[ios] Support for a customizable zoom gesture centering strategy.

platform/ios/src/MGLMapView.h

@@ -762,6 +762,25 @@ IB_DESIGNABLE
  */
 - (MGLMapCamera *)cameraThatFitsCoordinateBounds:(MGLCoordinateBounds)bounds edgePadding:(UIEdgeInsets)insets;
 
+/**
+ Returns the point in this view's coordinate system on which to "anchor" in
+ response to a user-initiated gesture.
+
+ For example, a pinch-to-zoom gesture would anchor the map at the midpoint of
+ the pinch.
+
+ If the `userTrackingMode` property is not `MGLUserTrackingModeNone`, the
+ user annotation is used as the anchor point.
+
+ Subclasses may override this method to provide specialized behavior - for
+ example, anchoring on the map's center point to provide a "locked" zooming
+ mode.
+
+ @param gesture An anchorable user gesture.
+ @return The point on which to anchor in response to the gesture.
+ */
+- (CGPoint)anchorPointForGesture:(UIGestureRecognizer *)gesture;
+
 /**
  The distance from the edges of the map view’s frame to the edges of the map
  view’s logical viewport.

platform/ios/src/MGLMapView.mm

@@ -1235,11 +1235,7 @@ - (void)handlePinchGesture:(UIPinchGestureRecognizer *)pinch
 
  _mbglMap->cancelTransitions();
 
- CGPoint centerPoint = [pinch locationInView:pinch.view];
- if (self.userTrackingMode != MGLUserTrackingModeNone)
- {
- centerPoint = self.userLocationAnnotationViewCenter;
- }
+ CGPoint centerPoint = [self anchorPointForGesture:pinch];
 
  if (pinch.state == UIGestureRecognizerStateBegan)
  {
@@ -1311,7 +1307,7 @@ - (void)handlePinchGesture:(UIPinchGestureRecognizer *)pinch
  [self unrotateIfNeededForGesture];
  }
 
- _previousPinchCenterCoordinate = [self convertPoint:[pinch locationInView:pinch.view] toCoordinateFromView:self];
+ _previousPinchCenterCoordinate = [self convertPoint:centerPoint toCoordinateFromView:self];
  _previousPinchNumberOfTouches = pinch.numberOfTouches;
 }
 
@@ -1321,11 +1317,7 @@ - (void)handleRotateGesture:(UIRotationGestureRecognizer *)rotate
 
  _mbglMap->cancelTransitions();
 
- CGPoint centerPoint = [rotate locationInView:rotate.view];
- if (self.userTrackingMode != MGLUserTrackingModeNone)
- {
- centerPoint = self.userLocationAnnotationViewCenter;
- }
+ CGPoint centerPoint = [self anchorPointForGesture:rotate];
 
  if (rotate.state == UIGestureRecognizerStateBegan)
  {
@@ -1454,11 +1446,7 @@ - (void)handleDoubleTapGesture:(UITapGestureRecognizer *)doubleTap
  if (doubleTap.state == UIGestureRecognizerStateEnded)
  {
  [self trackGestureEvent:MGLEventGestureDoubleTap forRecognizer:doubleTap];
- CGPoint gesturePoint = [doubleTap locationInView:doubleTap.view];
- if (self.userTrackingMode != MGLUserTrackingModeNone)
- {
- gesturePoint = self.userLocationAnnotationViewCenter;
- }
+ CGPoint gesturePoint = [self anchorPointForGesture:doubleTap];
 
  mbgl::ScreenCoordinate center(gesturePoint.x, gesturePoint.y);
  _mbglMap->scaleBy(2, center, MGLDurationInSeconds(MGLAnimationDuration));
@@ -1486,11 +1474,7 @@ - (void)handleTwoFingerTapGesture:(UITapGestureRecognizer *)twoFingerTap
  }
  else if (twoFingerTap.state == UIGestureRecognizerStateEnded)
  {
- CGPoint gesturePoint = [twoFingerTap locationInView:twoFingerTap.view];
- if (self.userTrackingMode != MGLUserTrackingModeNone)
- {
- gesturePoint = self.userLocationAnnotationViewCenter;
- }
+ CGPoint gesturePoint = [self anchorPointForGesture:twoFingerTap];
 
  mbgl::ScreenCoordinate center(gesturePoint.x, gesturePoint.y);
  _mbglMap->scaleBy(0.5, center, MGLDurationInSeconds(MGLAnimationDuration));
@@ -1528,11 +1512,8 @@ - (void)handleQuickZoomGesture:(UILongPressGestureRecognizer *)quickZoom
 
  if (newZoom < _mbglMap->getMinZoom()) return;
 
- CGPoint centerPoint = self.contentCenter;
- if (self.userTrackingMode != MGLUserTrackingModeNone)
- {
- centerPoint = self.userLocationAnnotationViewCenter;
- }
+ CGPoint centerPoint = [self anchorPointForGesture:quickZoom];
+
  _mbglMap->scaleBy(powf(2, newZoom) / _mbglMap->getScale(),
  mbgl::ScreenCoordinate { centerPoint.x, centerPoint.y });
 
@@ -1564,11 +1545,8 @@ - (void)handleTwoFingerDragGesture:(UIPanGestureRecognizer *)twoFingerDrag
 
  CGFloat pitchNew = currentPitch - (gestureDistance / slowdown);
 
- CGPoint centerPoint = self.contentCenter;
- if (self.userTrackingMode != MGLUserTrackingModeNone)
- {
- centerPoint = self.userLocationAnnotationViewCenter;
- }
+ CGPoint centerPoint = [self anchorPointForGesture:twoFingerDrag];
+
  _mbglMap->setPitch(pitchNew, mbgl::ScreenCoordinate { centerPoint.x, centerPoint.y });
 
  [self notifyMapChange:mbgl::MapChangeRegionIsChanging];
@@ -1578,7 +1556,21 @@ - (void)handleTwoFingerDragGesture:(UIPanGestureRecognizer *)twoFingerDrag
  [self notifyGestureDidEndWithDrift:NO];
  [self unrotateIfNeededForGesture];
  }
+}
 
+- (CGPoint)anchorPointForGesture:(UIGestureRecognizer *)gesture {
+ if (self.userTrackingMode != MGLUserTrackingModeNone)
+ {
+ return self.userLocationAnnotationViewCenter;
+ }
+
+ // Special case for two-finger drag and quickzoom
+ if ([gesture isKindOfClass:[UIPanGestureRecognizer class]] || [gesture isKindOfClass:[UILongPressGestureRecognizer class]])
+ {
+ return self.contentCenter;
+ }
+
+ return [gesture locationInView:gesture.view];
 }
 
 - (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer