[ios] Introduce visible annotations API

This adds an API for getting all annotations that are currently
visible on the map. In the future, an additional API can be added
to query by a specific rect. That API can be used internally to
power this one.

This also adds a new option to iosapp to test the feature. At the
time of this commit, the underlying `queryPointAnnotations` does
not appear to work as expected.

platform/ios/app/MBXViewController.m

@@ -207,6 +207,7 @@ - (IBAction)showSettings:(__unused id)sender
         ((_customUserLocationAnnnotationEnabled)
          ? @"Disable Custom User Dot"
          : @"Enable Custom User Dot"),
+        @"Query Annotations",
         nil];
 
     if (self.debugLoggingEnabled)
@@ -294,6 +295,10 @@ - (void)actionSheet:(UIActionSheet *)actionSheet didDismissWithButtonIndex:(NSIn
         self.mapView.showsUserLocation = NO;
         self.mapView.userTrackingMode = MGLUserTrackingModeFollow;
     }
+    else if (buttonIndex == actionSheet.firstOtherButtonIndex + 18)
+    {
+        [self testQueryPointAnnotations];
+    }
     else if (buttonIndex == actionSheet.numberOfButtons - 2 && self.debugLoggingEnabled)
     {
         NSString *fileContents = [NSString stringWithContentsOfFile:[self telemetryDebugLogfilePath] encoding:NSUTF8StringEncoding error:nil];
@@ -541,6 +546,20 @@ - (void)styleRoadLayer
     roadLayer.minimumZoomLevel = 13;
 }
 
+- (void)testQueryPointAnnotations {
+    NSNumber *visibleAnnotationCount = @(self.mapView.visibleAnnotations.count);
+    NSString *message;
+    if ([visibleAnnotationCount integerValue] == 1) {
+        message = [NSString stringWithFormat:@"There is %@ visible annotation.", visibleAnnotationCount];
+    } else {
+        message = [NSString stringWithFormat:@"There are %@ visible annotations.", visibleAnnotationCount];
+    }
+
+    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"Visible Annotations" message:message preferredStyle:UIAlertControllerStyleAlert];
+    [alertController addAction:[UIAlertAction actionWithTitle:@"Ok" style:UIAlertActionStyleCancel handler:nil]];
+    [self presentViewController:alertController animated:YES completion:nil];
+}
+
 - (IBAction)handleLongPress:(UILongPressGestureRecognizer *)longPress
 {
     if (longPress.state == UIGestureRecognizerStateBegan)

platform/ios/src/MGLMapView.h

@@ -923,6 +923,16 @@ IB_DESIGNABLE
  */
 @property (nonatomic, readonly, nullable) NS_ARRAY_OF(id <MGLAnnotation>) *annotations;
 
+/**
+ The complete list of annotations associated with the receiver that are 
+ currently visible. (read-only)
+ 
+ The objects in this array must adopt the `MGLAnnotation` protocol. If no
+ annotations are associated with the map view or if no annotations associated
+ with the map view are currently visible, the value of this property is `nil`.
+ */
+@property (nonatomic, readonly, nullable) NS_ARRAY_OF(id <MGLAnnotation>) *visibleAnnotations;
+
 /**
  Adds an annotation to the map view.
  

platform/ios/src/MGLMapView.mm

@@ -2787,6 +2787,30 @@ - (void)removeStyleClass:(NSString *)styleClass
     return [NSArray arrayWithObjects:&annotations[0] count:annotations.size()];
 }
 
+- (nullable NS_ARRAY_OF(id <MGLAnnotation>) *)visibleAnnotations
+{
+    if (_annotationContextsByAnnotationTag.empty())
+    {
+        return nil;
+    }
+
+    std::vector<MGLAnnotationTag> annotationTags = [self annotationTagsInRect:self.bounds];
+    if (annotationTags.size())
+    {
+        NSMutableArray *annotations = [NSMutableArray arrayWithCapacity:annotationTags.size()];
+
+        for (auto const& annotationTag: annotationTags)
+        {
+            MGLAnnotationContext annotationContext = _annotationContextsByAnnotationTag[annotationTag];
+            [annotations addObject:annotationContext.annotation];
+        }
+
+        return [annotations copy];
+    }
+
+    return nil;
+}
+
 /// Returns the annotation assigned the given tag. Cheap.
 - (id <MGLAnnotation>)annotationWithTag:(MGLAnnotationTag)tag
 {