Revert "[ios] Removed MGLAnnotationView.flat"

This reverts commit f2fdc23.

platform/ios/Mapbox.playground/Contents.swift

@@ -75,6 +75,7 @@ class MapDelegate: NSObject, MGLMapViewDelegate {
             let av = PlaygroundAnnotationView(reuseIdentifier: "annotation")
             av.frame = CGRect(x: 0, y: 0, width: 30, height: 30)
             av.centerOffset = CGVector(dx: -15, dy: -15)
+            av.flat = true
             let centerView = UIView(frame: CGRectInset(av.bounds, 3, 3))
             centerView.backgroundColor = UIColor.whiteColor()
             av.addSubview(centerView)

platform/ios/app/MBXViewController.m

@@ -588,6 +588,10 @@ - (MGLAnnotationView *)mapView:(MGLMapView *)mapView viewForAnnotation:(id<MGLAn
         annotationView = [[MBXAnnotationView alloc] initWithReuseIdentifier:MBXViewControllerAnnotationViewReuseIdentifer];
         annotationView.frame = CGRectMake(0, 0, 10, 10);
         annotationView.backgroundColor = [UIColor whiteColor];
+
+        // uncomment to flatten the annotation view against the map when the map is tilted
+        // this currently causes severe performance issues when more than 2k annotations are visible
+        // annotationView.flat = YES;
 
         // uncomment to make the annotation view draggable
         // also note that having two long press gesture recognizers on overlapping views (`self.view` & `annotationView`) will cause weird behaviour

platform/ios/src/MGLAnnotationView.h

@@ -126,6 +126,21 @@ typedef NS_ENUM(NSUInteger, MGLAnnotationViewDragState) {
  */
 @property (nonatomic) CGVector centerOffset;
 
+/**
+ A Boolean value indicating whether the view lies flat against the map as it
+ tilts.
+ 
+ If this option is unset, the annotation view remains unchanged as the map’s
+ pitch increases, so that the view appears to stand upright on the tilted map.
+ If this option is set, the annotation view tilts as the map’s pitch increases,
+ so that the view appears to lie flat against the tilted map.
+ 
+ For example, you would set this option if the annotation view depicts an arrow
+ that should always point due south. You would unset this option if the arrow
+ should always point down towards the ground.
+ */
+@property (nonatomic, assign, getter=isFlat) BOOL flat;
+
 /**
  A Boolean value that determines whether the annotation view grows and shrinks
  as the distance between the viewpoint and the annotation view changes on a

platform/ios/src/MGLAnnotationView.mm

@@ -89,13 +89,14 @@ - (void)updateTransform
     }
 
     self.layer.transform = CATransform3DIdentity;
-    if ( ! self.scalesWithViewingDistance)
+    MGLMapCamera *camera = self.mapView.camera;
+    if (self.flat)
     {
-        return;
+        self.layer.transform = CATransform3DRotate(self.layer.transform, MGLRadiansFromDegrees(camera.pitch), 1.0, 0, 0);
     }
 
     CGFloat superviewHeight = CGRectGetHeight(self.superview.frame);
-    if (superviewHeight > 0.0) {
+    if ( ! self.scalesWithViewingDistance && superviewHeight > 0.0) {
         // Find the maximum amount of scale reduction to apply as the view's center moves from the top
         // of the superview to the bottom. For example, if this view's center has moved 25% of the way
         // from the top of the superview towards the bottom then the maximum scale reduction is 1 - .25
@@ -108,16 +109,15 @@ - (void)updateTransform
         // as the map view will allow). The map view's maximum pitch is defined in `mbgl::util::PITCH_MAX`.
         // Since it is possible for the map view to report a pitch less than 0 due to the nature of
         // how the gesture information is captured, the value is guarded with MAX.
-        CGFloat pitchIntensity = MAX(self.mapView.camera.pitch, 0) / MGLDegreesFromRadians(mbgl::util::PITCH_MAX);
+        CGFloat pitchIntensity = MAX(camera.pitch, 0) / MGLDegreesFromRadians(mbgl::util::PITCH_MAX);
 
         // The pitch adjusted scale is the inverse proportion of the maximum possible scale reduction
         // multiplied by the pitch intensity. For example, if the maximum scale reduction is 75% and the
         // map view is 50% pitched then the annotation view should be reduced by 37.5% (.75 * .5). The
         // reduction is then normalized for a scale of 1.0.
         CGFloat pitchAdjustedScale = 1.0 - maxScaleReduction * pitchIntensity;
 
-        CATransform3D transform = self.layer.transform;
-        self.layer.transform = CATransform3DScale(transform, pitchAdjustedScale, pitchAdjustedScale, 1);
+        self.layer.transform = CATransform3DScale(self.layer.transform, pitchAdjustedScale, pitchAdjustedScale, 1);
     }
 }