[ios] MGLAnnotationView annotation property made writable (#8139)

platform/ios/src/MGLAnnotationView.h

@@ -89,7 +89,7 @@ typedef NS_ENUM(NSUInteger, MGLAnnotationViewDragState) {
  contains a non-`nil` value while the annotation view is visible on the map. If
  the view is queued, waiting to be reused, the value is `nil`.
  */
-@property (nonatomic, readonly, nullable) id <MGLAnnotation> annotation;
+@property (nonatomic, nullable) id <MGLAnnotation> annotation;
 
 /**
  The string that identifies that this annotation view is reusable.

platform/ios/src/MGLAnnotationView.mm

@@ -10,7 +10,6 @@
 @interface MGLAnnotationView () <UIGestureRecognizerDelegate>
 
 @property (nonatomic, readwrite, nullable) NSString *reuseIdentifier;
-@property (nonatomic, readwrite, nullable) id <MGLAnnotation> annotation;
 @property (nonatomic, readwrite) CATransform3D lastAppliedScaleTransform;
 @property (nonatomic, weak) UIPanGestureRecognizer *panGestureRecognizer;
 @property (nonatomic, weak) UILongPressGestureRecognizer *longPressRecognizer;
@@ -240,6 +239,7 @@
     }
     else if (dragState == MGLAnnotationViewDragStateCanceling)
     {
+        NSAssert(self.annotation, @"Annotation property should not be nil.");
         self.panGestureRecognizer.enabled = NO;
         self.longPressRecognizer.enabled = NO;
         self.center = [self.mapView convertCoordinate:self.annotation.coordinate toPointToView:self.mapView];

platform/ios/src/MGLAnnotationView_Private.h

@@ -8,7 +8,6 @@ NS_ASSUME_NONNULL_BEGIN
 @interface MGLAnnotationView (Private)
 
 @property (nonatomic, readwrite, nullable) NSString *reuseIdentifier;
-@property (nonatomic, readwrite, nullable) id <MGLAnnotation> annotation;
 @property (nonatomic, weak) MGLMapView *mapView;
 
 @end

platform/ios/src/MGLMapView.mm

@@ -1516,6 +1516,7 @@ public:
     {
         if (view.centerOffset.dx != 0 || view.centerOffset.dy != 0) {
             if (CGRectContainsPoint(view.frame, tapPoint)) {
+                NSAssert(view.annotation, @"Annotation's view annotation property should not be nil.");
                 CGPoint annotationPoint = [self convertCoordinate:view.annotation.coordinate toPointToView:self];
                 tapPoint = annotationPoint;
             }