RecyclerView Item 设为不可见后无法刷新
无法下拉刷新，想起以前添加 decoration 以后，就可以正常刷新了。
试了一下果然，这一次寻找一下原因，最后发现还是因为隐藏某个 item 后造成的，就不应该隐藏。
调试后发现
```
在
com.aspsine.swipetoloadlayout.SwipeToLoadLayout#onInterceptTouchEvent
中调用的 onCheckCanRefresh 始终返回 false，也就是不可以刷新。
com.aspsine.swipetoloadlayout.SwipeToLoadLayout#onCheckCanRefresh
com.aspsine.swipetoloadlayout.SwipeToLoadLayout#canChildScrollUp 始终返回 true
com.aspsine.swipetoloadlayout.SwipeToLoadLayout2#canChildScrollUp 重写的
com.aspsine.swipetoloadlayout.SwipeToLoadLayout2#canViewScrollUp
发现在计算 recyclerView 时
//⑥
android.support.v4.view.ViewCompat#canScrollVertically(view,-1) 返回 true
android.view.View#canScrollVertically
/**
* Check if this view can be scrolled vertically in a certain direction.
*
* @param direction Negative to check scrolling up, positive to check scrolling down.
* @return true if this view can be scrolled in the specified direction, false otherwise.
*/
public boolean canScrollVertically(int direction) {
//④
final int offset = computeVerticalScrollOffset();
final int range = computeVerticalScrollRange() - computeVerticalScrollExtent();
if (range == 0) return false;
if (direction < 0) {
//⑤
return offset > 0;
} else {
return offset < range - 1;
}
}
看 recyclerView 是如何计算 computeVerticalScrollOffset() 的
android.support.v7.widget.RecyclerView#computeVerticalScrollOffset
转给 LayoutManager
android.support.v7.widget.LinearLayoutManager#computeVerticalScrollOffset
android.support.v7.widget.LinearLayoutManager#computeScrollOffset
private int computeScrollOffset(RecyclerView.State state) {
if (getChildCount() == 0) {
return 0;
}
ensureLayoutState();
return ScrollbarHelper.computeScrollOffset(state, mOrientationHelper,
findFirstVisibleChildClosestToStart(!mSmoothScrollbarEnabled, true),
findFirstVisibleChildClosestToEnd(!mSmoothScrollbarEnabled, true),
this, mSmoothScrollbarEnabled, mShouldReverseLayout);
}
计算第一个可见 child
android.support.v7.widget.LinearLayoutManager#findFirstVisibleChildClosestToStart
android.support.v7.widget.LinearLayoutManager#findOneVisibleChild
View findOneVisibleChild(int fromIndex, int toIndex, boolean completelyVisible,
boolean acceptPartiallyVisible) {
ensureLayoutState();
final int start = mOrientationHelper.getStartAfterPadding();
final int end = mOrientationHelper.getEndAfterPadding();
final int next = toIndex > fromIndex ? 1 : -1;
View partiallyVisible = null;
for (int i = fromIndex; i != toIndex; i+=next) {
final View child = getChildAt(i);
final int childStart = mOrientationHelper.getDecoratedStart(child);
// ①
final int childEnd = mOrientationHelper.getDecoratedEnd(child);
if (childStart < end && childEnd > start) {
if (completelyVisible) {
if (childStart >= start && childEnd <= end) {
return child;
} else if (acceptPartiallyVisible && partiallyVisible == null) {
partiallyVisible = child;
}
} else {
return child;
}
}
}
return partiallyVisible;
}
android.support.v7.widget.OrientationHelper#getDecoratedEnd
android.support.v7.widget.ScrollbarHelper#computeScrollOffset
static int computeScrollOffset(RecyclerView.State state, OrientationHelper orientation,
View startChild, View endChild, RecyclerView.LayoutManager lm,
boolean smoothScrollbarEnabled, boolean reverseLayout) {
if (lm.getChildCount() == 0 || state.getItemCount() == 0 || startChild == null ||
endChild == null) {
return 0;
}
//②
final int minPosition = Math.min(lm.getPosition(startChild),
lm.getPosition(endChild));
final int maxPosition = Math.max(lm.getPosition(startChild),
lm.getPosition(endChild));
//③
final int itemsBefore = reverseLayout
? Math.max(0, state.getItemCount() - maxPosition - 1)
: Math.max(0, minPosition);
if (!smoothScrollbarEnabled) {
return itemsBefore;
}
final int laidOutArea = Math.abs(orientation.getDecoratedEnd(endChild) -
orientation.getDecoratedStart(startChild));
final int itemRange = Math.abs(lm.getPosition(startChild) -
lm.getPosition(endChild)) + 1;
final float avgSizePerRow = (float) laidOutArea / itemRange;
return Math.round(itemsBefore * avgSizePerRow + (orientation.getStartAfterPadding()
- orientation.getDecoratedStart(startChild)));
}
设置了 decoration ，使得①中第0个隐藏的 headerView 可见，②③得出的 itemsBefore 为0，返回到④为0，⑤返回 false，到⑥返回 false 可以刷新。
示设置 decoration , 使得①中第0个隐藏的 headerView 不可见，②③得出的 itemsBefore 为1，返回到④有值，⑤返回 true，到⑥返回 false 不可以刷新。
```
到此处，就和之前研初过的 item 设置为 GONE 时仍占用高度的问题连起来了。
要想让 item 设为 GONE，应该在外面包裹一层 ViewGroup，然后手动设置为 GONE
但设置为 GONE，又会导致计算 itemsBefore 时，认为前面还有一个item。
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