Make it possible for the scroll offset delegate to intercept fling.

android_webview/java/src/org/chromium/android_webview/AwScrollOffsetManager.java

Index: android_webview/java/src/org/chromium/android_webview/AwScrollOffsetManager.java
diff --git a/android_webview/java/src/org/chromium/android_webview/AwScrollOffsetManager.java b/android_webview/java/src/org/chromium/android_webview/AwScrollOffsetManager.java
index 4564b42ff04c4f76e2bf45c42611cd21bae64bb0..ead226c1adf9e134e7ff8496c40c143d1d3de669 100644
--- a/android_webview/java/src/org/chromium/android_webview/AwScrollOffsetManager.java
+++ b/android_webview/java/src/org/chromium/android_webview/AwScrollOffsetManager.java
@@ -4,6 +4,8 @@
 
 package org.chromium.android_webview;
 
+import android.widget.OverScroller;
+
 import org.chromium.base.CalledByNative;
 
 /**
@@ -24,8 +26,11 @@ public class AwScrollOffsetManager {
         // operation, the native side shouldn't synchronously alter the scroll offset from within
         // this call.
         void scrollNativeTo(int x, int y);
+
         int getContainerViewScrollX();
         int getContainerViewScrollY();
+
+        void invalidate();
     }
 
     private final Delegate mDelegate;
@@ -42,14 +47,24 @@ public class AwScrollOffsetManager {
     private int mContainerViewWidth;
     private int mContainerViewHeight;
 
+    // Whether we're in the middle of processing a touch event.
     private boolean mProcessingTouchEvent;
 
+    // Whether (and to what value) to update the native side scroll offset after we've finished
+    // provessing a touch event.
     private boolean mApplyDeferredNativeScroll;
     private int mDeferredNativeScrollX;
     private int mDeferredNativeScrollY;
 
-    public AwScrollOffsetManager(Delegate delegate) {
+    // The velocity of the last recorded fling,
+    private int mLastFlingVx;
+    private int mLastFlingVy;
+
+    private OverScroller mScroller;
+
+    public AwScrollOffsetManager(Delegate delegate, OverScroller overScroller) {
         mDelegate = delegate;
+        mScroller = overScroller;
     }
 
     //----- Scroll range and extent calculation methods -------------------------------------------
@@ -133,7 +148,9 @@ public class AwScrollOffsetManager {
     }
 
     // Called by the native side to over-scroll the container view.
-    public void overscrollBy(int deltaX, int deltaY) {
+    public void overScrollBy(int deltaX, int deltaY) {
+        if (deltaX == 0 && deltaY == 0) return;
+
         final int scrollX = mDelegate.getContainerViewScrollX();
         final int scrollY = mDelegate.getContainerViewScrollY();
         final int scrollRangeX = computeMaximumHorizontalScrollOffset();
@@ -201,4 +218,59 @@ public class AwScrollOffsetManager {
 
         mDelegate.scrollNativeTo(x, y);
     }
+
+    /**
+     * Called at the beginning of every fling gesture.
+     */
+    public void onFlingStartGestureStart(int vx, int vy) {
+        mLastFlingVx = vx;
+        mLastFlingVy = vy;
+    }
+
+    /**
+     * Called when a fling gesture is not handled by the renderer.
+     * We explicitly ask the renderer not to handle fling gestures targeted at the root
+     * scroll layer.
+     */
+    public void onUnhandledFlingStartEvent() {
+        final int scrollX = mDelegate.getContainerViewScrollX();
+        final int scrollY = mDelegate.getContainerViewScrollY();
+        final int rangeX = computeMaximumHorizontalScrollOffset();
+        final int rangeY = computeMaximumVerticalScrollOffset();
+
+        mScroller.fling(scrollX, scrollY, -mLastFlingVx, -mLastFlingVy, 0, rangeX, 0, rangeY);
+        mDelegate.invalidate();
+    }
+
+    /**
+     * Called immediately before the draw to update the scroll offset.
+     */
+    public void computeScrollAndAbsorbGlow(OverScrollGlow overScrollGlow) {
+        if (!mScroller.computeScrollOffset()) {
+            return;
+        }
+
+        final int oldX = mDelegate.getContainerViewScrollX();
+        final int oldY = mDelegate.getContainerViewScrollY();
+        int x = mScroller.getCurrX();
+        int y = mScroller.getCurrY();
+
+        int rangeX = computeMaximumHorizontalScrollOffset();
+        int rangeY = computeMaximumVerticalScrollOffset();
+
+        if (overScrollGlow != null) {
+            overScrollGlow.absorbGlow(x, y, oldX, oldY, rangeX, rangeY,
+                    mScroller.getCurrVelocity());
+        }
+
+        overScrollBy(x - oldX, y - oldY);
+
+        mDelegate.invalidate();
+    }
+
+    public void onTouchDown() {
+        // TODO(mkosiba): Support speeding up a fling by flinging again.
+        // http://crbug.com/265841
+        mScroller.forceFinished(true);
+    }
 }