Android——UI（一）：UI绘制流程

UI绘制流程

1、为什么调用setContentView之后就可以显示我们想要的布局？

进入setContentView方法可以看到

    public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

其中getWindow()返回了一个Window对象，Window是一个抽象类，Window中有很多抽象方法，最常见的就是我们经常写的findViewById。
通过注释

/**
* Abstract base class for a top-level window look and behavior policy. An
* instance of this class should be used as the top-level view added to the
* window manager. It provides standard UI policies such as a background, title
* area, default key processing, etc.
*
*

The only existing implementation of this abstract class is
* android.view.PhoneWindow, which you should instantiate when needing a
* Window.
*/

可以看到Window处于最顶层，并且在Android中只有PhoneWindow一个子类。

进入PhoneWindow的setContentView可以看到

    @Override
    public void setContentView(int layoutResID) {
        // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
        // decor, when theme attributes and the like are crystalized. Do not check the feature
        // before this happens.
        if (mContentParent == null) {
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        //Activity的转场动画
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }

首先是对mContentParent 进行判空，这个mContentParent 是一个ViewGroup，通过注释

    // This is the view in which the window contents are placed. It is either
    // mDecor itself, or a child of mDecor where the contents go.

可以看到mContentParent 就是Window的内容，注释中的mDecor是DecorView，Android中最顶层的View。

往下看会看到mLayoutInflater.inflate(layoutResID, mContentParent);
将id设置到ViewGroup中，这里暂不做深入。

当mContentParent为空时会执行installDecor();
进去之后，找到

        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);

继续查看generateLayout(mDecor)

TypedArray a = getWindowStyle();

        if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {
            requestFeature(FEATURE_NO_TITLE);
        } else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {
            // Don't allow an action bar if there is no title.
            requestFeature(FEATURE_ACTION_BAR);
        }

这里代码是不是很熟悉，在这里就会获取到Window的style。
继续往下看

 int layoutResource;
        int features = getLocalFeatures();
        // System.out.println("Features: 0x" + Integer.toHexString(features));
        if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
            layoutResource = R.layout.screen_swipe_dismiss;
        } else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
            if (mIsFloating) {
                TypedValue res = new TypedValue();
                getContext().getTheme().resolveAttribute(
                        R.attr.dialogTitleIconsDecorLayout, res, true);
                layoutResource = res.resourceId;
            } else {
                layoutResource = R.layout.screen_title_icons;
            }

在这里通过不同的features去加载不同的layout，同时也说明了，requestFeature必须在setContentView之前进行调用。

再来看layoutResource即将加载的布局是什么，在这里以R.layout.screen_simple为例，进去之后可以看到

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    android:orientation="vertical">
    <ViewStub android:id="@+id/action_mode_bar_stub"
              android:inflatedId="@+id/action_mode_bar"
              android:layout="@layout/action_mode_bar"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:theme="?attr/actionBarTheme" />
     //显示我们布局的地方
    <FrameLayout
         android:id="@android:id/content"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:foregroundInsidePadding="false"
         android:foregroundGravity="fill_horizontal|top"
         android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

小结：

• 每个Activity都有一个关联的Window抽象类，用来描述应用程序窗口。
• 在Android中，Window有且仅有一个实现类PhoneWindow
• PhoneWindow中包含了一个DecorView
• 我们自己设置的布局在DecorView下的FrameLayout中
• 最后通过mLayoutInflater.inflate将id与mContentParent进行关联

---

2、LayoutInflater如何把xml转成View？

• 为什么include不能作为xml资源文件布局根节点？
• 为什么merge只能作为xml资源文件布局根节点？

带着疑问继续看源码。

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
        final Resources res = getContext().getResources();
        if (DEBUG) {
            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                    + Integer.toHexString(resource) + ")");
        }
        //获取xml解析器
        final XmlResourceParser parser = res.getLayout(resource);
        try {
        //使用解析器获取View
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }

进入inflate(parser, root, attachToRoot)继续看，

//获取xml中的属性集
final AttributeSet attrs = Xml.asAttributeSet(parser);

    int type;
    //获取根节点
    while ((type = parser.next()) != XmlPullParser.START_TAG &&
          type != XmlPullParser.END_DOCUMENT) {
          // Empty
                }

final String name = parser.getName();

  if (TAG_MERGE.equals(name)) {
      if (root == null || !attachToRoot) {
            throw new InflateException("<merge /> can be used only with a valid " + "ViewGroup root and attachToRoot=true");
                    }
       rInflate(parser, root, inflaterContext, attrs, false);
         } else {

 final View temp = createViewFromTag(root, name, inflaterContext, attrs);

       ViewGroup.LayoutParams params = null;

   if (root != null) {
   if (DEBUG) {
     System.out.println("Creating params from root: " + root);
                        }
    // 获取xml上的属性，在自定义属性时经常用到
     params = root.generateLayoutParams(attrs);
    if (!attachToRoot) {
    // 设置属性
       temp.setLayoutParams(params);
                        }
                    }

// 解析完父容器之后继续解析子控件
    rInflateChildren(parser, temp, attrs, true);

    final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
            boolean finishInflate) throws XmlPullParserException, IOException {
        rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
    }

 void rInflate(XmlPullParser parser, View parent, Context context,
            AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {

        final int depth = parser.getDepth();
        int type;
        //遍历节点
        while (((type = parser.next()) != XmlPullParser.END_TAG ||
                parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {

            if (type != XmlPullParser.START_TAG) {
                continue;
            }

            final String name = parser.getName();

            if (TAG_REQUEST_FOCUS.equals(name)) {
                parseRequestFocus(parser, parent);
            } else if (TAG_TAG.equals(name)) {
                parseViewTag(parser, parent, attrs);
            } else if (TAG_INCLUDE.equals(name)) {
                //判断是否是跟标签
                if (parser.getDepth() == 0) {
                //include不能作为跟标签
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, context, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {
            //merge必须作为跟标签
                throw new InflateException("<merge /> must be the root element");
            } else {
            //最终将子布局添加到父布局中
                final View view = createViewFromTag(parent, name, context, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflateChildren(parser, view, attrs, true);
                viewGroup.addView(view, params);
            }
        }

        if (finishInflate) {
            parent.onFinishInflate();
        }
    }