StaggeredGridView 实现分析--首次填充过程（一）
ExtendableListView.java
1， 因为是首次layout， changed为true， childCount是 0，
//ExtendableListView.java
/**
* {@inheritDoc}
*/
@Override
protected void onLayout(final boolean changed, final int l, final int t, final int r, final int b) {
// super.onLayout(changed, l, t, r, b); - skipping base AbsListView implementation on purpose
// haven't set an adapter yet? get to it
if (mAdapter == null) {
return;
}
if (changed) {
int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
getChildAt(i).forceLayout();
}
mRecycleBin.markChildrenDirty();
}
// TODO get the height of the view??
mInLayout = true;
layoutChildren();
mInLayout = false;
}
2， layoutChildren， 在步骤1中可知 在layoutChildren（）期间 变量mInLayout 一直为true。
在attachToWindow() 和 setAdapter(..)时 ，如果adapter不为null， 则mDataChanged 设置为 true, 所以假设这里的mDataChanged是true .
在handleDataChanged() 中， mLayoutMode 被设置为 LAYOUT_FORTCE_TOP ， 所以 mFirstPosition 为 0， 然后执行 fillFromTop( top) ,假设没有headerview， 那么 top值是0 或者 listview的 topPadding值。
//ExtendableListView.java
/**
* {@inheritDoc}
*/
@Override
protected void layoutChildren() {
if (mBlockLayoutRequests) return;
mBlockLayoutRequests = true;
try {
super.layoutChildren();
invalidate();
...
int childrenTop = getListPaddingTop();
int childCount = getChildCount();
View oldFirst = null;
// our last state so we keep our position
if (mLayoutMode == LAYOUT_NORMAL) {
oldFirst = getChildAt(0);
}
boolean dataChanged = mDataChanged;
if (dataChanged) {
handleDataChanged();
}
...
// Pull all children into the RecycleBin.
// These views will be reused if possible
final int firstPosition = mFirstPosition;
final RecycleBin recycleBin = mRecycleBin;
if (dataChanged) { // true now
for (int i = 0; i < childCount; i++) { // childcount is 0 now
recycleBin.addScrapView(getChildAt(i), firstPosition + i);
}
}
else {
recycleBin.fillActiveViews(childCount, firstPosition);
}
// Clear out old views
detachAllViewsFromParent();
recycleBin.removeSkippedScrap();
switch (mLayoutMode) {
case LAYOUT_FORCE_TOP: {
mFirstPosition = 0;
resetToTop();
adjustViewsUpOrDown();
fillFromTop(childrenTop);
adjustViewsUpOrDown();
break;
}
case LAYOUT_SYNC: {
fillSpecific(mSyncPosition, mSpecificTop);
break;
}
case LAYOUT_NORMAL:
default: {
if (childCount == 0) {
fillFromTop(childrenTop);
}
else if (mFirstPosition < mItemCount) {
fillSpecific(mFirstPosition,
oldFirst == null ? childrenTop : oldFirst.getTop());
}
else {
fillSpecific(0, childrenTop);
}
break;
}
}
// Flush any cached views that did not get reused above
recycleBin.scrapActiveViews();
mDataChanged = false;
mNeedSync = false;
mLayoutMode = LAYOUT_NORMAL;
invokeOnItemScrollListener();
} finally {
mBlockLayoutRequests = false;
}
}
////////////////////////////////////////////////////////
2' , override:
这里将mColumnBottoms[] 和 mColumnTops[]是一样的值（每个值都是0， 或者listview的 topPadding值）， 因为还没任何child view
//StaggeredGridView.java
@Override
protected void layoutChildren() {
preLayoutChildren();
super.layoutChildren();
}
//mNeedSync 在 onRestoreInstanceState 时才会被设置为true，其他情况均为false
private void preLayoutChildren() {
// on a major re-layout reset for our next layout pass
if (!mNeedSync) {
Arrays.fill(mColumnBottoms, 0);
}
else {
mNeedSync = false;
}
// copy the tops into the bottom
// since we're going to redo a layout pass that will draw down from
// the top
System.arraycopy(mColumnTops, 0, mColumnBottoms, 0, mColumnCount);
}
3， 回顾2, ExtendableListView.java， fillFromTop， 其参数是0 或者 listView的 topPadding值；mFirstPosition是0
在fillDown中， 此时 itemPos 是0 ； nextTop是顶部位置: topPadding 或 0 ， 通过while循环这一过程，将有足够多的child view 被生成以填满整个listview。
//ExtendableListView.java
/**
* Fills the list from top to bottom, starting with mFirstPosition
*/
private View fillFromTop(int nextTop) {
mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
if (mFirstPosition < 0) {
mFirstPosition = 0;
}
return fillDown(mFirstPosition, nextTop);
}
private View fillDown(int itemPos, int nextTop) { //(0,topPadding)
if (DBG) Log.d(TAG, "fillDown - pos:" + pos + " nextTop:" + nextTop);
View selectedView = null;
int end = getHeight();
if (mClipToPadding) {
end -= getListPaddingBottom();
}
while ((nextTop < end || hasSpaceDown()) && pos < mItemCount) {
// TODO : add selection support
// 目前不支持select ， 所以selected一律为false
makeAndAddView(itemPos, nextTop, true, false);
itemPos++;
nextTop = getNextChildDownsTop(itemPos); // = child.getBottom();
}
return selectedView;
}
4, makeAndAddView(0, 0, true, false):
执行obtainView
//ExtendableListView.java
/**
* Gets a view either a new view an unused view?? or a recycled view and adds it to our children
*/
private View makeAndAddView(int position, int y, boolean flowDown, boolean selected) {
View child;
onChildCreated(position, flowDown);
if (!mDataChanged) {// now mDataChanged is true , so it is skipped here
// Try to use an existing view for this position
child = mRecycleBin.getActiveView(position);
if (child != null) {
// Found it -- we're using an existing child
// This just needs to be positioned
setupChild(child, position, y, flowDown, 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, flowDown, selected, mIsScrap[0]);
return child;
}
4.1 onChildCreated( 0, true): ExtendableListView 的实现为空， 所以直接看StaggeredGridView的实现：
执行if 部分，需要看 getChildColumn 和 setPositionColumn 。
//StaggeredGridView.java
@Override
protected void onChildCreated(final int position, final boolean flowDown) {
super.onChildCreated(position, flowDown);
if (!isHeaderOrFooter(position)) {
// do we already have a column for this position?
final int column = getChildColumn(position, flowDown);
setPositionColumn(position, column);
}
else {
setPositionIsHeaderFooter(position);
}
}