Purge caches on low memory on iOS (#9491)

shell/common/rasterizer.cc

@@ -68,6 +68,19 @@ void Rasterizer::Teardown() {
   last_layer_tree_.reset();
 }
 
+void Rasterizer::NotifyLowMemoryWarning() const {
+  if (!surface_) {
+    FML_DLOG(INFO) << "Rasterizer::PurgeCaches called with no surface.";
+    return;
+  }
+  auto context = surface_->GetContext();
+  if (!context) {
+    FML_DLOG(INFO) << "Rasterizer::PurgeCaches called with no GrContext.";
+    return;
+  }
+  context->freeGpuResources();
+}
+
 flutter::TextureRegistry* Rasterizer::GetTextureRegistry() {
   return &compositor_context_->texture_registry();
 }

shell/common/rasterizer.h

@@ -46,6 +46,11 @@ class Rasterizer final : public SnapshotDelegate {
 
   void Teardown();
 
+  // Frees up Skia GPU resources.
+  //
+  // This method must be called from the GPU task runner.
+  void NotifyLowMemoryWarning() const;
+
   fml::WeakPtr<Rasterizer> GetWeakPtr() const;
 
   fml::WeakPtr<SnapshotDelegate> GetSnapshotDelegate() const;

shell/common/shell.cc

@@ -369,6 +369,17 @@ Shell::~Shell() {
   platform_latch.Wait();
 }
 
+void Shell::NotifyLowMemoryWarning() const {
+  task_runners_.GetGPUTaskRunner()->PostTask(
+      [rasterizer = rasterizer_->GetWeakPtr()]() {
+        if (rasterizer) {
+          rasterizer->NotifyLowMemoryWarning();
+        }
+      });
+  // The IO Manager uses resource cache limits of 0, so it is not necessary
+  // to purge them.
+}
+
 bool Shell::IsSetup() const {
   return is_setup_;
 }

shell/common/shell.h

@@ -78,6 +78,13 @@ class Shell final : public PlatformView::Delegate,
 
   DartVM* GetDartVM();
 
+  // Embedders should call this under low memory conditions to free up
+  // internal caches used.
+  //
+  // This method posts a task to the GPU threads to signal the Rasterizer to
+  // free resources.
+  void NotifyLowMemoryWarning() const;
+
   bool IsSetup() const;
 
   Rasterizer::Screenshot Screenshot(Rasterizer::ScreenshotType type,

shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -93,11 +93,21 @@
 
   [self setupChannels];
 
+  NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
+  [center addObserver:self
+             selector:@selector(onMemoryWarning:)
+                 name:UIApplicationDidReceiveMemoryWarningNotification
+               object:nil];
+
   return self;
 }
 
 - (void)dealloc {
   [_pluginPublications release];
+
+  NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
+  [center removeObserver:self name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
+
   [super dealloc];
 }
 
@@ -574,6 +584,15 @@
   return _pluginPublications[pluginKey];
 }
 
+#pragma mark - Memory Notifications
+
+- (void)onMemoryWarning:(NSNotification*)notification {
+  if (_shell) {
+    _shell->NotifyLowMemoryWarning();
+  }
+  [_systemChannel sendMessage:@{@"type" : @"memoryPressure"}];
+}
+
 @end
 
 @implementation FlutterEngineRegistrar {

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -208,11 +208,6 @@ NSNotificationName const FlutterSemanticsUpdateNotification = @"FlutterSemantics
                  name:UIAccessibilityBoldTextStatusDidChangeNotification
                object:nil];
 
-  [center addObserver:self
-             selector:@selector(onMemoryWarning:)
-                 name:UIApplicationDidReceiveMemoryWarningNotification
-               object:nil];
-
   [center addObserver:self
              selector:@selector(onUserSettingsChanged:)
                  name:UIContentSizeCategoryDidChangeNotification
@@ -794,12 +789,6 @@ static flutter::PointerData::DeviceKind DeviceKindFromTouchType(UITouch* touch)
 #endif
 }
 
-#pragma mark - Memory Notifications
-
-- (void)onMemoryWarning:(NSNotification*)notification {
-  [[_engine.get() systemChannel] sendMessage:@{@"type" : @"memoryPressure"}];
-}
-
 #pragma mark - Locale updates
 
 - (void)onLocaleUpdated:(NSNotification*)notification {