[ios] Fix up center coordinate after lopsided pinch gesture

The gesture recognizer only reports the gesture’s current center point, so use the previous center point to anchor the transition.

Fixes #4315.

1 files changed, 18 insertions, 0 deletions

diff --git a/platform/ios/src/MGLMapView.mm b/platform/ios/src/MGLMapView.mm
index adfb635312..1d7a5090d5 100644
--- a/platform/ios/src/MGLMapView.mm
+++ b/platform/ios/src/MGLMapView.mm
@@ -211,6 +211,10 @@ public:
     
     NSUInteger _changeDelimiterSuppressionDepth;
     
+    /// Center coordinate of the pinch gesture on the previous iteration of the gesture.
+    CLLocationCoordinate2D _previousPinchCenterCoordinate;
+    NSUInteger _previousPinchNumberOfTouches;
+    
     BOOL _delegateHasAlphasForShapeAnnotations;
     BOOL _delegateHasStrokeColorsForShapeAnnotations;
     BOOL _delegateHasFillColorsForShapeAnnotations;
@@ -1133,6 +1137,17 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
         if (log2(newScale) < _mbglMap->getMinZoom()) return;
         
         _mbglMap->setScale(newScale, { centerPoint.x, centerPoint.y });
+        
+        // The gesture recognizer only reports the gesture’s current center
+        // point, so use the previous center point to anchor the transition.
+        // If the number of touches has changed, the remembered center point is
+        // meaningless.
+        if (self.userTrackingMode == MGLUserTrackingModeNone && pinch.numberOfTouches == _previousPinchNumberOfTouches)
+        {
+            CLLocationCoordinate2D centerCoordinate = _previousPinchCenterCoordinate;
+            _mbglMap->setLatLng(MGLLatLngFromLocationCoordinate2D(centerCoordinate),
+                                { centerPoint.x, centerPoint.y });
+        }
 
         [self notifyMapChange:mbgl::MapChangeRegionIsChanging];
     }
@@ -1176,6 +1191,9 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
 
         [self unrotateIfNeededForGesture];
     }
+    
+    _previousPinchCenterCoordinate = [self convertPoint:[pinch locationInView:pinch.view] toCoordinateFromView:self];
+    _previousPinchNumberOfTouches = pinch.numberOfTouches;
 }
 
 - (void)handleRotateGesture:(UIRotationGestureRecognizer *)rotate