ListView 源码详解 有这一篇就够了！！！（二）

本篇文章是接着上篇文章继续分析
上篇文章地址
接下来主要讨论ListView 由用户改变行为，分一下几个方面

（5） Listview 的滑动过程
（6）setAdapter 原理
（7）notifyDataSetChanged 原理

（5） Listview 的滑动过程

Listview 和 GirdView 滑动事件的处理是在父类AbsListView完成对view销毁缓存和复用的， 滑动是Listview 实现数据展示一个重要的功能。我们现来看onTouchEvent方法，同样也是对重要的方法进行解析

 @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();
        }

        if (mPositionScroller != null) {
            mPositionScroller.stop();
        }

        if (mIsDetaching || !isAttachedToWindow()) {
            // Something isn't right.
            // Since we rely on being attached to get data set change notifications,
            // don't risk doing anything where we might try to resync and find things
            // in a bogus state.
            return false;
        }

        startNestedScroll(SCROLL_AXIS_VERTICAL);

        if (mFastScroll != null && mFastScroll.onTouchEvent(ev)) {
            return true;
        }

        initVelocityTrackerIfNotExists();
        final MotionEvent vtev = MotionEvent.obtain(ev);

        final int actionMasked = ev.getActionMasked();
        if (actionMasked == MotionEvent.ACTION_DOWN) {
            mNestedYOffset = 0;
        }
        vtev.offsetLocation(0, mNestedYOffset);
        switch (actionMasked) {
            case MotionEvent.ACTION_DOWN: {
                onTouchDown(ev);
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                onTouchMove(ev, vtev);
                break;
            }

            case MotionEvent.ACTION_UP: {
                onTouchUp(ev);
                break;
            }

            case MotionEvent.ACTION_CANCEL: {
                onTouchCancel();
                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
                    final View child = getChildAt(motionPosition - mFirstPosition);
                    mMotionViewOriginalTop = child.getTop();
                    mMotionPosition = motionPosition;
                }
                mLastY = y;
                break;
            }

            case MotionEvent.ACTION_POINTER_DOWN: {
                // New pointers take over dragging duties
                final int index = ev.getActionIndex();
                final int id = ev.getPointerId(index);
                final int x = (int) ev.getX(index);
                final int y = (int) ev.getY(index);
                mMotionCorrection = 0;
                mActivePointerId = id;
                mMotionX = x;
                mMotionY = y;
                final int motionPosition = pointToPosition(x, y);
                if (motionPosition >= 0) {
                    // Remember where the motion event started
                    final View child = getChildAt(motionPosition - mFirstPosition);
                    mMotionViewOriginalTop = child.getTop();
                    mMotionPosition = motionPosition;
                }
                mLastY = y;
                break;
            }
        }

        if (mVelocityTracker != null) {
            mVelocityTracker.addMovement(vtev);
        }
        vtev.recycle();
        return true;
    }

当ListView在滚动时，直接进入onTouchMove 方法

private void onTouchMove(MotionEvent ev, MotionEvent vtev) {
        if (mHasPerformedLongPress) {
            // Consume all move events following a successful long press.
            return;
        }

        int pointerIndex = ev.findPointerIndex(mActivePointerId);
        if (pointerIndex == -1) {
            pointerIndex = 0;
            mActivePointerId = ev.getPointerId(pointerIndex);
        }

        if (mDataChanged) {
            // Re-sync everything if data has been changed
            // since the scroll operation can query the adapter.
            layoutChildren();
        }

        final int y = (int) ev.getY(pointerIndex);

        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. If so, we'll enter scrolling mode.
                if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, vtev)) {
                    break;
                }
                // Otherwise, check containment within list bounds. If we're
                // outside bounds, cancel any active presses.
                final View motionView = getChildAt(mMotionPosition - mFirstPosition);
                final float x = ev.getX(pointerIndex);
                if (!pointInView(x, y, mTouchSlop)) {
                    setPressed(false);
                    if (motionView != null) {
                        motionView.setPressed(false);
                    }
                    removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ?
                            mPendingCheckForTap : mPendingCheckForLongPress);
                    mTouchMode = TOUCH_MODE_DONE_WAITING;
                    updateSelectorState();
                } else if (motionView != null) {
                    // Still within bounds, update the hotspot.
                    final float[] point = mTmpPoint;
                    point[0] = x;
                    point[1] = y;
                    transformPointToViewLocal(point, motionView);
                    motionView.drawableHotspotChanged(point[0], point[1]);
                }
                break;
            case TOUCH_MODE_SCROLL:
            case TOUCH_MODE_OVERSCROLL:
                scrollIfNeeded((int) ev.getX(pointerIndex), y, vtev);
                break;
        }
    }

当手指在屏幕上滑动时，TouchMode是等于TOUCH_MODE_SCROLL这个值的，至于为什么是这个值，大家可以自己查源码，这里主要解析滚动的源码解析， 这里根据touchMode会直接进入scrollIfNeeded方法

private void scrollIfNeeded(int x, int y, MotionEvent vtev) {
        int rawDeltaY = y - mMotionY;
        int scrollOffsetCorrection = 0;
        int scrollConsumedCorrection = 0;
        if (mLastY == Integer.MIN_VALUE) {
            rawDeltaY -= mMotionCorrection;
        }
        if (dispatchNestedPreScroll(0, mLastY != Integer.MIN_VALUE ? mLastY - y : -rawDeltaY,
                mScrollConsumed, mScrollOffset)) {
            rawDeltaY += mScrollConsumed[1];
            scrollOffsetCorrection = -mScrollOffset[1];
            scrollConsumedCorrection = mScrollConsumed[1];
            if (vtev != null) {
                vtev.offsetLocation(0, mScrollOffset[1]);
                mNestedYOffset += mScrollOffset[1];
            }
        }
        final int deltaY = rawDeltaY;
        int incrementalDeltaY =
                mLastY != Integer.MIN_VALUE ? y - mLastY + scrollConsumedCorrection : deltaY;
        int lastYCorrection = 0;

        if (mTouchMode == TOUCH_MODE_SCROLL) {
            if (PROFILE_SCROLLING) {
                if (!mScrollProfilingStarted) {
                    Debug.startMethodTracing("AbsListViewScroll");
                    mScrollProfilingStarted = true;
                }
            }

            if (mScrollStrictSpan == null) {
                // If it's non-null, we're already in a scroll.
                mScrollStrictSpan = StrictMode.enterCriticalSpan("AbsListView-scroll");
            }

            if (y != mLastY) {
                // We may be here after stopping a fling and continuing to scroll.
                // If so, we haven't disallowed intercepting touch events yet.
                // Make sure that we do so in case we're in a parent that can intercept.
                if ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) == 0 &&
                        Math.abs(rawDeltaY) > mTouchSlop) {
                    final ViewParent parent = getParent();
                    if (parent != null) {
                        parent.requestDisallowInterceptTouchEvent(true);
                    }
                }

                final int motionIndex;
                if (mMotionPosition >= 0) {
                    motionIndex = mMotionPosition - mFirstPosition;
                } else {
                    // If we don't have a motion position that we can reliably track,
                    // pick something in the middle to make a best guess at things below.
                    motionIndex = getChildCount() / 2;
                }

                int motionViewPrevTop = 0;
                View motionView = this.getChildAt(motionIndex);
                if (motionView != null) {
                    motionViewPrevTop = motionView.getTop();
                }

                // 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
                motionView = this.getChildAt(motionIndex);
                if (motionView != null) {
                    // Check if the top of the motion view is where it is
                    // supposed to be
                    final int motionViewRealTop = motionView.getTop();
                    if (atEdge) {
                        // Apply overscroll

                        int overscroll = -incrementalDeltaY -
                                (motionViewRealTop - motionViewPrevTop);
                        if (dispatchNestedScroll(0, overscroll - incrementalDeltaY, 0, overscroll,
                                mScrollOffset)) {
                            lastYCorrection -= mScrollOffset[1];
                            if (vtev != null) {
                                vtev.offsetLocation(0, mScrollOffset[1]);
                                mNestedYOffset += mScrollOffset[1];
                            }
                        } else {
                            final boolean atOverscrollEdge = overScrollBy(0, overscroll,
                                    0, mScrollY, 0, 0, 0, mOverscrollDistance, true);

                            if (atOverscrollEdge && mVelocityTracker != null) {
                                // Don't allow overfling if we're at the edge
                                mVelocityTracker.clear();
                            }

                            final int overscrollMode = getOverScrollMode();
                            if (overscrollMode == OVER_SCROLL_ALWAYS ||
                                    (overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS &&
                                            !contentFits())) {
                                if (!atOverscrollEdge) {
                                    mDirection = 0; // Reset when entering overscroll.
                                    mTouchMode = TOUCH_MODE_OVERSCROLL;
                                }
                                if (incrementalDeltaY > 0) {
                                    mEdgeGlowTop.onPull((float) -overscroll / getHeight(),
                                            (float) x / getWidth());
                                    if (!mEdgeGlowBottom.isFinished()) {
                                        mEdgeGlowBottom.onRelease();
                                    }
                                    invalidateTopGlow();
                                } else if (incrementalDeltaY < 0) {
                                    mEdgeGlowBottom.onPull((float) overscroll / getHeight(),
                                            1.f - (float) x / getWidth());
                                    if (!mEdgeGlowTop.isFinished()) {
                                        mEdgeGlowTop.onRelease();
                                    }
                                    invalidateBottomGlow();
                                }
                            }
                        }
                    }
                    mMotionY = y + lastYCorrection + scrollOffsetCorrection;
                }
                mLastY = y + lastYCorrection + scrollOffsetCorrection;
            }
        } else if (mTouchMode == TOUCH_MODE_OVERSCROLL) {
            if (y != mLastY) {
                final int oldScroll = mScrollY;
                final int newScroll = oldScroll - incrementalDeltaY;
                int newDirection = y > mLastY ? 1 : -1;

                if (mDirection == 0) {
                    mDirection = newDirection;
                }

                int overScrollDistance = -incrementalDeltaY;
                if ((newScroll < 0 && oldScroll >= 0) || (newScroll > 0 && oldScroll <= 0)) {
                    overScrollDistance = -oldScroll;
                    incrementalDeltaY += overScrollDistance;
                } else {
                    incrementalDeltaY = 0;
                }

                if (overScrollDistance != 0) {
                    overScrollBy(0, overScrollDistance, 0, mScrollY, 0, 0,
                            0, mOverscrollDistance, true);
                    final int overscrollMode = getOverScrollMode();
                    if (overscrollMode == OVER_SCROLL_ALWAYS ||
                            (overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS &&
                                    !contentFits())) {
                        if (rawDeltaY > 0) {
                            mEdgeGlowTop.onPull((float) overScrollDistance / getHeight(),
                                    (float) x / getWidth());
                            if (!mEdgeGlowBottom.isFinished()) {
                                mEdgeGlowBottom.onRelease();
                            }
                            invalidateTopGlow();
                        } else if (rawDeltaY < 0) {
                            mEdgeGlowBottom.onPull((float) overScrollDistance / getHeight(),
                                    1.f - (float) x / getWidth());
                            if (!mEdgeGlowTop.isFinished()) {
                                mEdgeGlowTop.onRelease();
                            }
                            invalidateBottomGlow();
                        }
                    }
                }

                if (incrementalDeltaY != 0) {
                    // Coming back to 'real' list scrolling
                    if (mScrollY != 0) {
                        mScrollY = 0;
                        invalidateParentIfNeeded();
                    }

                    trackMotionScroll(incrementalDeltaY, incrementalDeltaY);

                    mTouchMode = TOUCH_MODE_SCROLL;

                    // We did not scroll the full amount. Treat this essentially like the
                    // start of a new touch scroll
                    final int motionPosition = findClosestMotionRow(y);

                    mMotionCorrection = 0;
                    View motionView = getChildAt(motionPosition - mFirstPosition);
                    mMotionViewOriginalTop = motionView != null ? motionView.getTop() : 0;
                    mMotionY =  y + scrollOffsetCorrection;
                    mMotionPosition = motionPosition;
                }
                mLastY = y + lastYCorrection + scrollOffsetCorrection;
                mDirection = newDirection;
            }
        }
    }

这个方法很长，需要处理屏幕上大大小小的事件，这里我们需要研究的是trackMotionScroll方法，我们手指在屏幕上又一点的滑动都会触发这个方法。
可以从代码的19行看出，incrementalDeltaY 表示触发事件在Y轴上的偏移量，我们会根据他的正负表示向上还是向下滑动，deltaY表示从手指按下到当前手指位置的距离。

 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;

        // "effective padding" In this case is the amount of padding that affects
        // how much space should not be filled by items. If we don't clip to padding
        // there is no effective padding.
        int effectivePaddingTop = 0;
        int effectivePaddingBottom = 0;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            effectivePaddingTop = listPadding.top;
            effectivePaddingBottom = listPadding.bottom;
        }

         // FIXME account for grid vertical spacing too?
        final int spaceAbove = effectivePaddingTop - firstTop;
        final int end = getHeight() - effectivePaddingBottom;
        final int spaceBelow = lastBottom - end;

        final int height = getHeight() - mPaddingBottom - mPaddingTop;
        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;

        // Update our guesses for where the first and last views are
        if (firstPosition == 0) {
            mFirstPositionDistanceGuess = firstTop - listPadding.top;
        } else {
            mFirstPositionDistanceGuess += incrementalDeltaY;
        }
        if (firstPosition + childCount == mItemCount) {
            mLastPositionDistanceGuess = lastBottom + listPadding.bottom;
        } else {
            mLastPositionDistanceGuess += incrementalDeltaY;
        }

        final boolean cannotScrollDown = (firstPosition == 0 &&
                firstTop >= listPadding.top && incrementalDeltaY >= 0);
        final boolean cannotScrollUp = (firstPosition + childCount == mItemCount &&
                lastBottom <= getHeight() - listPadding.bottom && incrementalDeltaY <= 0);

        if (cannotScrollDown || cannotScrollUp) {
            return incrementalDeltaY != 0;
        }

        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) {
            int top = -incrementalDeltaY;
            if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
                top += listPadding.top;
            }
            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) {
                        // The view will be rebound to new data, clear any
                        // system-managed transient state.
                        child.clearAccessibilityFocus();
                        mRecycler.addScrapView(child, position);
                    }
                }
            }
        } else {
            int bottom = getHeight() - incrementalDeltaY;
            if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
                bottom -= listPadding.bottom;
            }
            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) {
                        // The view will be rebound to new data, clear any
                        // system-managed transient state.
                        child.clearAccessibilityFocus();
                        mRecycler.addScrapView(child, position);
                    }
                }
            }
        }

        mMotionViewNewTop = mMotionViewOriginalTop + deltaY;

        mBlockLayoutRequests = true;

        if (count > 0) {
            detachViewsFromParent(start, count);
            mRecycler.removeSkippedScrap();
        }

        // invalidate before moving the children to avoid unnecessary invalidate
        // calls to bubble up from the children all the way to the top
        if (!awakenScrollBars()) {
           invalidate();
        }

        offsetChildrenTopAndBottom(incrementalDeltaY);

        if (down) {
            mFirstPosition += count;
        }

        final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
        if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
            fillGap(down);
        }

        mRecycler.fullyDetachScrapViews();
        if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {
            final int childIndex = mSelectedPosition - mFirstPosition;
            if (childIndex >= 0 && childIndex < getChildCount()) {
                positionSelector(mSelectedPosition, getChildAt(childIndex));
            }
        } else if (mSelectorPosition != INVALID_POSITION) {
            final int childIndex = mSelectorPosition - mFirstPosition;
            if (childIndex >= 0 && childIndex < getChildCount()) {
                positionSelector(INVALID_POSITION, getChildAt(childIndex));
            }
        } else {
            mSelectorRect.setEmpty();
        }

        mBlockLayoutRequests = false;

        invokeOnItemScrollListener();

        return false;
    }

如果incrementalDeltaY < 0， 表示向下滑动， down = true； 进入一个for循环，一旦child.getBottom() < top，表示 view滑出屏幕就会调用addScrapView，将view添加到mScrapViews数组中; 同样如果 down = false，一样的处理，进行view的缓存； 之后会调用detachViewsFromParent对 view 资源进行释放，因为屏幕外还有很多view等着展示，已经不展示的数据就可以直接释放了；
之后根据下面这段代码判断

 if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
     fillGap(down);
  }

如果第一个ListView的第一个View的顶部移入屏幕，或者最后一个View的底部移入屏幕，就会调用fillGap 方法，因为可以判断fillGap方法是用来加载屏幕外数据加载到屏幕上的。现在我们来看下fillGap方法到底藏了什么武器

AbsListView 的 fillGap方法是个抽象类，应该在ListView 和GridView 中分别实现的

abstract void fillGap(boolean down);

ListView 的fillGap方法：

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

fillGap方法 根据 用户滑动的方向分别使用 fillDown 和fillUp 处理，从上篇文章分析我们知道fillDown 方法使用一个循环调用主makeAndAddView获得view 会在 adapter 的getView 方法中判断是重新LayoutInflater.from(this).inflate() 布局 还是复用convertView。
到这里我们就分析完 ListView 在滑动的时候是怎么处理数据的了，对于即将要滑出屏幕的数据使用RecycleBin的addScrapView方法缓存，并且调用detachViewsFromParent将屏幕外的view detach掉， 对于屏幕上的view根据fillGap方法调用fillDown或者fillUp进而调用makeAndAddView，再调用obtinView获取view，最后使用setupChild 将 view 通过attachViewToParent或者addViewInLayout 方法 添加到屏幕上。

（6） setAdapter 原理

 @Override
    public void setAdapter(ListAdapter adapter) {
    // 当ListView之前绑定过adapter信息时，在这里会清除原有Adapter和数据集观察者等信息，
        if (mAdapter != null && mDataSetObserver != null) {
            mAdapter.unregisterDataSetObserver(mDataSetObserver);
        }

        resetList();
        mRecycler.clear();

        if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
            mAdapter = wrapHeaderListAdapterInternal(mHeaderViewInfos, mFooterViewInfos, adapter);
        } else {
            mAdapter = adapter;
        }

        mOldSelectedPosition = INVALID_POSITION;
        mOldSelectedRowId = INVALID_ROW_ID;

        // AbsListView#setAdapter will update choice mode states.
        super.setAdapter(adapter);

        if (mAdapter != null) {
            mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
            mOldItemCount = mItemCount;
            mItemCount = mAdapter.getCount();
            checkFocus();
            // 重新绑定新的数据集观察者
            mDataSetObserver = new AdapterDataSetObserver();
            mAdapter.registerDataSetObserver(mDataSetObserver);

            mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

            int position;
            // 
            if (mStackFromBottom) {
                position = lookForSelectablePosition(mItemCount - 1, false);
            } else {
                position = lookForSelectablePosition(0, true);
            }
            setSelectedPositionInt(position);
            setNextSelectedPositionInt(position);

            if (mItemCount == 0) {
                // Nothing selected
                checkSelectionChanged();
            }
        } else {
            mAreAllItemsSelectable = true;
            checkFocus();
            // Nothing selected
            checkSelectionChanged();
        }
        // 重绘
        requestLayout();
    }

ListView的setAdapter方法，首先会清楚之前adapter的相关设置，之后重新设置数据观察者，之后再调用requestLayout() 重新进行绘制过程。

（7）notifyDataSetChanged 原理

adapter的notifyDataSetChanged 会先调用onRestoreInstanceState进行数据恢复，再 调到requestLayout方法进行重绘。setAdapter 没有当前状态的保存，直接进行requestLayout 进行重绘。

我们先进入

BaseAdapter 中的 notifyDataSetChanged 
public void notifyDataSetChanged() {
        mDataSetObservable.notifyChanged();
    }


mDataSetObservable 的 notifyChanged
 public void notifyChanged() {
        synchronized(mObservers) {
            // since onChanged() is implemented by the app, it could do anything, including
            // removing itself from {@link mObservers} - and that could cause problems if
            // an iterator is used on the ArrayList {@link mObservers}.
            // to avoid such problems, just march thru the list in the reverse order.
            for (int i = mObservers.size() - 1; i >= 0; i--) {
                mObservers.get(i).onChanged();
            }
        }
    }


mObservers 的onChangde 方法是个空方法。
  public void onChanged() {
        // Do nothing
    }

要想涨到onChanged方法的具体实现，这样从setAdapter 方法开始追朔。

@Override
    public void setAdapter(ListAdapter adapter) {
        if (mAdapter != null && mDataSetObserver != null) {
            mAdapter.unregisterDataSetObserver(mDataSetObserver);
        }

        resetList();
        mRecycler.clear();

        if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
            mAdapter = wrapHeaderListAdapterInternal(mHeaderViewInfos, mFooterViewInfos, adapter);
        } else {
            mAdapter = adapter;
        }

        mOldSelectedPosition = INVALID_POSITION;
        mOldSelectedRowId = INVALID_ROW_ID;

        // AbsListView#setAdapter will update choice mode states.
        super.setAdapter(adapter);

        if (mAdapter != null) {
            mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
            mOldItemCount = mItemCount;
            mItemCount = mAdapter.getCount();
            checkFocus();

            mDataSetObserver = new AdapterDataSetObserver();
            // 调用Adapter 注册 观察者
            mAdapter.registerDataSetObserver(mDataSetObserver);

            mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

            int position;
            if (mStackFromBottom) {
                position = lookForSelectablePosition(mItemCount - 1, false);
            } else {
                position = lookForSelectablePosition(0, true);
            }
            setSelectedPositionInt(position);
            setNextSelectedPositionInt(position);

            if (mItemCount == 0) {
                // Nothing selected
                checkSelectionChanged();
            }
        } else {
            mAreAllItemsSelectable = true;
            checkFocus();
            // Nothing selected
            checkSelectionChanged();
        }

        requestLayout();
    }

AdapterDataSetObserver 通过mAdapter.registerDataSetObserver(mDataSetObserver); 将观察者添加到mObservers集合中，自然，onChanged 方法是在 AdapterDataSetObserver 的。
AdapterDataSetObserver 是 AdapterView 的内部类

class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount();

            // Detect the case where a cursor that was previously invalidated has
            // been repopulated with new data.
            if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                    && mOldItemCount == 0 && mItemCount > 0) {
                AdapterView.this.onRestoreInstanceState(mInstanceState);
                mInstanceState = null;
            } else {
                rememberSyncState();
            }
            checkFocus();
            requestLayout();
        }
        ...
}

到这里，我们就找到了onChanged 方法的真身了，从代码个 AdapterView.this.onRestoreInstanceState(mInstanceState); 在开始先判断是否有状态保存，先恢复状态，再requestLayout();进行重绘

目前为止，对ListView的源码有了基本的认识。如果有那些地方需要讨论，希望大家指正。哈哈