大致的讲解了下ListView的复用的原理，了解手势触摸的时候大致做了什么操作
1.首先查看onTouchEvent，实现是在AbsListView里面
case MotionEvent.ACTION_MOVE:
{
onTouchMove(ev, vtev);
break;
}
2.onTouchMove方法中，实现的过程,其中的mTouchMode 在滑动的时候取值为TOUCH_MODE_SCROLL
private void onTouchMove(MotionEvent ev, MotionEvent vtev)
{
case TOUCH_MODE_SCROLL:
case TOUCH_MODE_OVERSCROLL:
scrollIfNeeded((int) ev.getX(pointerIndex), y, vtev);
break;
}
3.scrollIfNeeded方法的实现为
private void scrollIfNeeded(int x, int y, MotionEvent vtev)
{
。。。
final int deltaY = rawDeltaY;
int incrementalDeltaY = mLastY != Integer.MIN_VALUE ? y - mLastY + scrollConsumedCorrection : deltaY;
int overScrollDistance = -incrementalDeltaY;
trackMotionScroll(incrementalDeltaY, incrementalDeltaY);
。。。
}
4.trackMotionScroll方法的实现为这里传递的为滑动的距离
boolean trackMotionScroll(int deltaY, int incrementalDeltaY)
{
//如果incrementalDeltay小于0，代表向下滑动，如果大于0，代表向上滑动
final boolean down = incrementalDeltaY < 0;
final int headerViewsCount = getHeaderViewsCount();
final int footerViewsStart = mItemCount - getFooterViewsCount();
int start = 0;
int count = 0;
if (down) {
//向上滑动，界面会往上移动，注意第一个item消失的时机点，top这里为正数，代表要滑动的距离，如果top > child.getBottom代表当前的item就要从屏幕上消失了,就可以添加到缓存机制中
int top = -incrementalDeltaY;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
top += listPadding.top;
}
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
//如果第一个childViewd的都不满足消失的点，就没有必要继续往下遍历了，优化。。。。
if (child.getBottom() >= top) {
break;
} else {
//child.getBottom < top 代表当前的item要从屏幕上消失了
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
// The view will be rebound to new data, clear any
// system-managed transient state.
child.clearAccessibilityFocus();
//将当前要移除的item，添加到缓存机制中
mRecycler.addScrapView(child, position);
}
}
}
} else {
//如果是往下滑动，最后的一个item将会从屏幕上消失，这里要缓存起来,这里的incrementalDeltay为正数，这里的bottom代表最后一个item的bottom值
int bottom = getHeight() - incrementalDeltaY;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
bottom -= listPadding.bottom;
}
//这里是从下往上遍历，也是一种优化
for (int i = childCount - 1; i >= 0; i--) {
final View child = getChildAt(i);
if (child.getTop() <= bottom) { //最后一个item都不满足，就没有必要再次遍历了
break;
} else {
// child.getTop() > bottom 最后一个item要消失了，缓存
start = i;
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
// The view will be rebound to new data, clear any
// system-managed transient state.
child.clearAccessibilityFocus();
mRecycler.addScrapView(child, position);
}
}
}
if (count > 0) {
//將当前要移除屏幕的item，从父布局中移除
detachViewsFromParent(start, count);
mRecycler.removeSkippedScrap();
}
// invalidate before moving the children to avoid unnecessary invalidate
// calls to bubble up from the children all the way to the top
if (!awakenScrollBars()) {
invalidate();
}
offsetChildrenTopAndBottom(incrementalDeltaY);
if (down) {
mFirstPosition += count;
}
//取的是滑动的距离，这里是绝对值
final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
fillGap(down);
}
}
}
5.fillGap(down) 函数 在AbsListView中为 abstract void fillGap(boolean down); 是一个抽象的方法，所以实现类在具体的子类里面，这里为ListView，传递的参数down代表是否是从下往上的填充布局
/**
* {@inheritDoc}
*/
@Override
void fillGap(boolean down) {
final int count = getChildCount();
if (down) {
int paddingTop = 0;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
paddingTop = getListPaddingTop();
}
final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :
paddingTop;
fillDown(mFirstPosition + count, startOffset);
correctTooHigh(getChildCount());
} else {
int paddingBottom = 0;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
paddingBottom = getListPaddingBottom();
}
final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :
getHeight() - paddingBottom;
fillUp(mFirstPosition - 1, startOffset);
correctTooLow(getChildCount());
}
}
6.这里的函数有俩个主要的函数 fillDown(mFirstPosition + count, startOffset);或者 fillUp(mFirstPosition - 1, startOffset);我们查看那一个即可
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;
}
//这个可以保证childView是有限个的，只是一个屏幕的范围
while (nextTop < end && pos < mItemCount) {
// is this the selected item?
boolean selected = pos == mSelectedPosition;
//得到一个view，会先去从缓存中获取到，如果获取的为空话，适配器自己去创建一个View
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;
}
7.makeAndAddView(pos, nextTop, true, mListPadding.left, selected);函数的实现为
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.
//得到一个itemView
final View child = obtainView(position, mIsScrap);
// This needs to be positioned and measured.
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
return child;
}
8.关注函数的 final View child = obtainView(position, mIsScrap);的实现为：
View obtainView(int position, boolean[] outMetadata) {
.......
//这里是从我们的缓存池中获取回收的view，有可能为空，所以我们在适配器中的getView中要对scrapView中要判断是否为空，当为空的时候，就自己创建一个view,这就是listView的复用机制
final View scrapView = mRecycler.getScrapView(position);
final View child = mAdapter.getView(position, scrapView, this);
......
}
