Bug 1589046 - Propagate the return value from AttemptScroll to DispatchScroll and CallDispatchScroll. r=botond

No functional changes here.

Differential Revision: https://phabricator.services.mozilla.com/D69623

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -2623,7 +2623,7 @@ static bool TransformDisplacement(APZCTreeManager* aTreeManager,
   return true;
 }
 
-void APZCTreeManager::DispatchScroll(
+bool APZCTreeManager::DispatchScroll(
     AsyncPanZoomController* aPrev, ParentLayerPoint& aStartPoint,
     ParentLayerPoint& aEndPoint,
     OverscrollHandoffState& aOverscrollHandoffState) {
@@ -2635,19 +2635,19 @@ void APZCTreeManager::DispatchScroll(
   // nothing more to scroll, so we ignore the rest of the pan gesture.
   if (overscrollHandoffChainIndex >= overscrollHandoffChain.Length()) {
     // Nothing more to scroll - ignore the rest of the pan gesture.
-    return;
+    return false;
   }
 
   next = overscrollHandoffChain.GetApzcAtIndex(overscrollHandoffChainIndex);
 
   if (next == nullptr || next->IsDestroyed()) {
-    return;
+    return false;
   }
 
   // Convert the start and end points from |aPrev|'s coordinate space to
   // |next|'s coordinate space.
   if (!TransformDisplacement(this, aPrev, next, aStartPoint, aEndPoint)) {
-    return;
+    return false;
   }
 
   // Scroll |next|. If this causes overscroll, it will call DispatchScroll()
@@ -2662,7 +2662,11 @@ void APZCTreeManager::DispatchScroll(
     if (!TransformDisplacement(this, next, aPrev, aStartPoint, aEndPoint)) {
       NS_WARNING("Failed to untransform scroll points during dispatch");
     }
+    return false;
   }
+
+  // Return true to indicate the scroll was consumed entirely.
+  return true;
 }
 
 ParentLayerPoint APZCTreeManager::DispatchFling(

gfx/layers/apz/src/APZCTreeManager.h

@@ -328,10 +328,10 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
    * overscroll to the next APZC. Note that because of scroll grabbing, the
    * first APZC to scroll may not be the one that is receiving the touch events.
    *
-   * |aAPZC| is the APZC that received the touch events triggering the scroll
+   * |aPrev| is the APZC that received the touch events triggering the scroll
    *   (in the case of an initial scroll), or the last APZC to scroll (in the
    *   case of overscroll)
-   * |aStartPoint| and |aEndPoint| are in |aAPZC|'s transformed screen
+   * |aStartPoint| and |aEndPoint| are in |aPrev|'s transformed screen
    *   coordinates (i.e. the same coordinates in which touch points are given to
    *   APZCs). The amount of (over)scroll is represented by two points rather
    *   than a displacement because with certain 3D transforms, the same
@@ -348,6 +348,12 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
    * an overscrolled state if no APZC further up in the handoff chain accepted
    * the entire scroll.
    *
+   * The function will return true if the entire scroll was consumed, and
+   * false otherwise. As this function also modifies aStartPoint and aEndPoint,
+   * when scroll is consumed, it should always the case that this function
+   * returns true if and only if IsZero(aStartPoint - aEndPoint), using the
+   * modified aStartPoint and aEndPoint after the function returns.
+   *
    * The way this method works is best illustrated with an example.
    * Consider three nested APZCs, A, B, and C, with C being the innermost one.
    * Say B is scroll-grabbing.
@@ -376,7 +382,7 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
    * Note: this should be used for panning only. For handing off overscroll for
    *       a fling, use DispatchFling().
    */
-  void DispatchScroll(AsyncPanZoomController* aApzc,
+  bool DispatchScroll(AsyncPanZoomController* aPrev,
                       ParentLayerPoint& aStartPoint,
                       ParentLayerPoint& aEndPoint,
                       OverscrollHandoffState& aOverscrollHandoffState);

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -3264,12 +3264,14 @@ bool AsyncPanZoomController::AttemptScroll(
     // Note: "+ overscroll" rather than "- overscroll" because "overscroll"
     // is what's left of "displacement", and "displacement" is "start - end".
     ++aOverscrollHandoffState.mChainIndex;
-    CallDispatchScroll(aStartPoint, aEndPoint, aOverscrollHandoffState);
-
-    overscroll = aStartPoint - aEndPoint;
-    if (IsZero(overscroll)) {
+    bool consumed =
+        CallDispatchScroll(aStartPoint, aEndPoint, aOverscrollHandoffState);
+    if (consumed) {
       return true;
     }
+
+    overscroll = aStartPoint - aEndPoint;
+    MOZ_ASSERT(!IsZero(overscroll));
   }
 
   // If there is no APZC later in the handoff chain that accepted the
@@ -3501,7 +3503,7 @@ void AsyncPanZoomController::StartOverscrollAnimation(
   StartAnimation(new OverscrollAnimation(*this, aVelocity));
 }
 
-void AsyncPanZoomController::CallDispatchScroll(
+bool AsyncPanZoomController::CallDispatchScroll(
     ParentLayerPoint& aStartPoint, ParentLayerPoint& aEndPoint,
     OverscrollHandoffState& aOverscrollHandoffState) {
   // Make a local copy of the tree manager pointer and check if it's not
@@ -3509,7 +3511,7 @@ void AsyncPanZoomController::CallDispatchScroll(
   // Destroy(), which nulls out mTreeManager, could be called concurrently.
   APZCTreeManager* treeManagerLocal = GetApzcTreeManager();
   if (!treeManagerLocal) {
-    return;
+    return false;
   }
 
   // Obey overscroll-behavior.
@@ -3524,12 +3526,12 @@ void AsyncPanZoomController::CallDispatchScroll(
     }
     if (aStartPoint == endPoint) {
       // Handoff not allowed in either direction - don't even bother.
-      return;
+      return false;
     }
   }
 
-  treeManagerLocal->DispatchScroll(this, aStartPoint, endPoint,
-                                   aOverscrollHandoffState);
+  return treeManagerLocal->DispatchScroll(this, aStartPoint, endPoint,
+                                          aOverscrollHandoffState);
 }
 
 void AsyncPanZoomController::RecordScrollPayload(const TimeStamp& aTimeStamp) {

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -1534,7 +1534,7 @@ class AsyncPanZoomController {
    * Guards against the case where the APZC is being concurrently destroyed
    * (and thus mTreeManager is being nulled out).
    */
-  void CallDispatchScroll(ParentLayerPoint& aStartPoint,
+  bool CallDispatchScroll(ParentLayerPoint& aStartPoint,
                           ParentLayerPoint& aEndPoint,
                           OverscrollHandoffState& aOverscrollHandoffState);