[ios] fixes #5406 view for annotation (#5413)

platform/ios/src/MGLMapView.h

@@ -951,6 +951,15 @@ IB_DESIGNABLE
  */
 - (void)removeAnnotations:(NS_ARRAY_OF(id <MGLAnnotation>) *)annotations;
 
+/**
+ Returns an `MGLAnnotationView` if the given annotation is currently associated
+ with a view, otherwise nil.
+ 
+ @param annotation The annotation associated with the view. 
+    Annotation must conform to the `MGLAnnotation` protocol.
+ */
+- (nullable MGLAnnotationView *)viewForAnnotation:(id <MGLAnnotation>)annotation;
+
 /**
  Returns a reusable annotation image object associated with its identifier.
  

platform/ios/src/MGLMapView.mm

@@ -2984,6 +2984,15 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
     return annotationView;
 }
 
+- (nullable MGLAnnotationView *)viewForAnnotation:(id<MGLAnnotation>)annotation
+{
+    MGLAnnotationTag annotationTag = [self annotationTagForAnnotation:annotation];
+    if (annotationTag == MGLAnnotationTagNotFound) return nil;
+    
+    MGLAnnotationContext &annotationContext = _annotationContextsByAnnotationTag.at(annotationTag);
+    return annotationContext.annotationView;
+}
+
 - (double)alphaForShapeAnnotation:(MGLShape *)annotation
 {
     if (_delegateHasAlphasForShapeAnnotations)