accelerate consecutive flings

Use the fact that the scroller keeps track of the current velocity
to accelerate subsequent flings. The current velocity is added to
the new velocity if the flings are in the same general direction
and if the fling animation hasn't been aborted.

This makes it easier to move to the top or bottom of very large
pages (either in overview mode or reading mode) by taking
consecutive flings as a cue from the user that they want to
page faster. The user can stop the fling with any action as
before.

fixes http://b/issue?id=2066090

core/java/android/webkit/WebView.java

@@ -342,6 +342,9 @@ public class WebView extends AbsoluteLayout
      */
     VelocityTracker mVelocityTracker;
     private int mMaximumFling;
+    private float mLastVelocity;
+    private float mLastVelX;
+    private float mLastVelY;
 
     // use this flag to control whether enabling the new double tap zoom
     static final boolean ENABLE_DOUBLETAP_ZOOM = true;
@@ -1795,6 +1798,13 @@ public class WebView extends AbsoluteLayout
                 , contentToView(x.right), contentToView(x.bottom));
     }
 
+    // stop the scroll animation, and don't let a subsequent fling add
+    // to the existing velocity
+    private void abortAnimation() {
+        mScroller.abortAnimation();
+        mLastVelocity = 0;
+    }
+
     /* call from webcoreview.draw(), so we're still executing in the UI thread
     */
     private void recordNewContentSize(int w, int h, boolean updateLayout) {
@@ -1819,7 +1829,7 @@ public class WebView extends AbsoluteLayout
                 mScrollY = pinLocY(mScrollY);
                 // android.util.Log.d("skia", "recordNewContentSize -
                 // abortAnimation");
-                mScroller.abortAnimation(); // just in case
+                abortAnimation(); // just in case
                 if (oldX != mScrollX || oldY != mScrollY) {
                     sendOurVisibleRect();
                 }
@@ -2339,7 +2349,7 @@ public class WebView extends AbsoluteLayout
                     animationDuration > 0 ? animationDuration : computeDuration(dx, dy));
             invalidate();
         } else {
-            mScroller.abortAnimation(); // just in case
+            abortAnimation(); // just in case
             scrollTo(x, y);
         }
         return true;
@@ -2963,7 +2973,7 @@ public class WebView extends AbsoluteLayout
         mLastTouchX = halfW;
         int halfH = height >> 1;
         mLastTouchY = halfH;
-        mScroller.abortAnimation();
+        abortAnimation();
         mZoomScrollStart = System.currentTimeMillis();
         Rect zoomFrame = scrollZoomFrame(width, height
                 , scrollZoomMagScale(mZoomScrollInvLimit));
@@ -3850,6 +3860,9 @@ public class WebView extends AbsoluteLayout
                     mLastScrollY = mZoomScrollY;
                     // If two taps are close, ignore the first tap
                 } else if (!mScroller.isFinished()) {
+                    // stop the current scroll animation, but if this is
+                    // the start of a fling, allow it to add to the current
+                    // fling's velocity
                     mScroller.abortAnimation();
                     mTouchMode = TOUCH_DRAG_START_MODE;
                     mPrivateHandler.removeMessages(RESUME_WEBCORE_UPDATE);
@@ -4110,6 +4123,7 @@ public class WebView extends AbsoluteLayout
                             doFling();
                             break;
                         }
+                        mLastVelocity = 0;
                         WebViewCore.resumeUpdate(mWebViewCore);
                         break;
                     case TOUCH_DRAG_START_MODE:
@@ -4475,6 +4489,27 @@ public class WebView extends AbsoluteLayout
             vx = vx * 3 / 4;
             vy = vy * 3 / 4;
         }
+        float currentVelocity = mScroller.getCurrVelocity();
+        if (mLastVelocity > 0 && currentVelocity > 0) {
+            float deltaR = (float) (Math.abs(Math.atan2(mLastVelY, mLastVelX)
+                    - Math.atan2(vy, vx)));
+            final float circle = (float) (Math.PI) * 2.0f;
+            if (deltaR > circle * 0.9f || deltaR < circle * 0.1f) {
+                vx += currentVelocity * mLastVelX / mLastVelocity;
+                vy += currentVelocity * mLastVelY / mLastVelocity;
+                if (DebugFlags.WEB_VIEW) {
+                    Log.v(LOGTAG, "doFling vx= " + vx + " vy=" + vy);
+                }
+            } else if (DebugFlags.WEB_VIEW) {
+                Log.v(LOGTAG, "doFling missed " + deltaR / circle);
+            }
+        } else if (DebugFlags.WEB_VIEW) {
+            Log.v(LOGTAG, "doFling start last=" + mLastVelocity
+                    + " current=" + currentVelocity);
+        }
+        mLastVelX = vx;
+        mLastVelY = vy;
+        mLastVelocity = (float) Math.hypot(vx, vy);
 
         mScroller.fling(mScrollX, mScrollY, -vx, -vy, 0, maxX, 0, maxY);
         // TODO: duration is calculated based on velocity, if the range is
@@ -4683,7 +4718,7 @@ public class WebView extends AbsoluteLayout
         mLastTouchY = y + (float) (mWebTextView.getTop() - mScrollY);
         mLastTouchTime = eventTime;
         if (!mScroller.isFinished()) {
-            mScroller.abortAnimation();
+            abortAnimation();
             mPrivateHandler.removeMessages(RESUME_WEBCORE_UPDATE);
         }
         mSnapScrollMode = SNAP_NONE;

core/java/android/widget/Scroller.java

@@ -132,6 +132,17 @@ public class Scroller  {
         return mCurrY;
     }
     
+    /**
+     * @hide
+     * Returns the current velocity.
+     *
+     * @return The original velocity less the deceleration. Result may be
+     * negative.
+     */
+    public float getCurrVelocity() {
+        return mVelocity - mDeceleration * timePassed() / 2000.0f;
+    }
+
     /**
      * Returns the start X offset in the scroll. 
      *