[ios] Use transparent image for annotations backed by views (#5461)

The current implementation of view backed annotations mostly relies
on the existing annotation model manager and the associated underlying
boost query implementations for annotation management. However, since
the representation of the model is a view, an image was not installed
for view backed annotations. Because the underlying annotation
management implementation is not entire comfortable working without
an image, it would report
`[INFO] {Worker}[Sprite]: Can't find sprite named 'default_marker'`
when annotation models without an image were encountered.

This updates the annotation adding logic in to create (or reuse) and
install a small, invisible image for each view backed annotation.

1 files changed, 32 insertions, 4 deletions

diff --git a/platform/ios/src/MGLMapView.mm b/platform/ios/src/MGLMapView.mm
index 79a5f961b2..c0dacb50fe 100644
--- a/platform/ios/src/MGLMapView.mm
+++ b/platform/ios/src/MGLMapView.mm
@@ -88,6 +88,10 @@ const CLLocationDirection MGLToleranceForSnappingToNorth = 7;
 /// Reuse identifier and file name of the default point annotation image.
 static NSString * const MGLDefaultStyleMarkerSymbolName = @"default_marker";
 
+/// Reuse identifier and file name of the invisible point annotation image used
+/// by annotations that are visually backed by MGLAnnotationView objects
+static NSString * const MGLInvisibleStyleMarkerSymbolName = @"invisible_marker";
+
 /// Prefix that denotes a sprite installed by MGLMapView, to avoid collisions
 /// with style-defined sprites.
 NSString *const MGLAnnotationSpritePrefix = @"com.mapbox.sprites.";
@@ -2861,6 +2865,14 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
                     annotationView.annotation = annotation;
                     annotationView.center = [self convertCoordinate:annotation.coordinate toPointToView:self];
                     [newAnnotationViews addObject:annotationView];
+                  
+                    MGLAnnotationImage *annotationImage = self.invisibleAnnotationImage;
+                    symbolName = annotationImage.styleIconIdentifier;
+                    annotationImagesForAnnotation[annotationValue] = annotationImage;
+                    if ( ! self.annotationImagesByIdentifier[annotationImage.reuseIdentifier])
+                    {
+                        [self installAnnotationImage:annotationImage];
+                    }
                 }
             }
             
@@ -2897,22 +2909,21 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
 
             MGLAnnotationTag annotationTag = _mbglMap->addAnnotation(mbgl::SymbolAnnotation {
                 MGLPointFromLocationCoordinate2D(annotation.coordinate),
-                symbolName.UTF8String ?: ""
+                symbolName.UTF8String
             });
 
             MGLAnnotationContext context;
             context.annotation = annotation;
             MGLAnnotationImage *annotationImage = annotationImagesForAnnotation[annotationValue];
+            context.imageReuseIdentifier = annotationImage.reuseIdentifier;
 
-            if (annotationImage) {
-                context.imageReuseIdentifier = annotationImage.reuseIdentifier;
-            }
             if (annotationView) {
                 context.annotationView = annotationView;
                 context.viewReuseIdentifier = annotationView.reuseIdentifier;
             }
             
             _annotationContextsByAnnotationTag[annotationTag] = context;
+
             if ([annotation isKindOfClass:[NSObject class]]) {
                 NSAssert(![annotation isKindOfClass:[MGLMultiPoint class]], @"Point annotation should not be MGLMultiPoint.");
                 [(NSObject *)annotation addObserver:self forKeyPath:@"coordinate" options:0 context:(void *)(NSUInteger)annotationTag];
@@ -2962,6 +2973,23 @@ mbgl::Duration MGLDurationInSeconds(NSTimeInterval duration)
     return annotationImage;
 }
 
+- (MGLAnnotationImage *)invisibleAnnotationImage
+{
+    MGLAnnotationImage *annotationImage = [self dequeueReusableAnnotationImageWithIdentifier:MGLInvisibleStyleMarkerSymbolName];
+    
+    if (!annotationImage)
+    {
+        UIGraphicsBeginImageContext(CGSizeMake(1, 1));
+        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
+        UIGraphicsEndImageContext();
+        annotationImage = [MGLAnnotationImage annotationImageWithImage:image
+                                                                           reuseIdentifier:MGLInvisibleStyleMarkerSymbolName];
+        annotationImage.styleIconIdentifier = [MGLAnnotationSpritePrefix stringByAppendingString:annotationImage.reuseIdentifier];
+    }
+    
+    return annotationImage;
+}
+
 - (MGLAnnotationView *)annotationViewForAnnotation:(id<MGLAnnotation>)annotation
 {
     MGLAnnotationView *annotationView = [self.delegate mapView:self viewForAnnotation:annotation];