[ios] Made annotations accessible

Lazily create and cache accessibility elements for visible annotations as UIAccessibility asks about them.

Sort the annotations’ accessibility elements by the annotations’ distance from the logical center (the center after accounting for padding, or the user dot when user tracking mode is on).

Updated the changelog.

Fixes #1493.

platform/ios/src/MGLMapView.mm

@@ -129,12 +129,23 @@ mbgl::Color MGLColorObjectFromUIColor(UIColor *color)
     return {{ (float)r, (float)g, (float)b, (float)a }};
 }
 
+@interface MGLAnnotationAccessibilityElement : UIAccessibilityElement
+
+@property (nonatomic) MGLAnnotationTag tag;
+
+@end
+
+@implementation MGLAnnotationAccessibilityElement
+
+@end
+
 /// Lightweight container for metadata about an annotation, including the annotation itself.
 class MGLAnnotationContext {
 public:
     id <MGLAnnotation> annotation;
     /// The annotation’s image’s reuse identifier.
     NSString *imageReuseIdentifier;
+    MGLAnnotationAccessibilityElement *accessibilityElement;
 };
 
 #pragma mark - Private -
@@ -1789,6 +1800,105 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
     return frame;
 }
 
+- (NSInteger)accessibilityElementCount
+{
+    std::vector<MGLAnnotationTag> visibleAnnotations = [self annotationTagsInRect:self.bounds];
+    return visibleAnnotations.size() + 2 /* compass, attributionButton */;
+}
+
+- (id)accessibilityElementAtIndex:(NSInteger)index
+{
+    std::vector<MGLAnnotationTag> visibleAnnotations = [self annotationTagsInRect:self.bounds];
+    
+    // Ornaments
+    if (index == 0)
+    {
+        return self.compassView;
+    }
+    if (index > 0 && (NSUInteger)index == visibleAnnotations.size() + 1 /* compass */)
+    {
+        return self.attributionButton;
+    }
+    
+    std::sort(visibleAnnotations.begin(), visibleAnnotations.end());
+    CGPoint centerPoint = self.contentCenter;
+    if (self.userTrackingMode != MGLUserTrackingModeNone)
+    {
+        centerPoint = self.userLocationAnnotationViewCenter;
+    }
+    CLLocationCoordinate2D currentCoordinate = [self convertPoint:centerPoint toCoordinateFromView:self];
+    std::sort(visibleAnnotations.begin(), visibleAnnotations.end(), [&](const MGLAnnotationTag tagA, const MGLAnnotationTag tagB) {
+        CLLocationCoordinate2D coordinateA = [[self annotationWithTag:tagA] coordinate];
+        CLLocationCoordinate2D coordinateB = [[self annotationWithTag:tagB] coordinate];
+        CLLocationDegrees deltaA = hypot(coordinateA.latitude - currentCoordinate.latitude,
+                                         coordinateA.longitude - currentCoordinate.longitude);
+        CLLocationDegrees deltaB = hypot(coordinateB.latitude - currentCoordinate.latitude,
+                                         coordinateB.longitude - currentCoordinate.longitude);
+        return deltaA < deltaB;
+    });
+    
+    NSUInteger annotationIndex = MGLAnnotationTagNotFound;
+    if (index >= 0 && (NSUInteger)index < visibleAnnotations.size())
+    {
+        annotationIndex = index - 1 /* compass */;
+    }
+    MGLAnnotationTag annotationTag = visibleAnnotations[annotationIndex];
+    NSAssert(annotationTag != MGLAnnotationTagNotFound, @"Can’t get accessibility element for nonexistent or invisible annotation at index %li.", index);
+    NSAssert(_annotationContextsByAnnotationTag.count(annotationTag), @"Missing annotation for tag %u.", annotationTag);
+    MGLAnnotationContext &annotationContext = _annotationContextsByAnnotationTag.at(annotationTag);
+    id <MGLAnnotation> annotation = annotationContext.annotation;
+    MGLAnnotationAccessibilityElement *element = annotationContext.accessibilityElement;
+    
+    // Lazily create an accessibility element for the found annotation.
+    if ( ! element)
+    {
+        element = [[MGLAnnotationAccessibilityElement alloc] initWithAccessibilityContainer:self];
+        element.tag = annotationTag;
+        element.accessibilityTraits = UIAccessibilityTraitButton;
+        if ([annotation respondsToSelector:@selector(title)])
+        {
+            element.accessibilityLabel = annotation.title;
+        }
+        if ([annotation respondsToSelector:@selector(subtitle)])
+        {
+            element.accessibilityValue = annotation.subtitle;
+        }
+        annotationContext.accessibilityElement = element;
+    }
+    
+    // Update the accessibility element’s frame.
+    MGLAnnotationImage *annotationImage = [self imageOfAnnotationWithTag:annotationTag];
+    CGRect annotationFrame = [self frameOfImage:annotationImage.image centeredAtCoordinate:annotation.coordinate];
+    CGRect screenRect = UIAccessibilityConvertFrameToScreenCoordinates(annotationFrame, self);
+    element.accessibilityFrame = screenRect;
+    
+    return element;
+}
+
+- (NSInteger)indexOfAccessibilityElement:(id)element
+{
+    if (element == self.compassView)
+    {
+        return 0;
+    }
+    if ( ! [element isKindOfClass:[MGLAnnotationAccessibilityElement class]] &&
+        element != self.attributionButton)
+    {
+        return NSNotFound;
+    }
+    
+    std::vector<MGLAnnotationTag> visibleAnnotations = [self annotationTagsInRect:self.bounds];
+    if (element == self.attributionButton)
+    {
+        return visibleAnnotations.size();
+    }
+    std::sort(visibleAnnotations.begin(), visibleAnnotations.end());
+    auto foundElement = std::find(visibleAnnotations.begin(), visibleAnnotations.end(),
+                                  ((MGLAnnotationAccessibilityElement *)element).tag);
+    if (foundElement == visibleAnnotations.end()) return NSNotFound;
+    else return std::distance(visibleAnnotations.begin(), foundElement) + 1;
+}
+
 #pragma mark - Geography -
 
 + (NS_SET_OF(NSString *) *)keyPathsForValuesAffectingCenterCoordinate
@@ -2551,6 +2661,7 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
     }
     
     [self didChangeValueForKey:@"annotations"];
+    UIAccessibilityPostNotification(UIAccessibilityLayoutChangedNotification, nil);
 }
 
 /// Initialize and return a default annotation image that depicts a round pin
@@ -2685,6 +2796,7 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
         [self willChangeValueForKey:@"annotations"];
         _mbglMap->removeAnnotations(annotationTagsToRemove);
         [self didChangeValueForKey:@"annotations"];
+        UIAccessibilityPostNotification(UIAccessibilityLayoutChangedNotification, nil);
     }
 }