[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.

CHANGELOG.md

@@ -72,6 +72,7 @@ Known issues:
 - Tiles and other resources are cached in the same file that holds offline resources. The combined cache file is located in a subdirectory of the user’s Application Support directory, which means iOS will not delete the file when disk space runs low. ([#4377](https://github.com/mapbox/mapbox-gl-native/pull/4377))
 - The user dot no longer disappears after panning the map across the antimeridian at low zoom levels. ([#4275](https://github.com/mapbox/mapbox-gl-native/pull/4275))
 - The map no longer recoils when panning quickly at low zoom levels. ([#4214](https://github.com/mapbox/mapbox-gl-native/pull/4214))
+- Fixed an issue causing the map to pan the wrong way when the user pinches unevenly. ([#4427](https://github.com/mapbox/mapbox-gl-native/pull/4427))
 - An icon laid out along a line no longer appears if it would extend past the end of the line. Some one-way arrows no longer point the wrong way. ([#3839](https://github.com/mapbox/mapbox-gl-native/pull/3839))
 - Fixed warping of dashed lines near sharp corners. ([#3914](https://github.com/mapbox/mapbox-gl-native/pull/3914))
 - Telemetry location gathering now occurs only when the device is in motion. ([#4115](https://github.com/mapbox/mapbox-gl-native/pull/4115))

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