AbsListView: 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: Iad52a35120212c871ffd35df6184aeb678ee44aa
Signed-off-by: Alex Naidis <alex.naidis@linux.com>

1 files changed, 14 insertions, 2 deletions

diff --git a/core/java/android/widget/AbsListView.java b/core/java/android/widget/AbsListView.java
index 3fd0f037f3b0..3ebe2eca3278 100755
--- a/core/java/android/widget/AbsListView.java
+++ b/core/java/android/widget/AbsListView.java
@@ -666,6 +666,7 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
     private int mMinimumVelocity;
     @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P, trackingBug = 124051740)
     private int mMaximumVelocity;
+    private int mDecacheThreshold;
     private float mVelocityScale = 1.0f;
 
     final boolean[] mIsScrap = new boolean[1];
@@ -957,6 +958,7 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
         }
         mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
         mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
+        mDecacheThreshold = mMaximumVelocity / 2;
         mOverscrollDistance = configuration.getScaledOverscrollDistance();
         mOverflingDistance = configuration.getScaledOverflingDistance();
 
@@ -4811,7 +4813,7 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
                     // 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);
                 }
@@ -4827,6 +4829,11 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
         // Use AbsListView#fling(int) instead
         @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P)
         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);
@@ -4907,6 +4914,10 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
         // To interrupt a fling early you should use smoothScrollBy(0,0) instead
         @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P)
         void endFling() {
+            endFling(true);
+        }
+
+        void endFling(boolean clearCache) {
             mTouchMode = TOUCH_MODE_REST;
 
             removeCallbacks(this);
@@ -4915,7 +4926,8 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
             if (!mSuppressIdleStateChangeCall) {
                 reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
             }
-            clearScrollingCache();
+            if (clearCache)
+                clearScrollingCache();
             mScroller.abortAnimation();
 
             if (mFlingStrictSpan != null) {