[core] use raii to guard backend deactivation

include/mbgl/map/backend.hpp

@@ -10,6 +10,8 @@ namespace gl {
 class Context;
 } // namespace gl
 
+class BackendScope;
+
 class Backend {
 public:
     Backend();
@@ -18,6 +20,14 @@ public:
     // Returns the backend's context which manages OpenGL state.
     gl::Context& getContext();
 
+    // Called when the map needs to be rendered; the backend should call Map::render() at some point
+    // in the near future. (Not called for Map::renderStill() mode.)
+    virtual void invalidate() = 0;
+
+    // Notifies a watcher of map x/y/scale/rotation changes.
+    virtual void notifyMapChange(MapChange change);
+
+protected:
     // Called when the backend's GL context needs to be made active or inactive. These are called,
     // as a matched pair, in four situations:
     //
@@ -31,15 +41,24 @@ public:
     virtual void activate() = 0;
     virtual void deactivate() = 0;
 
-    // Called when the map needs to be rendered; the backend should call Map::render() at some point
-    // in the near future. (Not called for Map::renderStill() mode.)
-    virtual void invalidate() = 0;
-
-    // Notifies a watcher of map x/y/scale/rotation changes.
-    virtual void notifyMapChange(MapChange change);
-
 private:
     const std::unique_ptr<gl::Context> context;
+
+    friend class BackendScope;
+};
+
+class BackendScope {
+public:
+    BackendScope(Backend& backend_) : backend(backend_) {
+        backend.activate();
+    }
+
+    ~BackendScope() {
+        backend.deactivate();
+    }
+
+private:
+    Backend& backend;
 };
 
 } // namespace mbgl

platform/android/src/native_map_view.cpp

@@ -190,7 +190,7 @@ void NativeMapView::invalidate() {
 }
 
 void NativeMapView::render() {
-    activate();
+    BackendScope guard(*this);
 
     updateViewBinding();
     map->render(*this);
@@ -235,8 +235,6 @@ void NativeMapView::render() {
     } else {
         mbgl::Log::Info(mbgl::Event::Android, "Not swapping as we are not ready");
     }
-
-    deactivate();
 }
 
 mbgl::Map &NativeMapView::getMap() { return *map; }
@@ -421,7 +419,7 @@ void NativeMapView::createSurface(ANativeWindow *window_) {
     if (!firstTime) {
         firstTime = true;
 
-        activate();
+        BackendScope guard(*this);
 
         if (!eglMakeCurrent(display, surface, surface, context)) {
             mbgl::Log::Error(mbgl::Event::OpenGL, "eglMakeCurrent() returned error %d",
@@ -442,8 +440,6 @@ void NativeMapView::createSurface(ANativeWindow *window_) {
         mbgl::gl::InitializeExtensions([] (const char * name) {
              return reinterpret_cast<mbgl::gl::glProc>(eglGetProcAddress(name));
         });
-
-        deactivate();
     }
 }
 

platform/android/src/native_map_view.hpp

@@ -23,8 +23,6 @@ public:
     void updateViewBinding();
     void bind() override;
 
-    void activate() override;
-    void deactivate() override;
     void invalidate() override;
 
     void notifyMapChange(mbgl::MapChange) override;
@@ -53,6 +51,10 @@ public:
 
     void scheduleTakeSnapshot();
 
+protected:
+    void activate() override;
+    void deactivate() override;
+
 private:
     EGLConfig chooseConfig(const EGLConfig configs[], EGLint numConfigs);
 

src/mbgl/map/map.cpp

@@ -140,7 +140,7 @@ Map::Impl::Impl(Backend& backend_,
 }
 
 Map::~Map() {
-    impl->backend.activate();
+    BackendScope guard(impl->backend);
 
     impl->styleRequest = nullptr;
 
@@ -149,8 +149,6 @@ Map::~Map() {
     impl->style.reset();
     impl->annotationManager.reset();
     impl->painter.reset();
-
-    impl->backend.deactivate();
 }
 
 void Map::renderStill(View& view, StillImageCallback callback) {
@@ -275,9 +273,8 @@ void Map::Impl::update() {
         backend.invalidate();
     } else if (stillImageRequest && style->isLoaded()) {
         // TODO: determine whether we need activate/deactivate
-        backend.activate();
+        BackendScope guard(backend);
         render(stillImageRequest->view);
-        backend.deactivate();
     }
 
     updateFlags = Update::Nothing;
@@ -848,12 +845,10 @@ void Map::addLayer(std::unique_ptr<Layer> layer, const optional<std::string>& be
     }
 
     impl->styleMutated = true;
-    impl->backend.activate();
+    BackendScope guard(impl->backend);
 
     impl->style->addLayer(std::move(layer), before);
     impl->onUpdate(Update::Classes);
-
-    impl->backend.deactivate();
 }
 
 std::unique_ptr<Layer> Map::removeLayer(const std::string& id) {
@@ -862,13 +857,11 @@ std::unique_ptr<Layer> Map::removeLayer(const std::string& id) {
     }
 
     impl->styleMutated = true;
-    impl->backend.activate();
+    BackendScope guard(impl->backend);
 
     auto removedLayer = impl->style->removeLayer(id);
     impl->onUpdate(Update::Classes);
 
-    impl->backend.deactivate();
-
     return removedLayer;
 }
 
@@ -1033,9 +1026,8 @@ void Map::setSourceTileCacheSize(size_t size) {
 
 void Map::onLowMemory() {
     if (impl->painter) {
-        impl->backend.activate();
+        BackendScope guard(impl->backend);
         impl->painter->cleanup();
-        impl->backend.deactivate();
     }
     if (impl->style) {
         impl->style->onLowMemory();