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Fix unexpected ScrollView fling behavior due to Android P bug workaround #34233

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Fix unexpected ScrollView fling behavior due to Android P bug workaround #34233

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33 changes: 21 additions & 12 deletions ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,7 @@
import android.graphics.Rect;
import android.graphics.drawable.ColorDrawable;
import android.graphics.drawable.Drawable;
import android.os.Build;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.View;
Expand Down Expand Up @@ -479,18 +480,7 @@ public boolean getChildVisibleRect(View child, Rect r, android.graphics.Point of

@Override
public void fling(int velocityY) {
// Workaround.
// On Android P if a ScrollView is inverted, we will get a wrong sign for
// velocityY (see https://issuetracker.google.com/issues/112385925).
// At the same time, mOnScrollDispatchHelper tracks the correct velocity direction.
//
// Hence, we can use the absolute value from whatever the OS gives
// us and use the sign of what mOnScrollDispatchHelper has tracked.
float signum = Math.signum(mOnScrollDispatchHelper.getYFlingVelocity());
if (signum == 0) {
signum = Math.signum(velocityY);
}
final int correctedVelocityY = (int) (Math.abs(velocityY) * signum);
final int correctedVelocityY = correctFlingVelocityY(velocityY);

if (mPagingEnabled) {
flingAndSnap(correctedVelocityY);
Expand Down Expand Up @@ -529,6 +519,25 @@ public void fling(int velocityY) {
handlePostTouchScrolling(0, correctedVelocityY);
}

private int correctFlingVelocityY(int velocityY) {
if (Build.VERSION.SDK_INT != Build.VERSION_CODES.P) {
return velocityY;
}

// Workaround.
// On Android P if a ScrollView is inverted, we will get a wrong sign for
// velocityY (see https://issuetracker.google.com/issues/112385925).
// At the same time, mOnScrollDispatchHelper tracks the correct velocity direction.
//
// Hence, we can use the absolute value from whatever the OS gives
// us and use the sign of what mOnScrollDispatchHelper has tracked.
float signum = Math.signum(mOnScrollDispatchHelper.getYFlingVelocity());
if (signum == 0) {
signum = Math.signum(velocityY);
}
return (int) (Math.abs(velocityY) * signum);
}

private void enableFpsListener() {
if (isScrollPerfLoggingEnabled()) {
Assertions.assertNotNull(mFpsListener);
Expand Down