[ios] Added centerOffset to MGLAnnotationView (#5062)

* [ios] Added centerOffset to MGLAnnotationView

* [ios] Reposition immediately after changing the centerOffset

closes #5059

platform/ios/CHANGELOG.md

@@ -26,6 +26,7 @@ Mapbox welcomes participation and contributions from everyone.  Please read [CON
 - Removed unused SVG files from the SDK’s resource bundle. ([#4641](https://github.com/mapbox/mapbox-gl-native/pull/4641))
 - Deprecated `-[MGLMapView emptyMemoryCache]`. ([#4725](https://github.com/mapbox/mapbox-gl-native/pull/4725))
 - Added `MGLCoordinateInCoordinateBounds()`, a function that tests whether or not a coordinate is in a given bounds. ([#5053](https://github.com/mapbox/mapbox-gl-native/pull/5053))
+- An MGLAnnotationView can be repositioned in relation to the associated MGLAnnotation.coordinate by changing its `centerOffset` property. ([#5059](https://github.com/mapbox/mapbox-gl-native/issues/5059))
 
 ## 3.2.2
 

platform/ios/Mapbox.playground/Contents.swift

@@ -52,6 +52,7 @@ class MapDelegate: NSObject, MGLMapViewDelegate {
         if (annotationView == nil) {
             let av = PlaygroundAnnotationView(reuseIdentifier: "annotation")
             av.frame = CGRect(x: 0, y: 0, width: 30, height: 30)
+            av.centerOffset = CGVector(dx: -15, dy: -15)
             let centerView = UIView(frame: CGRectInset(av.bounds, 3, 3))
             centerView.backgroundColor = UIColor.whiteColor()
             av.addSubview(centerView)

platform/ios/src/MGLAnnotationView.h

@@ -26,6 +26,14 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property (nonatomic, readonly, nullable) NSString *reuseIdentifier;
 
+/**
+ Annotation view is centered at the coordinate point of the associated annotation.
+ 
+ By changing this property you can reposition the view as needed. The offset is measured in points.
+ Positive offset moves the annotation view towards the bottom right, while negative offset moves it towards the top left.
+ */
+@property (nonatomic) CGVector centerOffset;
+
 
 /**
  Called when the view is removed from the reuse queue.

platform/ios/src/MGLAnnotationView.m

@@ -27,6 +27,19 @@
     // Intentionally left blank. The default implementation of this method does nothing.
 }
 
+- (void)setCenterOffset:(CGVector)centerOffset
+{
+    _centerOffset = centerOffset;
+    self.center = self.center;
+}
+
+- (void)setCenter:(CGPoint)center
+{
+    center.x += _centerOffset.dx;
+    center.y += _centerOffset.dy;
+    [super setCenter:center];
+}
+
 - (id<CAAction>)actionForLayer:(CALayer *)layer forKey:(NSString *)event
 {
     // Allow mbgl to drive animation of this view’s bounds.