Improve scrolling cache

Scrolling cache helps make short scrolls/flings smooth but will
cause stutter when long flings are made. This patch disables
scrolling cache when long flings are made.

This patch also fixes a related bug where scrolling cache will
not be enabled properly when transitioning from flinging to scrolling.

Patch Set 2: Calculate threshold based on maximum velocity (Sang Tae Park)

Change-Id: I1fdbaad92263c9f983a32900e74eba1f0367902b

core/java/android/widget/AbsListView.java

@@ -581,6 +581,7 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
     Runnable mPositionScrollAfterLayout;
     private int mMinimumVelocity;
     private int mMaximumVelocity;
+    private int mDecacheThreshold;
     private float mVelocityScale = 1.0f;
 
     final boolean[] mIsScrap = new boolean[1];
@@ -810,6 +811,7 @@ private void initAbsListView() {
         mTouchSlop = configuration.getScaledTouchSlop();
         mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
         mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
+        mDecacheThreshold = mMaximumVelocity / 2;
         mOverscrollDistance = configuration.getScaledOverscrollDistance();
         mOverflingDistance = configuration.getScaledOverflingDistance();
 
@@ -4027,7 +4029,7 @@ public void run() {
                     // Keep the fling alive a little longer
                     postDelayed(this, FLYWHEEL_TIMEOUT);
                 } else {
-                    endFling();
+                    endFling(false); // Don't disable the scrolling cache right after it was enabled
                     mTouchMode = TOUCH_MODE_SCROLL;
                     reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL);
                 }
@@ -4041,6 +4043,11 @@ public void run() {
         }
 
         void start(int initialVelocity) {
+            if (Math.abs(initialVelocity) > mDecacheThreshold) {
+                // For long flings, scrolling cache causes stutter, so don't use it
+                clearScrollingCache();
+            }
+
             int initialY = initialVelocity < 0 ? Integer.MAX_VALUE : 0;
             mLastFlingY = initialY;
             mScroller.setInterpolator(null);
@@ -4113,13 +4120,18 @@ void startScroll(int distance, int duration, boolean linear) {
         }
 
         void endFling() {
+            endFling(true);
+        }
+
+        void endFling(boolean clearCache) {
             mTouchMode = TOUCH_MODE_REST;
 
             removeCallbacks(this);
             removeCallbacks(mCheckFlywheel);
 
             reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
-            clearScrollingCache();
+            if (clearCache)
+                clearScrollingCache();
             mScroller.abortAnimation();
 
             if (mFlingStrictSpan != null) {