ListView希望通过复用看不见的View来达到目的。其中最最重要的就是这个
2.RecycleBin--回收站.
回收站并不是垃圾站。
最重要的是两个成员变量 mScrapViews 和mActiveViews， mScrapViews 表示回收站的View. mActiveViews表示激活的View，激活的View是可以直接拿来显示的，不想要重新调用getView填充数据。激活的View（mActiveViews）和废弃的View（mScrapViews） 不同，激活的View不想要调用getView填充数据，而废弃的View需要。
/**
* Unsorted views that can be used by the adapter as a convert view.
*/
private ArrayList<View>[] mScrapViews;
private View[] mActiveViews = new View[0];
看不见的View都会放进这里面。主要是通过以下方法 addScrapView 加进去的
/**
* Put a view into the ScapViews list. These views are unordered.
*
* @param scrap The view to add
*/
void addScrapView(View scrap, int position) {
AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
if (lp == null) {
return;
}
// Don't put header or footer views or views that should be ignored
// into the scrap heap
int viewType = lp.viewType;
if (!shouldRecycleViewType(viewType)) {
if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
removeDetachedView(scrap, false);
}
return;
}
lp.scrappedFromPosition = position;
if (mViewTypeCount == 1) {
scrap.dispatchStartTemporaryDetach();
mCurrentScrap.add(scrap);
} else {
scrap.dispatchStartTemporaryDetach();
mScrapViews[viewType].add(scrap);
}
if (mRecyclerListener != null) {
mRecyclerListener.onMovedToScrapHeap(scrap);
}
}
3.回收站算法
makeAndAddView 这个方法显示了回收站的使用算法。如果没有dadachange，那么就直接从getActiveView 方法取得那个View，如果成功，就调整其位置。如果datachange，就需要重新绑定数据到这个View上。这里的 重新绑定数据 方法obtainView 同时考虑是否使用回收站的View.
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
View child;
if (!mDataChanged) {
// Try to use an existing view for this position
child = mRecycler.getActiveView(position);
if (child != null) {
if (ViewDebug.TRACE_RECYCLER) {
ViewDebug.trace(child, ViewDebug.RecyclerTraceType.RECYCLE_FROM_ACTIVE_HEAP,
position, getChildCount());
}
// Found it -- we're using an existing child
// This just needs to be positioned
setupChild(child, position, y, flow, childrenLeft, selected, true);
return child;
}
}
// Make a new view for this position, or convert an unused view if possible
child = obtainView(position, mIsScrap);
// This needs to be positioned and measured
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
return child;
}
4.通过ObtainView方法绑定数据
其实也很简单。主要任务是复用mScrapViews。
View obtainView(int position, boolean[] isScrap) {
isScrap[0] = false;
View scrapView;
scrapView = mRecycler.getScrapView(position);
View child;
if (scrapView != null) {
if (ViewDebug.TRACE_RECYCLER) {
ViewDebug.trace(scrapView, ViewDebug.RecyclerTraceType.RECYCLE_FROM_SCRAP_HEAP,
position, -1);
}
child = mAdapter.getView(position, scrapView, this);
if (ViewDebug.TRACE_RECYCLER) {
ViewDebug.trace(child, ViewDebug.RecyclerTraceType.BIND_VIEW,
position, getChildCount());
}
if (child != scrapView) {
mRecycler.addScrapView(scrapView, position);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
if (ViewDebug.TRACE_RECYCLER) {
ViewDebug.trace(scrapView, ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP,
position, -1);
}
} else {
isScrap[0] = true;
child.dispatchFinishTemporaryDetach();
}
} else {
child = mAdapter.getView(position, null, this);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
if (ViewDebug.TRACE_RECYCLER) {
ViewDebug.trace(child, ViewDebug.RecyclerTraceType.NEW_VIEW,
position, getChildCount());
}
}
return child;
}
5.布局listView
fillDown 方法用得最多，其目的就是从pos这个index和nextTop这个View top坐标，向下填充Views直到底部。
/**
* 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++;
}
return selectedView;
}
6.触发fillDown的切入点
layoutChildren
trackMotionScroll
7.不得不说的LayoutParams
/**
* AbsListView extends LayoutParams to provide a place to hold the view type.
*/
public static class LayoutParams extends ViewGroup.LayoutParams {
/**
* View type for this view, as returned by
* {@link android.widget.Adapter#getItemViewType(int) }
*/
@ViewDebug.ExportedProperty(category = "list", mapping = {
@ViewDebug.IntToString(from = ITEM_VIEW_TYPE_IGNORE, to = "ITEM_VIEW_TYPE_IGNORE"),
@ViewDebug.IntToString(from = ITEM_VIEW_TYPE_HEADER_OR_FOOTER, to = "ITEM_VIEW_TYPE_HEADER_OR_FOOTER")
})
int viewType;
/**
* When this boolean is set, the view has been added to the AbsListView
* at least once. It is used to know whether headers/footers have already
* been added to the list view and whether they should be treated as
* recycled views or not.
*/
@ViewDebug.ExportedProperty(category = "list")
boolean recycledHeaderFooter;
/**
* When an AbsListView is measured with an AT_MOST measure spec, it needs
* to obtain children views to measure itself. When doing so, the children
* are not attached to the window, but put in the recycler which assumes
* they've been attached before. Setting this flag will force the reused
* view to be attached to the window rather than just attached to the
* parent.
*/
@ViewDebug.ExportedProperty(category = "list")
boolean forceAdd;
/**
* The position the view was removed from when pulled out of the
* scrap heap.
* @hide
*/
int scrappedFromPosition;
public LayoutParams(Context c, AttributeSet attrs) {
super(c, attrs);
}
public LayoutParams(int w, int h) {
super(w, h);
}
public LayoutParams(int w, int h, int viewType) {
super(w, h);
this.viewType = viewType;
}
public LayoutParams(ViewGroup.LayoutParams source) {
super(source);
}
}
8.提高布局性能的函数 offsetLeftAndRight和 offsetTopAndBottom
在setupChild函数 中可以看到如下两行
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());
}
9. 提高性能的函数addViewInLayout
if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
attachViewToParent(child, flowDown ? -1 : 0, p);
} else {
p.forceAdd = false;
if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
p.recycledHeaderFooter = true;
}
addViewInLayout(child, flowDown ? -1 : 0, p, true);
}