split renderSync and transition nudging to allow client view syncing

include/mbgl/map/map.hpp

@@ -63,7 +63,10 @@ class Map : private util::noncopyable {
     void renderStill(StillImageCallback callback);
 
     // Triggers a synchronous or asynchronous render.
-    void renderSync();
+    bool renderSync();
+
+    // Nudges transitions one step, possibly notifying of the need for a rerender.
+    void nudgeTransitions(bool forceRerender);
 
     // Notifies the Map thread that the state has changed and an update might be necessary.
     void update(Update update = Update::Nothing);

platform/ios/MGLMapView.mm

@@ -682,7 +682,8 @@ - (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
 
         _mbglMap->setSourceTileCacheSize(cacheSize);
 
-        _mbglMap->renderSync();
+        bool needsRerender = _mbglMap->renderSync();
+        _mbglMap->nudgeTransitions(needsRerender);
     }
 }
 

src/mbgl/map/map.cpp

@@ -46,7 +46,7 @@ void Map::renderStill(StillImageCallback callback) {
     context->invoke(&MapContext::renderStill, transform->getState(), callback);
 }
 
-void Map::renderSync() {
+bool Map::renderSync() {
     if (renderState == RenderState::never) {
         view.notifyMapChange(MapChangeWillStartRenderingMap);
     }
@@ -67,9 +67,13 @@ void Map::renderSync() {
         view.notifyMapChange(MapChangeDidFinishRenderingMapFullyRendered);
     }
 
+    return result.needsRerender;
+}
+
+void Map::nudgeTransitions(bool forceRerender) {
     if (transform->needsTransition()) {
         update(Update(transform->updateTransitions(Clock::now())));
-    } else if (result.needsRerender) {
+    } else if (forceRerender) {
         update();
     }
 }