fixes #1157, #1255: cache parsed tiles in memory

include/mbgl/ios/MGLMapView.h

@@ -279,6 +279,9 @@ IB_DESIGNABLE
 /** Toggle the current value of debugActive. */
 - (void)toggleDebug;
 
+/** Empties the in-memory tile cache. */
+- (void)emptyMemoryCache;
+
 /** Resets the map to the minimum zoom level, a center coordinate of (0, 0), and a northern heading. */
 - (void)resetPosition;
 

include/mbgl/map/map.hpp

@@ -152,6 +152,11 @@ public:
     std::vector<uint32_t> getAnnotationsInBounds(const LatLngBounds&);
     LatLngBounds getBoundsForAnnotations(const std::vector<uint32_t>&);
 
+    // Memory
+    void setSourceTileCacheSize(size_t);
+    size_t getSourceTileCacheSize() const { return sourceCacheSize; }
+    void onLowMemory();
+
     // Debug
     void setDebug(bool value);
     void toggleDebug();
@@ -196,6 +201,8 @@ private:
 
     void updateAnnotationTiles(const std::vector<TileID>&);
 
+    size_t sourceCacheSize;
+
     enum class Mode : uint8_t {
         None, // we're not doing any processing
         Continuous, // continually updating map