already in the view. Otherwise make some up...  
    // noinspection unchecked  
    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();  
    if (p == null) {  
        p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,  
                ViewGroup.LayoutParams.WRAP_CONTENT, 0);  
    }  
    p.viewType = mAdapter.getItemViewType(position);  
    if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&  
            p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {  
        attachViewToParent(child, flowDown ? -1 : 0, p);  
    } else {  
        p.forceAdd = false;  
        if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {  
            p.recycledHeaderFooter = true;  
        }  
        addViewInLayout(child, flowDown ? -1 : 0, p, true);  
    }  
    if (updateChildSelected) {  
        child.setSelected(isSelected);  
    }  
    if (updateChildPressed) {  
        child.setPressed(isPressed);  
    }  
    if (needToMeasure) {  
        int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,  
                mListPadding.left + mListPadding.right, p.width);  
        int lpHeight = p.height;  
        int childHeightSpec;  
        if (lpHeight > 0) {  
            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);  
        } else {  
            childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);  
        }  
        child.measure(childWidthSpec, childHeightSpec);  
    } else {  
        cleanupLayoutState(child);  
    }  
    final int w = child.getMeasuredWidth();  
    final int h = child.getMeasuredHeight();  
    final int childTop = flowDown ? y : y - h;  
    if (needToMeasure) {  
        final int childRight = childrenLeft + w;  
        final int childBottom = childTop + h;  
        child.layout(childrenLeft, childTop, childRight, childBottom);  
    } else {  
        child.offsetLeftAndRight(childrenLeft - child.getLeft());  
        child.offsetTopAndBottom(childTop - child.getTop());  
    }  
    if (mCachingStarted && !child.isDrawingCacheEnabled()) {  
        child.setDrawingCacheEnabled(true);  
    }  
}

可以看到,setupChild()方法的最后一个参数是recycled,然后在第32行会对这个变量进行判断,由于recycled现在是true,所以会执行attachViewToParent()方法,而第一次Layout过程则是执行的else语句中的addViewInLayout()方法。这两个方法最大的区别在于,如果我们需要向ViewGroup中添加一个新的子View,应该调用addViewInLayout()方法,而如果是想要将一个之前detach的View重新attach到ViewGroup上,就应该调用attachViewToParent()方法。那么由于前面在layoutChildren()方法当中调用了detachAllViewsFromParent()方法,这样ListView中所有的子View都是处于detach状态的,所以这里attachViewToParent()方法是正确的选择。


经历了这样一个detach又attach的过程,ListView中所有的子View又都可以正常显示出来了,那么第二次Layout过程结束。


滑动加载更多数据

经历了两次Layout过程,虽说我们已经可以在ListView中看到内容了,然而关于ListView最神奇的部分我们却还没有接触到,因为目前ListView中只是加载并显示了第一屏的数据而已。比如说我们的Adapter当中有1000条数据,但是第一屏只显示了10条,ListView中也只有10个子View而已,那么剩下的990是怎样工作并显示到界面上的呢?这就要看一下ListView滑动部分的源码了,因为我们是通过手指滑动来显示更多数据的。


由于滑动部分的机制是属于通用型的,即ListView和GridView都会使用同样的机制,因此这部分代码就肯定是写在AbsListView当中的了。那么监听触控事件是在onTouchEvent()方法当中进行的,我们就来看一下AbsListView中的这个方法:

@Override  
public boolean onTouchEvent(MotionEvent ev) {  
    if (!isEnabled()) {  
        // A disabled view that is clickable still consumes the touch  
        // events, it just doesn't respond to them.  
        return isClickable() || isLongClickable();  
    }  
    final int action = ev.getAction();  
    View v;  
    int deltaY;  
    if (mVelocityTracker == null) {  
        mVelocityTracker = VelocityTracker.obtain();  
    }  
    mVelocityTracker.addMovement(ev);  
    switch (action & MotionEvent.ACTION_MASK) {  
    case MotionEvent.ACTION_DOWN: {  
        mActivePointerId = ev.getPointerId(0);  
        final int x = (int) ev.getX();  
        final int y = (int) ev.getY();  
        int motionPosition = pointToPosition(x, y);  
        if (!mDataChanged) {  
            if ((mTouchMode != TOUCH_MODE_FLING) && (motionPosition >= 0)  
                    && (getAdapter().isEnabled(motionPosition))) {  
                // User clicked on an actual view (and was not stopping a  
                // fling). It might be a  
                // click or a scroll. Assume it is a click until proven  
                // otherwise  
                mTouchMode = TOUCH_MODE_DOWN;  
                // FIXME Debounce  
                if (mPendingCheckForTap == null) {  
                    mPendingCheckForTap = new CheckForTap();  
                }  
                postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());  
            } else {  
                if (ev.getEdgeFlags() != 0 && motionPosition < 0) {  
                    // If we couldn't find a view to click on, but the down  
                    // event was touching  
                    // the edge, we will bail out and try again. This allows  
                    // the edge correcting  
                    // code in ViewRoot to try to find a nearby view to  
                    // select  
                    return false;  
                }  
  
                if (mTouchMode == TOUCH_MODE_FLING) {  
                    // Stopped a fling. It is a scroll.  
                    createScrollingCache();  
                    mTouchMode = TOUCH_MODE_SCROLL;  
                    mMotionCorrection = 0;  
                    motionPosition = findMotionRow(y);  
                    reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL);  
                }  
            }  
        }  
        if (motionPosition >= 0) {  
            // Remember where the motion event started  
            v = getChildAt(motionPosition - mFirstPosition);  
            mMotionViewOriginalTop = v.getTop();  
        }  
        mMotionX = x;  
        mMotionY = y;  
        mMotionPosition = motionPosition;  
        mLastY = Integer.MIN_VALUE;  
        break;  
    }  
    case MotionEvent.ACTION_MOVE: {  
        final int pointerIndex = ev.findPointerIndex(mActivePointerId);  
        final int y = (int) ev.getY(pointerIndex);  
        deltaY = y - mMotionY;  
        switch (mTouchMode) {  
        case TOUCH_MODE_DOWN:  
        case TOUCH_MODE_TAP:  
        case TOUCH_MODE_DONE_WAITING:  
            // Check if we have moved far enough that it looks more like a  
            // scroll than a tap  
            startScrollIfNeeded(deltaY);  
            break;  
        case TOUCH_MODE_SCROLL:  
            if (PROFILE_SCROLLING) {  
                if (!mScrollProfilingStarted) {  
                    Debug.startMethodTracing("AbsListViewScroll");  
                    mScrollProfilingStarted = true;  
                }  
            }  
            if (y != mLastY) {  
                deltaY -= mMotionCorrection;  
                int incrementalDeltaY = mLastY != Integer.MIN_VALUE ? y - mLastY : deltaY;  
                // No need to do all this work if we're not going to move  
                // anyway  
                boolean atEdge = false;  
                if (incrementalDeltaY != 0) {  
                    atEdge = trackMotionScroll(deltaY, incrementalDeltaY);  
                }  
                // Check to see if we have bumped into the scroll limit  
                if (atEdge && getChildCount() > 0) {  
                    // Treat this like we're starting a new scroll from the  
                    // current  
                    // position. This will let the user start scrolling back  
                    // into  
                    // content immediately rather than needing to scroll  
                    // back to the  
                    // point where they hit the limit first.  
                    int motionPosition = findMotionRow(y);  
                    if (motionPosition >= 0) {  
                        final View motionView = getChildAt(motionPosition - mFirstPosition);  
                        mMotionViewOriginalTop = motionView.getTop();  
                    }  
                    mMotionY = y;  
                    mMotionPosition = motionPosition;  
                    invalidate();  
                }  
                mLastY = y;  
            }  
            break;  
        }  
        break;  
    }  
    case MotionEvent.ACTION_UP: {  
        switch (mTouchMode) {  
        case TOUCH_MODE_DOWN:  
        case TOUCH_MODE_TAP:  
        case TOUCH_MODE_DONE_WAITING:  
            final int motionPosition = mMotionPosition;  
            final View child = getChildAt(motionPosition - mFirstPosition);  
            if (child != null && !child.hasFocusable()) {  
                if (mTouchMode != TOUCH_MODE_DOWN) {  
                    child.setPressed(false);  
                }  
                if (mPerformClick == null) {  
                    mPerformClick = new PerformClick();  
                }  
                final AbsListView.PerformClick performClick = mPerformClick;  
                performClick.mChild = child;  
                performClick.mClickMotionPosition = motionPosition;  
                performClick.rememberWindowAttachCount();  
                mResurrectToPosition = motionPosition;  
                if (mTouchMode == TOUCH_MODE_DOWN || mTouchMode == TOUCH_MODE_TAP) {  
                    final Handler handler = getHandler();  
                    if (handler != null) {  
                        handler.removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ? mPendingCheckForTap  
                                : mPendingCheckForLongPress);  
                    }  
                    mLayoutMode = LAYOUT_NORMAL;  
                    if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {  
                        mTouchMode = TOUCH_MODE_TAP;  
                        setSelectedPositionInt(mMotionPosition);  
                        layoutChildren();  
                        child.setPressed(true);  
                        positionSelector(child);  
                        setPressed(true);  
                        if (mSelector != null) {  
                            Drawable d = mSelector.getCurrent();  
                            if (d != null && d instanceof TransitionDrawable) {  
                                ((TransitionDrawable) d).resetTransition();  
                            }  
                        }  
                        postDelayed(new Runnable() {  
                            public void run() {  
                                child.setPressed(false);  
                                setPressed(false);  
                                if (!mDataChanged) {  
                                    post(performClick);  
                                }  
                                mTouchMode = TOUCH_MODE_REST;  
                            }  
                        }, ViewConfiguration.getPressedStateDuration());  
                    } else {  
                        mTouchMode = TOUCH_MODE_REST;  
                    }  
                    return true;  
                } else if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {  
                    post(performClick);  
                }  
            }  
            mTouchMode = TOUCH_MODE_REST;  
            break;  
        case TOUCH_MODE_SCROLL:  
            final int childCount = getChildCount();  
            if (childCount > 0) {  
                if (mFirstPosition == 0  
                        && getChildAt(0).getTop() >= mListPadding.top  
                        && mFirstPosition + childCount < mItemCount  
                        && getChildAt(childCount - 1).getBottom() <= getHeight()  
                                - mListPadding.bottom) {  
                    mTouchMode = TOUCH_MODE_REST;  
                    reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);  
                } else {  
                    final VelocityTracker velocityTracker = mVelocityTracker;  
                    velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);  
                    final int initialVelocity = (int) velocityTracker  
                            .getYVelocity(mActivePointerId);  
                    if (Math.abs(initialVelocity) > mMinimumVelocity) {  
                        if (mFlingRunnable == null) {  
                            mFlingRunnable = new FlingRunnable();  
                        }  
                        reportScrollStateChange(OnScrollListener.SCROLL_STATE_FLING);  
                        mFlingRunnable.start(-initialVelocity);  
                    } else {  
                        mTouchMode = TOUCH_MODE_REST;  
                        reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);  
                    }  
                }  
            } else {  
                mTouchMode = TOUCH_MODE_REST;  
                reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);  
            }  
            break;  
        }  
        setPressed(false);  
        // Need to redraw since we probably aren't drawing the selector  
        // anymore  
        invalidate();  
        final Handler handler = getHandler();  
        if (handler != null) {  
            handler.removeCallbacks(mPendingCheckForLongPress);  
        }  
        if (mVelocityTracker != null) {  
            mVelocityTracker.recycle();  
            mVelocityTracker = null;  
        }  
        mActivePointerId = INVALID_POINTER;  
        if (PROFILE_SCROLLING) {  
            if (mScrollProfilingStarted) {  
                Debug.stopMethodTracing();  
                mScrollProfilingStarted = false;  
            }  
        }  
        break;  
    }  
    case MotionEvent.ACTION_CANCEL: {  
        mTouchMode = TOUCH_MODE_REST;  
        setPressed(false);  
        View motionView = this.getChildAt(mMotionPosition - mFirstPosition);  
        if (motionView != null) {  
            motionView.setPressed(false);  
        }  
        clearScrollingCache();  
        final Handler handler = getHandler();  
        if (handler != null) {  
            handler.removeCallbacks(mPendingCheckForLongPress);  
        }  
        if (mVelocityTracker != null) {  
            mVelocityTracker.recycle();  
            mVelocityTracker = null;  
        }  
        mActivePointerId = INVALID_POINTER;  
        break;  
    }  
    case MotionEvent.ACTION_POINTER_UP: {  
        onSecondaryPointerUp(ev);  
        final int x = mMotionX;  
        final int y = mMotionY;  
        final int motionPosition = pointToPosition(x, y);  
        if (motionPosition >= 0) {  
            // Remember where the motion event started  
            v = getChildAt(motionPosition - mFirstPosition);  
            mMotionViewOriginalTop = v.getTop();  
            mMotionPosition = motionPosition;  
        }  
        mLastY = y;  
        break;  
    }  
    }  
    return true;  
}

这个方法中的代码就非常多了,因为它所处理的逻辑也非常多,要监听各种各样的触屏事件。但是我们目前所关心的就只有手指在屏幕上滑动这一个事件而已,对应的是ACTION_MOVE这个动作,那么我们就只看这部分代码就可以了。


可以看到,ACTION_MOVE这个case里面又嵌套了一个switch语句,是根据当前的TouchMode来选择的。那这里我可以直接告诉大家,当手指在屏幕上滑动时,TouchMode是等于TOUCH_MODE_SCROLL这个值的,至于为什么那又要牵扯到另外的好几个方法,这里限于篇幅原因就不再展开讲解了,喜欢寻根究底的朋友们可以自己去源码里找一找原因。


这样的话,代码就应该会走到第78行的这个case里面去了,在这个case当中并没有什么太多需要注意的东西,唯一一点非常重要的就是第92行调用的trackMotionScroll()方法,相当于我们手指只要在屏幕上稍微有一点点移动,这个方法就会被调用,而如果是正常在屏幕上滑动的话,那么这个方法就会被调用很多次。那么我们进入到这个方法中瞧一瞧,代码如下所示:

boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {  
    final int childCount = getChildCount();  
    if (childCount == 0) {  
        return true;  
    }  
    final int firstTop = getChildAt(0).getTop();  
    final int lastBottom = getChildAt(childCount - 1).getBottom();  
    final Rect listPadding = mListPadding;  
    final int spaceAbove = listPadding.top - firstTop;  
    final int end = getHeight() - listPadding.bottom;  
    final int spaceBelow = lastBottom - end;  
    final int height = getHeight() - getPaddingBottom() - getPaddingTop();  
    if (deltaY < 0) {  
        deltaY = Math.max(-(height - 1), deltaY);  
    } else {  
        deltaY = Math.min(height - 1, deltaY);  
    }  
    if (incrementalDeltaY < 0) {  
        incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);  
    } else {  
        incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);  
    }  
    final int firstPosition = mFirstPosition;  
    if (firstPosition == 0 && firstTop >= listPadding.top && deltaY >= 0) {  
        // Don't need to move views down if the top of the first position  
        // is already visible  
        return true;  
    }  
    if (firstPosition + childCount == mItemCount && lastBottom <= end && deltaY <= 0) {  
        // Don't need to move views up if the bottom of the last position  
        // is already visible  
        return true;  
    }  
    final boolean down = incrementalDeltaY < 0;  
    final boolean inTouchMode = isInTouchMode();  
    if (inTouchMode) {  
        hideSelector();  
    }  
    final int headerViewsCount = getHeaderViewsCount();  
    final int footerViewsStart = mItemCount - getFooterViewsCount();  
    int start = 0;  
    int count = 0;  
    if (down) {  
        final int top = listPadding.top - incrementalDeltaY;  
        for (int i = 0; i < childCount; i++) {  
            final View child = getChildAt(i);  
            if (child.getBottom() >= top) {  
                break;  
            } else {  
                count++;  
                int position = firstPosition + i;  
                if (position >= headerViewsCount && position < footerViewsStart) {  
                    mRecycler.addScrapView(child);  
                }  
            }  
        }  
    } else {  
        final int bottom = getHeight() - listPadding.bottom - incrementalDeltaY;  
        for (int i = childCount - 1; i >= 0; i--) {  
            final View child = getChildAt(i);  
            if (child.getTop() <= bottom) {  
                break;  
            } else {  
                start = i;  
                count++;  
                int position = firstPosition + i;  
                if (position >= headerViewsCount && position < footerViewsStart) {  
                    mRecycler.addScrapView(child);  
                }  
            }  
        }  
    }  
    mMotionViewNewTop = mMotionViewOriginalTop + deltaY;  
    mBlockLayoutRequests = true;  
    if (count > 0) {  
        detachViewsFromParent(start, count);  
    }  
    offsetChildrenTopAndBottom(incrementalDeltaY);  
    if (down) {  
        mFirstPosition += count;  
    }  
    invalidate();  
    final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);  
    if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {  
        fillGap(down);  
    }  
    if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {  
        final int childIndex = mSelectedPosition - mFirstPosition;  
        if (childIndex >= 0 && childIndex < getChildCount()) {  
            positionSelector(getChildAt(childIndex));  
        }  
    }  
    mBlockLayoutRequests = false;  
    invokeOnItemScrollListener();  
    awakenScrollBars();  
    return false;  
}

这个方法接收两个参数,deltaY表示从手指按下时的位置到当前手指位置的距离,incrementalDeltaY则表示据上次触发event事件手指在Y方向上位置的改变量,那么其实我们就可以通过incrementalDeltaY的正负值情况来判断用户是向上还是向下滑动的了。如第34行代码所示,如果incrementalDeltaY小于0,说明是向下滑动,否则就是向上滑动。


下面将会进行一个边界值检测的过程,可以看到,从第43行开始,当ListView向下滑动的时候,就会进入一个for循环当中,从上往下依次获取子View,第47行当中,如果该子View的bottom值已经小于top值了,就说明这个子View已经移出屏幕了,所以会调用RecycleBin的addScrapView()方法将这个View加入到废弃缓存当中,并将count计数器加1,计数器用于记录有多少个子View被移出了屏幕。那么如果是ListView向上滑动的话,其实过程是基本相同的,只不过变成了从下往上依次获取子View,然后判断该子View的top值是不是大于bottom值了,如果大于的话说明子View已经移出了屏幕,同样把它加入到废弃缓存中,并将计数器加1。


接下来在第76行,会根据当前计数器的值来进行一个detach操作,它的作用就是把所有移出屏幕的子View全部detach掉,在ListView的概念当中,所有看不到的View就没有必要为它进行保存,因为屏幕外还有成百上千条数据等着显示呢,一个好的回收策略才能保证ListView的高性能和高效率。紧接着在第78行调用了offsetChildrenTopAndBottom()方法,并将incrementalDeltaY作为参数传入,这个方法的作用是让ListView中所有的子View都按照传入的参数值进行相应的偏移,这样就实现了随着手指的拖动,ListView的内容也会随着滚动的效果。


然后在第84行会进行判断,如果ListView中最后一个View的底部已经移入了屏幕,或者ListView中第一个View的顶部移入了屏幕,就会调用fillGap()方法,那么因此我们就可以猜出fillGap()方法是用来加载屏幕外数据的,进入到这个方法中瞧一瞧,如下所示:

/** 
 * Fills the gap left open by a touch-scroll. During a touch scroll, 
 * children that remain on screen are shifted and the other ones are 
 * discarded. The role of this method is to fill the gap thus created by 
 * performing a partial layout in the empty space. 
 *  
 * @param down 
 *            true if the scroll is going down, false if it is going up 
 */  
abstract void fillGap(boolean down);

OK,AbsListView中的fillGap()是一个抽象方法,那么我们立刻就能够想到,它的具体实现肯定是在ListView中完成的了。回到ListView当中,fillGap()方法的代码如下所示:

void fillGap(boolean down) {  
    final int count = getChildCount();  
    if (down) {  
        final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :  
                getListPaddingTop();  
        fillDown(mFirstPosition + count, startOffset);  
        correctTooHigh(getChildCount());  
    } else {  
        final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :  
                getHeight() - getListPaddingBottom();  
        fillUp(mFirstPosition - 1, startOffset);  
        correctTooLow(getChildCount());  
    }  
}

down参数用于表示ListView是向下滑动还是向上滑动的,可以看到,如果是向下滑动的话就会调用fillDown()方法,而如果是向上滑动的话就会调用fillUp()方法。那么这两个方法我们都已经非常熟悉了,内部都是通过一个循环来去对ListView进行填充,所以这两个方法我们就不看了,但是填充ListView会通过调用makeAndAddView()方法来完成,又是makeAndAddView()方法,但这次的逻辑再次不同了,所以我们还是回到这个方法瞧一瞧:

/** 
 * Obtain the view and add it to our list of children. The view can be made 
 * fresh, converted from an unused view, or used as is if it was in the 
 * recycle bin. 
 * 
 * @param position Logical position in the list 
 * @param y Top or bottom edge of the view to add 
 * @param flow If flow is true, align top edge to y. If false, align bottom 
 *        edge to y. 
 * @param childrenLeft Left edge where children should be positioned 
 * @param selected Is this position selected? 
 * @return View that was added 
 */  
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,  
        boolean selected) {  
    View child;  
    if (!mDataChanged) {  
        // Try to use an exsiting view for this position  
        child = mRecycler.getActiveView(position);  
        if (child != null) {  
            // Found it -- we're using an existing child  
            // This just needs to be positioned  
            setupChild(child, position, y, flow, childrenLeft, selected, true);  
            return child;  
        }  
    }  
    // Make a new view for this position, or convert an unused view if possible  
    child = obtainView(position, mIsScrap);  
    // This needs to be positioned and measured  
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);  
    return child;  
}

不管怎么说,这里首先仍然是会尝试调用RecycleBin的getActiveView()方法来获取子布局,只不过肯定是获取不到的了,因为在第二次Layout过程中我们已经从mActiveViews中获取过了数据,而根据RecycleBin的机制,mActiveViews是不能够重复利用的,因此这里返回的值肯定是null。


既然getActiveView()方法返回的值是null,那么就还是会走到第28行的obtainView()方法当中,代码如下所示:

/** 
 * Get a view and have it show the data associated with the specified 
 * position. This is called when we have already discovered that the view is 
 * not available for reuse in the recycle bin. The only choices left are 
 * converting an old view or making a new one. 
 *  
 * @param position 
 *            The position to display 
 * @param isScrap 
 *            Array of at least 1 boolean, the first entry will become true 
 *            if the returned view was taken from the scrap heap, false if 
 *            otherwise. 
 *  
 * @return A view displaying the data associated with the specified position 
 */  
View obtainView(int position, boolean[] isScrap) {  
    isScrap[0] = false;  
    View scrapView;  
    scrapView = mRecycler.getScrapView(position);  
    View child;  
    if (scrapView != null) {  
        child = mAdapter.getView(position, scrapView, this);  
        if (child != scrapView) {  
            mRecycler.addScrapView(scrapView);  
            if (mCacheColorHint != 0) {  
                child.setDrawingCacheBackgroundColor(mCacheColorHint);  
            }  
        } else {  
            isScrap[0] = true;  
            dispatchFinishTemporaryDetach(child);  
        }  
    } else {  
        child = mAdapter.getView(position, null, this);  
        if (mCacheColorHint != 0) {  
            child.setDrawingCacheBackgroundColor(mCacheColorHint);  
        }  
    }  
    return child;  
}

这里在第19行会调用RecyleBin的getScrapView()方法来尝试从废弃缓存中获取一个View,那么废弃缓存有没有View呢?当然有,因为刚才在trackMotionScroll()方法中我们就已经看到了,一旦有任何子View被移出了屏幕,就会将它加入到废弃缓存中,而从obtainView()方法中的逻辑来看,一旦有新的数据需要显示到屏幕上,就会尝试从废弃缓存中获取View。所以它们之间就形成了一个生产者和消费者的模式,那么ListView神奇的地方也就在这里体现出来了,不管你有任意多条数据需要显示,ListView中的子View其实来来回回就那么几个,移出屏幕的子View会很快被移入屏幕的数据重新利用起来,因而不管我们加载多少数据都不会出现OOM的情况,甚至内存都不会有所增加。


那么另外还有一点是需要大家留意的,这里获取到了一个scrapView,然后我们在第22行将它作为第二个参数传入到了Adapter的getView()方法当中。那么第二个参数是什么意思呢?我们再次看一下一个简单的getView()方法示例:

@Override  
public View getView(int position, View convertView, ViewGroup parent) {  
    Fruit fruit = getItem(position);  
    View view;  
    if (convertView == null) {  
        view = LayoutInflater.from(getContext()).inflate(resourceId, null);  
    } else {  
        view = convertView;  
    }  
    ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);  
    TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);  
    fruitImage.setImageResource(fruit.getImageId());  
    fruitName.setText(fruit.getName());  
    return view;  
}

第二个参数就是我们最熟悉的convertView呀,难怪平时我们在写getView()方法是要判断一下convertView是不是等于null,如果等于null才调用inflate()方法来加载布局,不等于null就可以直接利用convertView,因为convertView就是我们之间利用过的View,只不过被移出屏幕后进入到了废弃缓存中,现在又重新拿出来使用而已。然后我们只需要把convertView中的数据更新成当前位置上应该显示的数据,那么看起来就好像是全新加载出来的一个布局一样,这背后的道理你是不是已经完全搞明白了?


之后的代码又都是我们熟悉的流程了,从缓存中拿到子View之后再调用setupChild()方法将它重新attach到ListView当中,因为缓存中的View也是之前从ListView中detach掉的,这部分代码就不再重复进行分析了。


为了方便大家理解,这里我再附上一张图解说明:

Android ListView工作原理完全解析,带你从源码的角度彻底理解

那么到目前为止,我们就把ListView的整个工作流程代码基本分析结束了,文章比较长,希望大家可以理解清楚

相关标签: ListView Android