Add option to prefetch low-resolution tiles (#14031)

* Add option to prefetch low-resolution tiles

platform/ios/CHANGELOG.md

@@ -5,6 +5,7 @@ Mapbox welcomes participation and contributions from everyone. Please read [CONT
 ## master
 
 * Client-side text rendering of CJK ideographs is now enabled by default. ([#13988](https://github.com/mapbox/mapbox-gl-native/pull/13988))
+* Added an MGLMapView.prefetchesTiles property that you can disable if you don’t want to prefetch simplified tiles as a performance optimization. ([#14031](https://github.com/mapbox/mapbox-gl-native/pull/14031))
 
 ## 4.9.0 - February 27, 2019
 

platform/ios/src/MGLMapView.h

@@ -352,6 +352,18 @@ MGL_EXPORT IB_DESIGNABLE
  */
 @property (nonatomic, assign) MGLMapViewPreferredFramesPerSecond preferredFramesPerSecond;
 
+/**
+ A Boolean value indicating whether the map should prefetch tiles.
+ 
+ When this property is set to YES, the map view prefetches loads tiles designed for a
+ low zoom level and displays them until receiving more detailed tiles for the current
+ zoom level. The prefetched tiles typically contain simplified versions of each shape,
+ improving the map view’s perceived performance.
+ 
+ The default value of this property is YES.
+ */
+@property (nonatomic, assign) BOOL prefetchesTiles;
+
 @property (nonatomic) NSArray<NSString *> *styleClasses __attribute__((unavailable("Support for style classes has been removed.")));
 
 - (BOOL)hasStyleClass:(NSString *)styleClass __attribute__((unavailable("Support for style classes has been removed.")));

platform/ios/src/MGLMapView.mm

@@ -2630,6 +2630,14 @@ public:
     }
 }
 
+- (void)setPrefetchesTiles:(BOOL)prefetchesTiles{
+    _mbglMap->setPrefetchZoomDelta(prefetchesTiles ? mbgl::util::DEFAULT_PREFETCH_ZOOM_DELTA : 0);
+}
+
+- (BOOL)prefetchesTiles{
+    return _mbglMap->getPrefetchZoomDelta() > 0 ? YES : NO;
+}
+
 #pragma mark - Accessibility -
 
 - (NSString *)accessibilityValue