Track offset of center child view in Gallery for later layouts

When apps request layouts while scrolling is in progress, Gallery
children can get offset in strange ways. Compensate for this by
tracking the last known offset and applying it during layout.

Bug 7245853

Change-Id: I9d746ae6bb06918e2d920c58052f72e749a7ffbf

Author: adamp

core/java/android/widget/Gallery.java

@@ -182,6 +182,12 @@ public void run() {
      */
     private boolean mIsRtl = true;
 
+    /**
+     * Offset between the center of the selected child view and the center of the Gallery.
+     * Used to reset position correctly during layout.
+     */
+    private int mSelectedCenterOffset;
+
     public Gallery(Context context) {
         this(context, null);
     }
@@ -395,6 +401,14 @@ void trackMotionScroll(int deltaX) {
 
         setSelectionToCenterChild();
 
+        final View selChild = mSelectedChild;
+        if (selChild != null) {
+            final int childLeft = selChild.getLeft();
+            final int childCenter = selChild.getWidth() / 2;
+            final int galleryCenter = getWidth() / 2;
+            mSelectedCenterOffset = childLeft + childCenter - galleryCenter;
+        }
+
         onScrollChanged(0, 0, 0, 0); // dummy values, View's implementation does not use these.
 
         invalidate();
@@ -537,6 +551,7 @@ private void onFinishedMovement() {
             // We haven't been callbacking during the fling, so do it now
             super.selectionChanged();
         }
+        mSelectedCenterOffset = 0;
         invalidate();
     }
 
@@ -650,7 +665,8 @@ void layout(int delta, boolean animate) {
         View sel = makeAndAddView(mSelectedPosition, 0, 0, true);
 
         // Put the selected child in the center
-        int selectedOffset = childrenLeft + (childrenWidth / 2) - (sel.getWidth() / 2);
+        int selectedOffset = childrenLeft + (childrenWidth / 2) - (sel.getWidth() / 2) +
+                mSelectedCenterOffset;
         sel.offsetLeftAndRight(selectedOffset);
 
         fillToGalleryRight();