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Fix bug with calculating Y offset in RecyclerViewBackedScrollView.

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Summary:
This change fixes a bug in getTopOffsetForItem method of RecyclerViewBackedScrollView that was causing Y offset of onScroll events to be invalid when user would scroll up. I'm also adding some comments to this method which may help to understand its complexity.
Closes https://github.com/facebook/react-native/pull/5610

Reviewed By: svcscm

Differential Revision: D2880743

Pulled By: androidtrunkagent

fb-gh-sync-id: 7183e3d6760fab5683afc49d454864239260fb91
This commit is contained in:
Krzysztof Magiera
2016-01-29 13:25:43 -08:00
committed by facebook-github-bot-4
parent 063e8b9ae5
commit 041fb59146
1 changed files with 39 additions and 7 deletions
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46
ReactAndroid/src/main/java/com/facebook/react/views/recyclerview/RecyclerViewBackedScrollView.java
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@@ -114,25 +114,57 @@ public class RecyclerViewBackedScrollView extends RecyclerView {
}
public int getTopOffsetForItem(int index) {
// This method is frequently called from the "onScroll" handler of the "RecyclerView" with an
// index of first visible item of the view. Implementation of this method takes advantage of
// that fact by caching the value for the last index that this method has been called with.
//
// There are a 2 cases that we optimize for:
// 1) The visible item doesn't change between subsequent "onScroll" calls, in that case we
// don't need to calculate anything, just return the cached value
// 2) The next visible item will be the one that is adjacent to the item that we store the
// cached value for: index + 1 when scrolling down or index - 1 when scrolling up. Then it
// is sufficient to add/subtract height of item at the "last index"
//
// The implementation accounts for the cases when next index is not necessarily a subsequent
// number of the cached one. In which case we try to minimize the number of rows we will loop
// through.
if (mLastRequestedPosition != index) {
int sum = 0;
int startIndex = 0;
if (mLastRequestedPosition != -1) {
sum = mOffsetForLastPosition;
startIndex = mLastRequestedPosition;
}
int sum;
if (mLastRequestedPosition < index) {
// This can either happen when we're scrolling down or if the cached value has never been
// calculated
int startIndex;
if (mLastRequestedPosition != -1) {
// We already have the value cached, let's use it and only add heights of the items
// starting at the index we have the cached value for
sum = mOffsetForLastPosition;
startIndex = mLastRequestedPosition;
} else {
sum = 0;
startIndex = 0;
}
for (int i = startIndex; i < index; i++) {
sum += mReactListAdapter.mViews.get(i).getMeasuredHeight();
}
} else {
// We are scrolling up, we can either use cached value and subtract heights of rows
// between mLastRequestPosition and index, or we can calculate the height starting from 0
// (this can be quite a frequent case as well, when the list implements "jump to the top"
// action). We just go for the option that require less calculations
if (index < (mLastRequestedPosition - index)) {
// index is relatively small, it's faster to calculate the sum starting from 0
sum = 0;
for (int i = 0; i < index; i++) {
sum += mReactListAdapter.mViews.get(i).getMeasuredHeight();
}
} else {
// index is "closer" to the last cached index than it is to 0. We can reuse cached sum
// and calculate the new sum by subtracting heights of the elements between
// "mLastRequestPosition" and "index"
sum = mOffsetForLastPosition;
for (int i = mLastRequestedPosition - 1; i >= index; i--) {
sum -= mReactListAdapter.mViews.get(i).getMeasuredHeight();
}