ListView 源码详解 有这一篇就够了！！！

虽然现在大家使用ListView的机会相对RecyclerView的机会较少，但官方并没有标注 ListView 类过期，哈哈，就说明它一定还是有他的特殊之处，这篇文章就来分析下，ListView的内部机制以及几个重要的点。
将从下面几个方面开始着手分析ListView，将会分两篇文章进行解析

1. 类的继承关系
2. ListView 的构造方法
3. RecycleBin主要方法讲解
4. ListView 的绘制原理以及缓存回收过程 （onMeasure onLayout onDraw）
5. Listview 的滑动过程
6. setAdapter 原理
7. notifyDataSetChanged 原理

（1）ListView 的继承关系

（2）ListView 的构造函数

public ListView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        // 这里是调用的父类AbsListView的构造方法
        super(context, attrs, defStyleAttr, defStyleRes);

        final TypedArray a = context.obtainStyledAttributes(
                attrs, R.styleable.ListView, defStyleAttr, defStyleRes);

        final CharSequence[] entries = a.getTextArray(R.styleable.ListView_entries);
        if (entries != null) {
            // ListView 的默认Adapter
            setAdapter(new ArrayAdapter<>(context, R.layout.simple_list_item_1, entries));
        }

        // 设置分割线样式 用户可以重写ListView_divider样式
        final Drawable d = a.getDrawable(R.styleable.ListView_divider);
        if (d != null) {
            // Use an implicit divider height which may be explicitly
            // overridden by android:dividerHeight further down.
            setDivider(d);
        }

        // 设置 ListView 的footer和header
        final Drawable osHeader = a.getDrawable(R.styleable.ListView_overScrollHeader);
        if (osHeader != null) {
            setOverscrollHeader(osHeader);
        }

        final Drawable osFooter = a.getDrawable(R.styleable.ListView_overScrollFooter);
        if (osFooter != null) {
            setOverscrollFooter(osFooter);
        }

        // 分割线高度
        // Use an explicit divider height, if specified.
        if (a.hasValueOrEmpty(R.styleable.ListView_dividerHeight)) {
            final int dividerHeight = a.getDimensionPixelSize(
                    R.styleable.ListView_dividerHeight, 0);
            if (dividerHeight != 0) {
                setDividerHeight(dividerHeight);
            }
        }

        mHeaderDividersEnabled = a.getBoolean(R.styleable.ListView_headerDividersEnabled, true);
        mFooterDividersEnabled = a.getBoolean(R.styleable.ListView_footerDividersEnabled, true);

        a.recycle();
    }

AbsListView.java 的构造方法

 public AbsListView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
        initAbsListView();
        ...
 }

 private void initAbsListView() {
        // Setting focusable in touch mode will set the focusable property to true
        // 设置可点击
        setClickable(true);
        // 设置 触摸可以获取焦点
        setFocusableInTouchMode(true);
        // 设置可以绘制
        setWillNotDraw(false);
        // 设置对于透明的地方,显示最底层的背景
        setAlwaysDrawnWithCacheEnabled(false);
        // 设置 是否缓存卷动项
        setScrollingCacheEnabled(true);
        // 设置相关事件变量初始化
        final ViewConfiguration configuration = ViewConfiguration.get(mContext);
        mTouchSlop = configuration.getScaledTouchSlop();
        mVerticalScrollFactor = configuration.getScaledVerticalScrollFactor();
        mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
        mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
        mOverscrollDistance = configuration.getScaledOverscrollDistance();
        mOverflingDistance = configuration.getScaledOverflingDistance();

        mDensityScale = getContext().getResources().getDisplayMetrics().density;
    }

ListView 的构造方法中主要做一些资源的初始化工作。

(3) RecycleBin主要方法讲解

RecycleBin是 AbsListView的内部类，它的代码不多，这里主要介绍几个变量含义和方法功能

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[] mScrapViews;

        private int mViewTypeCount;

        private ArrayList mCurrentScrap;

        private ArrayList mSkippedScrap;

        private SparseArray mTransientStateViews;
        private LongSparseArray mTransientStateViewsById;

        public void setViewTypeCount(int viewTypeCount) {
            if (viewTypeCount < 1) {
                throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
            }
            //noinspection unchecked
            ArrayList[] scrapViews = new ArrayList[viewTypeCount];
            for (int i = 0; i < viewTypeCount; i++) {
                scrapViews[i] = new ArrayList();
            }
            mViewTypeCount = viewTypeCount;
            mCurrentScrap = scrapViews[0];
            mScrapViews = scrapViews;
        }

        public void markChildrenDirty() {
            if (mViewTypeCount == 1) {
                final ArrayList scrap = mCurrentScrap;
                final int scrapCount = scrap.size();
                for (int i = 0; i < scrapCount; i++) {
                    scrap.get(i).forceLayout();
                }
            } else {
                final int typeCount = mViewTypeCount;
                for (int i = 0; i < typeCount; i++) {
                    final ArrayList scrap = mScrapViews[i];
                    final int scrapCount = scrap.size();
                    for (int j = 0; j < scrapCount; j++) {
                        scrap.get(j).forceLayout();
                    }
                }
            }
            if (mTransientStateViews != null) {
                final int count = mTransientStateViews.size();
                for (int i = 0; i < count; i++) {
                    mTransientStateViews.valueAt(i).forceLayout();
                }
            }
            if (mTransientStateViewsById != null) {
                final int count = mTransientStateViewsById.size();
                for (int i = 0; i < count; i++) {
                    mTransientStateViewsById.valueAt(i).forceLayout();
                }
            }
        }

        public boolean shouldRecycleViewType(int viewType) {
            return viewType >= 0;
        }

        /**
         * Clears the scrap heap.
         */
        void clear() {
            if (mViewTypeCount == 1) {
                final ArrayList scrap = mCurrentScrap;
                clearScrap(scrap);
            } else {
                final int typeCount = mViewTypeCount;
                for (int i = 0; i < typeCount; i++) {
                    final ArrayList scrap = mScrapViews[i];
                    clearScrap(scrap);
                }
            }

            clearTransientStateViews();
        }

        /**
         * 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;

            //noinspection MismatchedReadAndWriteOfArray
            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;
                    // Remember the position so that setupChild() doesn't reset state.
                    lp.scrappedFromPosition = firstActivePosition + i;
                }
            }
        }

        /**
         * 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;
        }

        View getTransientStateView(int position) {
            if (mAdapter != null && mAdapterHasStableIds && mTransientStateViewsById != null) {
                long id = mAdapter.getItemId(position);
                View result = mTransientStateViewsById.get(id);
                mTransientStateViewsById.remove(id);
                return result;
            }
            if (mTransientStateViews != null) {
                final int index = mTransientStateViews.indexOfKey(position);
                if (index >= 0) {
                    View result = mTransientStateViews.valueAt(index);
                    mTransientStateViews.removeAt(index);
                    return result;
                }
            }
            return null;
        }

        /**
         * Dumps and fully detaches any currently saved views with transient
         * state.
         */
        void clearTransientStateViews() {
            final SparseArray viewsByPos = mTransientStateViews;
            if (viewsByPos != null) {
                final int N = viewsByPos.size();
                for (int i = 0; i < N; i++) {
                    removeDetachedView(viewsByPos.valueAt(i), false);
                }
                viewsByPos.clear();
            }

            final LongSparseArray viewsById = mTransientStateViewsById;
            if (viewsById != null) {
                final int N = viewsById.size();
                for (int i = 0; i < N; i++) {
                    removeDetachedView(viewsById.valueAt(i), false);
                }
                viewsById.clear();
            }
        }

        /**
         * @return A view from the ScrapViews collection. These are unordered.
         */
        View getScrapView(int position) {
            final int whichScrap = mAdapter.getItemViewType(position);
            if (whichScrap < 0) {
                return null;
            }
            if (mViewTypeCount == 1) {
                return retrieveFromScrap(mCurrentScrap, position);
            } else if (whichScrap < mScrapViews.length) {
                return retrieveFromScrap(mScrapViews[whichScrap], position);
            }
            return null;
        }

        /**
         * Puts a view into the list of scrap views.
         * 

         * If the list data hasn't changed or the adapter has stable IDs, views
         * with transient state will be preserved for later retrieval.
         *
         * @param scrap The view to add
         * @param position The view's position within its parent
         */
        void addScrapView(View scrap, int position) {
            final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
            if (lp == null) {
                // Can't recycle, but we don't know anything about the view.
                // Ignore it completely.
                return;
            }

            lp.scrappedFromPosition = position;

            // Remove but don't scrap header or footer views, or views that
            // should otherwise not be recycled.
            final int viewType = lp.viewType;
            if (!shouldRecycleViewType(viewType)) {
                // Can't recycle. If it's not a header or footer, which have
                // special handling and should be ignored, then skip the scrap
                // heap and we'll fully detach the view later.
                if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                    getSkippedScrap().add(scrap);
                }
                return;
            }

            scrap.dispatchStartTemporaryDetach();

            // The the accessibility state of the view may change while temporary
            // detached and we do not allow detached views to fire accessibility
            // events. So we are announcing that the subtree changed giving a chance
            // to clients holding on to a view in this subtree to refresh it.
            notifyViewAccessibilityStateChangedIfNeeded(
                    AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE);

            // Don't scrap views that have transient state.
            final boolean scrapHasTransientState = scrap.hasTransientState();
            if (scrapHasTransientState) {
                if (mAdapter != null && mAdapterHasStableIds) {
                    // If the adapter has stable IDs, we can reuse the view for
                    // the same data.
                    if (mTransientStateViewsById == null) {
                        mTransientStateViewsById = new LongSparseArray<>();
                    }
                    mTransientStateViewsById.put(lp.itemId, scrap);
                } else if (!mDataChanged) {
                    // If the data hasn't changed, we can reuse the views at
                    // their old positions.
                    if (mTransientStateViews == null) {
                        mTransientStateViews = new SparseArray<>();
                    }
                    mTransientStateViews.put(position, scrap);
                } else {
                    // Otherwise, we'll have to remove the view and start over.
                    clearScrapForRebind(scrap);
                    getSkippedScrap().add(scrap);
                }
            } else {
                clearScrapForRebind(scrap);
                if (mViewTypeCount == 1) {
                    mCurrentScrap.add(scrap);
                } else {
                    mScrapViews[viewType].add(scrap);
                }

                if (mRecyclerListener != null) {
                    mRecyclerListener.onMovedToScrapHeap(scrap);
                }
            }
        }

        private ArrayList getSkippedScrap() {
            if (mSkippedScrap == null) {
                mSkippedScrap = new ArrayList<>();
            }
            return mSkippedScrap;
        }

        /**
         * Finish the removal of any views that skipped the scrap heap.
         */
        void removeSkippedScrap() {
            if (mSkippedScrap == null) {
                return;
            }
            final int count = mSkippedScrap.size();
            for (int i = 0; i < count; i++) {
                removeDetachedView(mSkippedScrap.get(i), false);
            }
            mSkippedScrap.clear();
        }

        /**
         * Move all views remaining in mActiveViews to mScrapViews.
         */
        void scrapActiveViews() {
            final View[] activeViews = mActiveViews;
            final boolean hasListener = mRecyclerListener != null;
            final boolean multipleScraps = mViewTypeCount > 1;

            ArrayList scrapViews = mCurrentScrap;
            final int count = activeViews.length;
            for (int i = count - 1; i >= 0; i--) {
                final View victim = activeViews[i];
                if (victim != null) {
                    final AbsListView.LayoutParams lp
                            = (AbsListView.LayoutParams) victim.getLayoutParams();
                    final int whichScrap = lp.viewType;

                    activeViews[i] = null;

                    if (victim.hasTransientState()) {
                        // Store views with transient state for later use.
                        victim.dispatchStartTemporaryDetach();

                        if (mAdapter != null && mAdapterHasStableIds) {
                            if (mTransientStateViewsById == null) {
                                mTransientStateViewsById = new LongSparseArray();
                            }
                            long id = mAdapter.getItemId(mFirstActivePosition + i);
                            mTransientStateViewsById.put(id, victim);
                        } else if (!mDataChanged) {
                            if (mTransientStateViews == null) {
                                mTransientStateViews = new SparseArray();
                            }
                            mTransientStateViews.put(mFirstActivePosition + i, victim);
                        } else if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                            // The data has changed, we can't keep this view.
                            removeDetachedView(victim, false);
                        }
                    } else if (!shouldRecycleViewType(whichScrap)) {
                        // Discard non-recyclable views except headers/footers.
                        if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                            removeDetachedView(victim, false);
                        }
                    } else {
                        // Store everything else on the appropriate scrap heap.
                        if (multipleScraps) {
                            scrapViews = mScrapViews[whichScrap];
                        }

                        lp.scrappedFromPosition = mFirstActivePosition + i;
                        removeDetachedView(victim, false);
                        scrapViews.add(victim);

                        if (hasListener) {
                            mRecyclerListener.onMovedToScrapHeap(victim);
                        }
                    }
                }
            }
            pruneScrapViews();
        }

        /**
         * At the end of a layout pass, all temp detached views should either be re-attached or
         * completely detached. This method ensures that any remaining view in the scrap list is
         * fully detached.
         */
        void fullyDetachScrapViews() {
            final int viewTypeCount = mViewTypeCount;
            final ArrayList[] scrapViews = mScrapViews;
            for (int i = 0; i < viewTypeCount; ++i) {
                final ArrayList scrapPile = scrapViews[i];
                for (int j = scrapPile.size() - 1; j >= 0; j--) {
                    final View view = scrapPile.get(j);
                    if (view.isTemporarilyDetached()) {
                        removeDetachedView(view, false);
                    }
                }
            }
        }

        /**
         * Makes sure that the size of mScrapViews does not exceed the size of
         * mActiveViews, which can happen if an adapter does not recycle its
         * views. Removes cached transient state views that no longer have
         * transient state.
         */
        private void pruneScrapViews() {
            final int maxViews = mActiveViews.length;
            final int viewTypeCount = mViewTypeCount;
            final ArrayList[] scrapViews = mScrapViews;
            for (int i = 0; i < viewTypeCount; ++i) {
                final ArrayList scrapPile = scrapViews[i];
                int size = scrapPile.size();
                while (size > maxViews) {
                    scrapPile.remove(--size);
                }
            }

            final SparseArray transViewsByPos = mTransientStateViews;
            if (transViewsByPos != null) {
                for (int i = 0; i < transViewsByPos.size(); i++) {
                    final View v = transViewsByPos.valueAt(i);
                    if (!v.hasTransientState()) {
                        removeDetachedView(v, false);
                        transViewsByPos.removeAt(i);
                        i--;
                    }
                }
            }

            final LongSparseArray transViewsById = mTransientStateViewsById;
            if (transViewsById != null) {
                for (int i = 0; i < transViewsById.size(); i++) {
                    final View v = transViewsById.valueAt(i);
                    if (!v.hasTransientState()) {
                        removeDetachedView(v, false);
                        transViewsById.removeAt(i);
                        i--;
                    }
                }
            }
        }

        /**
         * Puts all views in the scrap heap into the supplied list.
         */
        void reclaimScrapViews(List views) {
            if (mViewTypeCount == 1) {
                views.addAll(mCurrentScrap);
            } else {
                final int viewTypeCount = mViewTypeCount;
                final ArrayList[] scrapViews = mScrapViews;
                for (int i = 0; i < viewTypeCount; ++i) {
                    final ArrayList scrapPile = scrapViews[i];
                    views.addAll(scrapPile);
                }
            }
        }

        /**
         * Updates the cache color hint of all known views.
         *
         * @param color The new cache color hint.
         */
        void setCacheColorHint(int color) {
            if (mViewTypeCount == 1) {
                final ArrayList scrap = mCurrentScrap;
                final int scrapCount = scrap.size();
                for (int i = 0; i < scrapCount; i++) {
                    scrap.get(i).setDrawingCacheBackgroundColor(color);
                }
            } else {
                final int typeCount = mViewTypeCount;
                for (int i = 0; i < typeCount; i++) {
                    final ArrayList scrap = mScrapViews[i];
                    final int scrapCount = scrap.size();
                    for (int j = 0; j < scrapCount; j++) {
                        scrap.get(j).setDrawingCacheBackgroundColor(color);
                    }
                }
            }
            // Just in case this is called during a layout pass
            final View[] activeViews = mActiveViews;
            final int count = activeViews.length;
            for (int i = 0; i < count; ++i) {
                final View victim = activeViews[i];
                if (victim != null) {
                    victim.setDrawingCacheBackgroundColor(color);
                }
            }
        }

        private View retrieveFromScrap(ArrayList scrapViews, int position) {
            final int size = scrapViews.size();
            if (size > 0) {
                // See if we still have a view for this position or ID.
                // Traverse backwards to find the most recently used scrap view
                for (int i = size - 1; i >= 0; i--) {
                    final View view = scrapViews.get(i);
                    final AbsListView.LayoutParams params =
                            (AbsListView.LayoutParams) view.getLayoutParams();

                    if (mAdapterHasStableIds) {
                        final long id = mAdapter.getItemId(position);
                        if (id == params.itemId) {
                            return scrapViews.remove(i);
                        }
                    } else if (params.scrappedFromPosition == position) {
                        final View scrap = scrapViews.remove(i);
                        clearScrapForRebind(scrap);
                        return scrap;
                    }
                }
                final View scrap = scrapViews.remove(size - 1);
                clearScrapForRebind(scrap);
                return scrap;
            } else {
                return null;
            }
        }

        private void clearScrap(final ArrayList scrap) {
            final int scrapCount = scrap.size();
            for (int j = 0; j < scrapCount; j++) {
                removeDetachedView(scrap.remove(scrapCount - 1 - j), false);
            }
        }

        private void clearScrapForRebind(View view) {
            view.clearAccessibilityFocus();
            view.setAccessibilityDelegate(null);
        }

        private void removeDetachedView(View child, boolean animate) {
            child.setAccessibilityDelegate(null);
            AbsListView.this.removeDetachedView(child, animate);
        }
    }

变量：

RecycleBin中 mActiveViews：当前在屏幕上展示的view
mScrapViews ：存储滑出屏幕的View； mViewTypeCount>1的时候，ScrapViews数组就会有多个值，分别缓存一个类型的item
mCurrentScrap ：和mScrapViews 功能类似，mViewTypeCount=1，也就是listView中只有一种类型的item时(不包括header和footer)，item是存入到mCurrentScrap中。
mViewTypeCount ： 当前ListView 的item的种类

                if (mViewTypeCount == 1) {
                    mCurrentScrap.add(scrap);
                } else {
                    mScrapViews[viewType].add(scrap);
                }

方法：

fillActiveViews：根据传入的参数，要存储view的数量以及第一个可见元素的位置，往mActiveViews添加View
getActiveView: 和fillActiveViews相对，从mActiveViews获取View，从getActiveView方法可以看到，在读取match之后，就 activeViews[index] = null; 说明 activeViews 中的view只能复用一次。
getScrapView: 从mScrapViews 或者mCurrentScrap中获取尾部的View
addScrapView: 将滚出屏幕的View添加到mScrapViews 或者mCurrentScrap中

(4) ListView 的绘制原理以及缓存回收过程

在看过ListView的初始化操作之后，之后就是由ViemRootImpl的 performTraversals 方法主持View的整个绘制过程， 这里简单介绍下View的启动过程，用UML图表示他们的调用关系，具体的源码大家可以再细细研究。

ViemRootImpl的 performTraversals 完成依次执行performMeasure performLayout performDraw三个方法，其中performMeasure 中调用measure方法，在measure方法中又会调用onMeasure方法，在onMeasure方法中会对所有的子元素进行measure过程，这个时候measure流程就从父容器传递到子元素中，这样就完成了一次measure过程。接着子元素会重复父容器的measure过程，如此反复完成整个View树的遍历。同理，performLayout和performDraw传递流程和perwformMeasure类似，唯一不同的是，performDraw过程实在draw方法中通过dispatchOnDraw来实现的，之后再进行onDraw，不过这并没有本质的差别。

接下来就来剖析ListView的绘制过程。

onMeasure 测量过程和其他View功能类似，测量好view的宽度和高度以及对应的SpecMode SpecSize
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // Sets up mListPadding
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        // 初始化 宽度和高度 的模式和大小
        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        int childWidth = 0;
        int childHeight = 0;
        int childState = 0;

        ...

        setMeasuredDimension(widthSize, heightSize);

        mWidthMeasureSpec = widthMeasureSpec;
    }

ListView 的绘制过程主要在onLayout，ListView 中没有onLayout在父类AbsListView 中

AbsListView 中的onLayout
@Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);

        mInLayout = true;

        final int childCount = getChildCount();
        if (changed) {
            for (int i = 0; i < childCount; i++) {
                getChildAt(i).forceLayout();
            }
            mRecycler.markChildrenDirty();
        }

        layoutChildren();

        mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;

        // TODO: Move somewhere sane. This doesn't belong in onLayout().
        if (mFastScroll != null) {
            mFastScroll.onItemCountChanged(getChildCount(), mItemCount);
        }
        mInLayout = false;
    }
 //  AbsListView 中的layoutChildren的实现分别在其子类中
  protected void layoutChildren() {
  }

ListView 的 layoutChildren

   @Override
    protected void layoutChildren() {
        final boolean blockLayoutRequests = mBlockLayoutRequests;
        if (blockLayoutRequests) {
            return;
        }

        mBlockLayoutRequests = true;

        try {
            super.layoutChildren();

            invalidate();

            if (mAdapter == null) {
                resetList();
                invokeOnItemScrollListener();
                return;
            }

            final int childrenTop = mListPadding.top;
            final int childrenBottom = mBottom - mTop - mListPadding.bottom;
            // 一开始获取 count = 0;
            final int childCount = getChildCount();

            int index = 0;
            int delta = 0;

            View sel;
            View oldSel = null;
            View oldFirst = null;
            View newSel = 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. Make sure your adapter calls notifyDataSetChanged() "
                        + "when its content changes. [in ListView(" + getId() + ", " + getClass()
                        + ") with Adapter(" + mAdapter.getClass() + ")]");
            }

            setSelectedPositionInt(mNextSelectedPosition);

            AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null;
            View accessibilityFocusLayoutRestoreView = null;
            int accessibilityFocusPosition = INVALID_POSITION;

            // Remember which child, if any, had accessibility focus. This must
            // occur before recycling any views, since that will clear
            // accessibility focus.
            final ViewRootImpl viewRootImpl = getViewRootImpl();
            if (viewRootImpl != null) {
                final View focusHost = viewRootImpl.getAccessibilityFocusedHost();
                if (focusHost != null) {
                    final View focusChild = getAccessibilityFocusedChild(focusHost);
                    if (focusChild != null) {
                        if (!dataChanged || isDirectChildHeaderOrFooter(focusChild)
                                || (focusChild.hasTransientState() && mAdapterHasStableIds)) {
                            // The views won't be changing, so try to maintain
                            // focus on the current host and virtual view.
                            accessibilityFocusLayoutRestoreView = focusHost;
                            accessibilityFocusLayoutRestoreNode = viewRootImpl
                                    .getAccessibilityFocusedVirtualView();
                        }

                        // If all else fails, maintain focus at the same
                        // position.
                        accessibilityFocusPosition = getPositionForView(focusChild);
                    }
                }
            }

            View focusLayoutRestoreDirectChild = null;
            View focusLayoutRestoreView = null;

            // 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 ViewAncestor 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 re-layout
                // if the data hasn't changed, or if the focused position is a
                // header or footer.
                if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)
                        || focusedChild.hasTransientState() || mAdapterHasStableIds) {
                    focusLayoutRestoreDirectChild = focusedChild;
                    // Remember the specific view that had focus.
                    focusLayoutRestoreView = findFocus();
                    if (focusLayoutRestoreView != null) {
                        // Tell it we are going to mess with it.
                        focusLayoutRestoreView.dispatchStartTemporaryDetach();
                    }
                }
                requestFocus();
            }

            // Pull all children into the RecycleBin.
            // These views will be reused if possible
           //mFirstPosition是ListView的成员变量，存储着第一个显示的child所对应的adapter的position
            final int firstPosition = mFirstPosition;
            final RecycleBin recycleBin = mRecycler;
            if (dataChanged) {
                //如果数据发生了变化，那么就把ListView的所有子View都放入到RecycleBin的mScrapViews数组中
                for (int i = 0; i < childCount; i++) {
                    //addScrapView方法会传入一个View，以及这个View所对应的position
                    recycleBin.addScrapView(getChildAt(i), firstPosition+i);
                }
            } else {
                //如果数据没发生变化，那么把ListView的所有子View都放入到RecycleBin的mActiveViews数组中
                recycleBin.fillActiveViews(childCount, firstPosition);
            }

            // Clear out old views
            detachAllViewsFromParent();
            recycleBin.removeSkippedScrap();

            // mLayoutMode的值来决定布局模式，默认情况下都是普通模式LAYOUT_NORMAL
            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:
                final int selectedPosition = reconcileSelectedPosition();
                sel = fillSpecific(selectedPosition, mSpecificTop);
                /**
                 * When ListView is resized, FocusSelector requests an async selection for the
                 * previously focused item to make sure it is still visible. If the item is not
                 * selectable, it won't regain focus so instead we call FocusSelector
                 * to directly request focus on the view after it is visible.
                 */
                if (sel == null && mFocusSelector != null) {
                    final Runnable focusRunnable = mFocusSelector
                            .setupFocusIfValid(selectedPosition);
                    if (focusRunnable != null) {
                        post(focusRunnable);
                    }
                }
                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();

            // remove any header/footer that has been temp detached and not re-attached
            removeUnusedFixedViews(mHeaderViewInfos);
            removeUnusedFixedViews(mFooterViewInfos);

            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 != null &&
                            focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
                    if (!focusWasTaken) {
                        // Selected item didn't take focus, but we still want to
                        // make sure something else outside of the selected view
                        // has focus.
                        final View focused = getFocusedChild();
                        if (focused != null) {
                            focused.clearFocus();
                        }
                        positionSelector(INVALID_POSITION, sel);
                    } else {
                        sel.setSelected(false);
                        mSelectorRect.setEmpty();
                    }
                } else {
                    positionSelector(INVALID_POSITION, sel);
                }
                mSelectedTop = sel.getTop();
            } else {
                final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP
                        || mTouchMode == TOUCH_MODE_DONE_WAITING;
                if (inTouchMode) {
                    // If the user's finger is down, select the motion position.
                    final View child = getChildAt(mMotionPosition - mFirstPosition);
                    if (child != null) {
                        positionSelector(mMotionPosition, child);
                    }
                } else if (mSelectorPosition != INVALID_POSITION) {
                    // If we had previously positioned the selector somewhere,
                    // put it back there. It might not match up with the data,
                    // but it's transitioning out so it's not a big deal.
                    final View child = getChildAt(mSelectorPosition - mFirstPosition);
                    if (child != null) {
                        positionSelector(mSelectorPosition, child);
                    }
                } else {
                    // Otherwise, clear selection.
                    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();
                }
            }

            // Attempt to restore accessibility focus, if necessary.
            if (viewRootImpl != null) {
                final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost();
                if (newAccessibilityFocusedView == null) {
                    if (accessibilityFocusLayoutRestoreView != null
                            && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) {
                        final AccessibilityNodeProvider provider =
                                accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider();
                        if (accessibilityFocusLayoutRestoreNode != null && provider != null) {
                            final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId(
                                    accessibilityFocusLayoutRestoreNode.getSourceNodeId());
                            provider.performAction(virtualViewId,
                                    AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null);
                        } else {
                            accessibilityFocusLayoutRestoreView.requestAccessibilityFocus();
                        }
                    } else if (accessibilityFocusPosition != INVALID_POSITION) {
                        // Bound the position within the visible children.
                        final int position = MathUtils.constrain(
                                accessibilityFocusPosition - mFirstPosition, 0,
                                getChildCount() - 1);
                        final View restoreView = getChildAt(position);
                        if (restoreView != null) {
                            restoreView.requestAccessibilityFocus();
                        }
                    }
                }
            }

            // Tell focus view we are done mucking with it, if it is still in
            // our view hierarchy.
            if (focusLayoutRestoreView != null
                    && focusLayoutRestoreView.getWindowToken() != null) {
                focusLayoutRestoreView.dispatchFinishTemporaryDetach();
            }

            mLayoutMode = LAYOUT_NORMAL;
            mDataChanged = false;
            if (mPositionScrollAfterLayout != null) {
                post(mPositionScrollAfterLayout);
                mPositionScrollAfterLayout = null;
            }
            mNeedSync = false;
            setNextSelectedPositionInt(mSelectedPosition);

            updateScrollIndicators();

            if (mItemCount > 0) {
                checkSelectionChanged();
            }

            invokeOnItemScrollListener();
        } finally {
            if (mFocusSelector != null) {
                mFocusSelector.onLayoutComplete();
            }
            if (!blockLayoutRequests) {
                mBlockLayoutRequests = false;
            }
        }
    }

看到这个 layoutChildren()这么复杂，这里找重要的类去分析：
一开始还没有设置数据源， getChildCount 的值等于0;
之后会判断数据源是否变化, 分别使用RecyclerBin 的 addScrapView fillActiveViews 对数据进行缓存和删除处理.
之后根据mLayoutMode 的值判定布局模式， 一般会进入default 中处理，由于默认的布局顺序是从上往下，所以会进入fillFromTop 方法

 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 fillFromTop 方法可以看出主要方法是在filDown方法中，接下来再看filDown中是什么…

 private View fillDown(int pos, int nextTop) {
        View selectedView = null;

        int end = (mBottom - mTop);
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            end -= 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++;
        }

        setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);
        return selectedView;
    }

fillDown 方法通过一个while循环，主要的是makeAndAddView

 private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
            boolean selected) {
        if (!mDataChanged) {
            // Try to use an existing view for this position.
            final View activeView = mRecycler.getActiveView(position);
            if (activeView != null) {
                // Found it. We're reusing an existing child, so it just needs
                // to be positioned like a scrap view.
                setupChild(activeView, position, y, flow, childrenLeft, selected, true);
                return activeView;
            }
        }

        // Make a new view for this position, or convert an unused view if
        // possible.
        final View child = obtainView(position, mIsScrap);

        // This needs to be positioned and measured.
        setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

        return child;
    }

ListView 中的View的创建以及复用主要是在obtainView中实现

View obtainView(int position, boolean[] outMetadata) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");

        outMetadata[0] = false;

        // Check whether we have a transient state view. Attempt to re-bind the
        // data and discard the view if we fail.
        final View transientView = mRecycler.getTransientStateView(position);
        if (transientView != null) {
            final LayoutParams params = (LayoutParams) transientView.getLayoutParams();

            // If the view type hasn't changed, attempt to re-bind the data.
            if (params.viewType == mAdapter.getItemViewType(position)) {
                final View updatedView = mAdapter.getView(position, transientView, this);

                // If we failed to re-bind the data, scrap the obtained view.
                if (updatedView != transientView) {
                    setItemViewLayoutParams(updatedView, position);
                    mRecycler.addScrapView(updatedView, position);
                }
            }

            outMetadata[0] = true;

            // Finish the temporary detach started in addScrapView().
            transientView.dispatchFinishTemporaryDetach();
            return transientView;
        }

        final View scrapView = mRecycler.getScrapView(position);
        final View child = mAdapter.getView(position, scrapView, this);
        if (scrapView != null) {
            if (child != scrapView) {
                // Failed to re-bind the data, return scrap to the heap.
                mRecycler.addScrapView(scrapView, position);
            } else if (child.isTemporarilyDetached()) {
                outMetadata[0] = true;

                // Finish the temporary detach started in addScrapView().
                child.dispatchFinishTemporaryDetach();
            }
        }

        if (mCacheColorHint != 0) {
            child.setDrawingCacheBackgroundColor(mCacheColorHint);
        }

        if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
            child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
        }

        setItemViewLayoutParams(child, position);

        if (AccessibilityManager.getInstance(mContext).isEnabled()) {
            if (mAccessibilityDelegate == null) {
                mAccessibilityDelegate = new ListItemAccessibilityDelegate();
            }
            if (child.getAccessibilityDelegate() == null) {
                child.setAccessibilityDelegate(mAccessibilityDelegate);
            }
        }

        Trace.traceEnd(Trace.TRACE_TAG_VIEW);

        return child;
    }

obtainView 方法中 从 RecyclerBin中getScrapView 在数据展示过程中第一次读取的时候一定是null，因为第一屏数据还没有展示完全，不会缓存数据； 之后就调用adapter中的getView那去child，getView就是我们在自定义Adapter时重写的getView方法，最后通过调用LayoutInflater的inflate()方法来去加载一个布局。拿到View之后，将回到makeAndAddView方法中，之后将child传入到setupChild中。

private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
            boolean selected, boolean isAttachedToWindow) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");

        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 = !isAttachedToWindow || updateChildSelected
                || child.isLayoutRequested();

        // Respect layout params that are already in the view. Otherwise make
        // some up...
        AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
        if (p == null) {
            p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
        }
        p.viewType = mAdapter.getItemViewType(position);
        p.isEnabled = mAdapter.isEnabled(position);

        // Set up view state before attaching the view, since we may need to
        // rely on the jumpDrawablesToCurrentState() call that occurs as part
        // of view attachment.
        if (updateChildSelected) {
            child.setSelected(isSelected);
        }

        if (updateChildPressed) {
            child.setPressed(isPressed);
        }

        if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
            if (child instanceof Checkable) {
                ((Checkable) child).setChecked(mCheckStates.get(position));
            } else if (getContext().getApplicationInfo().targetSdkVersion
                    >= android.os.Build.VERSION_CODES.HONEYCOMB) {
                child.setActivated(mCheckStates.get(position));
            }
        }

        if ((isAttachedToWindow && !p.forceAdd) || (p.recycledHeaderFooter
                && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
            attachViewToParent(child, flowDown ? -1 : 0, p);

            // If the view was previously attached for a different position,
            // then manually jump the drawables.
            if (isAttachedToWindow
                    && (((AbsListView.LayoutParams) child.getLayoutParams()).scrappedFromPosition)
                            != position) {
                child.jumpDrawablesToCurrentState();
            }
        } else {
            p.forceAdd = false;
            if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                p.recycledHeaderFooter = true;
            }
            addViewInLayout(child, flowDown ? -1 : 0, p, true);
            // add view in layout will reset the RTL properties. We have to re-resolve them
            child.resolveRtlPropertiesIfNeeded();
        }

        if (needToMeasure) {
            final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
                    mListPadding.left + mListPadding.right, p.width);
            final int lpHeight = p.height;
            final int childHeightSpec;
            if (lpHeight > 0) {
                childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
            } else {
                childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
                        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);
        }

        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

setupChild 方法主要是调用addViewInLayout 将 view 添加到ListView中，这样通过fillDown的循环，就依次填满ListView 一屏的数据。 这样第一次新建View 添加到ListView 相对来说比较耗时，但之后将通过RecyclerBin回收和缓冲机制来展示LsitView。

在之后绘制过程中，从上面提到的layoutChildren 方法中的getChildCount将不是0，而是ListView中一屏可以显示的子View数量， 之前我们会在fillActiveViews方法中存储我们目前屏幕中展示的View； 之后又调用了调用了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。

之后的分居布局模式判断将走进default的else中，执行fillSpecific方法， 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；
之后会从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obtainView()方法，而是会直接进入setupChild()方法当中，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；
setupChild()方法的最后一个参数是isAttachedToWindow，然后在第32行会对这个变量进行判断，由于isAttachedToWindow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。

这一部分的分析是参考 郭神的博客分析的，这是郭神原创

ListView的onDraw过程和其他View类似。

下一篇将会分析 ListView 剩余的 5，6，7部分