ListView、RecyclerView缓存策略解析

public class DemoAdapter extends BaseAdapter {
    private Context mContext;
    private List<Data> mDataList;

    public DemoAdapter(Context context, List<Data> dataList) {
        mContext = context;
        mDataList = dataList;
    }

    @Override
    public int getCount() {
        return mDataList.size();
    }

    @Override
    public Object getItem(int position) {
        return null;
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder viewHolder;
        if (convertView == null) {
            convertView = LayoutInflater.from(mContext).inflate(R.layout.item_location_hot_city, null);
            viewHolder = new ViewHolder(convertView);
            convertView.setTag(viewHolder);
        } else {
            viewHolder = (ViewHolder) convertView.getTag();
        }

        final Data data = mDataList.get(position);
        viewHolder.tvTitle.setText(data.getName());

        return convertView;
    }

    private static class ViewHolder {
        View itemView;
        TextView tvTitle;

        public ViewHolder(View view) {
            itemView = view;
            tvTitle = (TextView) view.findViewById(R.id.tv_title);
        }
    }
}

/**
  * The position of the first view stored in mActiveViews.
  */
private int mFirstActivePosition;

/**
  * Views that were on screen at the start of layout. This array is populated at the start of
  * layout, and at the end of layout all view in mActiveViews are moved to mScrapViews.
  * Views in mActiveViews represent a contiguous range of Views, with position of the first
  * view store in mFirstActivePosition.
  */
private View[] mActiveViews = new View[0];

/**
  * Unsorted views that can be used by the adapter as a convert view.
  */
private ArrayList<View>[] mScrapViews;

private int mViewTypeCount;

private ArrayList<View> mCurrentScrap;

private ArrayList<View> mSkippedScrap;

private SparseArray<View> mTransientStateViews;
private LongSparseArray<View> mTransientStateViewsById;

/**
  * Subclasses should NOT override this method but
  *  {@link #layoutChildren()} instead.
  */
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    super.onLayout(changed, l, t, r, b);

    mInLayout = true;

    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;
}

@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;
        final int childCount = getChildCount();

        int index = 0;
        int delta = 0;

        View sel;
        View oldSel = null;
        View oldFirst = null;
        View newSel = null;

        ...

        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);

        ...

        // Pull all children into the RecycleBin.
        // These views will be reused if possible
        final int firstPosition = mFirstPosition;
        final RecycleBin recycleBin = mRecycler;
        if (dataChanged) {
            for (int i = 0; i < childCount; i++) {
                recycleBin.addScrapView(getChildAt(i), firstPosition+i);
            }
        } else {
            recycleBin.fillActiveViews(childCount, firstPosition);
        }

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

        switch (mLayoutMode) {
            ...
        case LAYOUT_MOVE_SELECTION:
            ...
        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);

        ...

        // 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;
        }
    }
}

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

/**
  * Fills the list from pos down to the end of the list view.
  *
  * @param pos The first position to put in the list
  *
  * @param nextTop The location where the top of the item associated with pos
  *        should be drawn
  *
  * @return The view that is currently selected, if it happens to be in the
  *         range that we draw.
  */
private View fillDown(int pos, int nextTop) {
    View selectedView = null;

    int end = (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;
}

/**
  * Obtains the view and adds it to our list of children. The view can be
  * made fresh, converted from an unused view, or used as is if it was in
  * the recycle bin.
  *
  * @param position logical position in the list
  * @param y top or bottom edge of the view to add
  * @param flow {@code true} to align top edge to y, {@code false} to align
  *             bottom edge to y
  * @param childrenLeft left edge where children should be positioned
  * @param selected {@code true} if the position is selected, {@code false}
  *                 otherwise
  * @return the view that was added
  */
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;
}

/**
 * Gets a view and have it show the data associated with the specified
 * position. This is called when we have already discovered that the view
 * is not available for reuse in the recycle bin. The only choices left are
 * converting an old view or making a new one.
 *
 * @param position the position to display
 * @param outMetadata an array of at least 1 boolean where the first entry
 *                    will be set {@code true} if the view is currently
 *                    attached to the window, {@code false} otherwise (e.g.
 *                    newly-inflated or remained scrap for multiple layout
 *                    passes)
 *
 * @return A view displaying the data associated with the specified position
 */
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);
    }

    ...

    setItemViewLayoutParams(child, position);

    ...

    Trace.traceEnd(Trace.TRACE_TAG_VIEW);

    return child;
}

/**
  * Adds a view as a child and make sure it is measured (if necessary) and
  * positioned properly.
  *
  * @param child the view to add
  * @param position the position of this child
  * @param y the y position relative to which this view will be positioned
  * @param flowDown {@code true} to align top edge to y, {@code false} to
  *                 align bottom edge to y
  * @param childrenLeft left edge where children should be positioned
  * @param selected {@code true} if the position is selected, {@code false}
  *                 otherwise
  * @param isAttachedToWindow {@code true} if the view is already attached
  *                           to the window, e.g. whether it was reused, or
  *                           {@code false} otherwise
  */
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);
}

@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;
        final int childCount = getChildCount();

        int index = 0;
        int delta = 0;

        View sel;
        View oldSel = null;
        View oldFirst = null;
        View newSel = null;

        ...

        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);

        ...

        // Pull all children into the RecycleBin.
        // These views will be reused if possible
        final int firstPosition = mFirstPosition;
        final RecycleBin recycleBin = mRecycler;
        if (dataChanged) {
            for (int i = 0; i < childCount; i++) {
                recycleBin.addScrapView(getChildAt(i), firstPosition+i);
            }
        } else {
            recycleBin.fillActiveViews(childCount, firstPosition);
        }

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

        switch (mLayoutMode) {
            ...
        case LAYOUT_MOVE_SELECTION:
            ...
        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);

        ...

        // 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;
        }
    }
}

/**
  * Put a specific item at a specific location on the screen and then build
  * up and down from there.
  *
  * @param position The reference view to use as the starting point
  * @param top Pixel offset from the top of this view to the top of the
  *        reference view.
  *
  * @return The selected view, or null if the selected view is outside the
  *         visible area.
  */
private View fillSpecific(int position, int top) {
    boolean tempIsSelected = position == mSelectedPosition;
    View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);
    // Possibly changed again in fillUp if we add rows above this one.
    mFirstPosition = position;

    View above;
    View below;

    final int dividerHeight = mDividerHeight;
    if (!mStackFromBottom) {
        above = fillUp(position - 1, temp.getTop() - dividerHeight);
        // This will correct for the top of the first view not touching the top of the list
        adjustViewsUpOrDown();
        below = fillDown(position + 1, temp.getBottom() + dividerHeight);
        int childCount = getChildCount();
        if (childCount > 0) {
            correctTooHigh(childCount);
        }
    } else {
        below = fillDown(position + 1, temp.getBottom() + dividerHeight);
        // This will correct for the bottom of the last view not touching the bottom of the list
        adjustViewsUpOrDown();
        above = fillUp(position - 1, temp.getTop() - dividerHeight);
        int childCount = getChildCount();
        if (childCount > 0) {
                correctTooLow(childCount);
        }
    }

    if (tempIsSelected) {
        return temp;
    } else if (above != null) {
        return above;
    } else {
        return below;
    }
}

/**
  * Obtains the view and adds it to our list of children. The view can be
  * made fresh, converted from an unused view, or used as is if it was in
  * the recycle bin.
  *
  * @param position logical position in the list
  * @param y top or bottom edge of the view to add
  * @param flow {@code true} to align top edge to y, {@code false} to align
  *             bottom edge to y
  * @param childrenLeft left edge where children should be positioned
  * @param selected {@code true} if the position is selected, {@code false}
  *                 otherwise
  * @return the view that was added
  */
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;
}

/**
  * Adds a view as a child and make sure it is measured (if necessary) and
  * positioned properly.
  *
  * @param child the view to add
  * @param position the position of this child
  * @param y the y position relative to which this view will be positioned
  * @param flowDown {@code true} to align top edge to y, {@code false} to
  *                 align bottom edge to y
  * @param childrenLeft left edge where children should be positioned
  * @param selected {@code true} if the position is selected, {@code false}
  *                 otherwise
  * @param isAttachedToWindow {@code true} if the view is already attached
  *                           to the window, e.g. whether it was reused, or
  *                           {@code false} otherwise
  */
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);
}

@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;
}