[Multiwindow] Renderers receive view IDs as argument (flutter/engine#36593)

* Impl

* Format

* TODO comments

* Fix compile

* Better empty indication

* Change invalid ID to -1

* fix doc

* Change to default view

* Better doc

* Update doc of FlutterFrameInfo

* Change to ID

* Address comment

* Fix comment

* To assertion

* Fix compile

engine/src/flutter/shell/platform/darwin/macos/framework/Headers/FlutterEngine.h

@@ -63,7 +63,10 @@ FLUTTER_DARWIN_EXPORT
 - (BOOL)runWithEntrypoint:(nullable NSString*)entrypoint;
 
 /**
- * The `FlutterViewController` associated with this engine, if any.
+ * The default `FlutterViewController` associated with this engine, if any.
+ *
+ * The default view always has ID kFlutterDefaultViewId, and is the view
+ * operated by the APIs that do not have a view ID specified.
  */
 @property(nonatomic, nullable, weak) FlutterViewController* viewController;
 

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

@@ -400,7 +400,6 @@ static void OnPlatformMessage(const FlutterPlatformMessage* message, FlutterEngi
 - (void)setViewController:(FlutterViewController*)controller {
   if (_viewController != controller) {
     _viewController = controller;
-    [_renderer setFlutterView:controller.flutterView];
 
     if (_semanticsEnabled && _bridge) {
       _bridge->UpdateDefaultViewController(_viewController);
@@ -425,10 +424,14 @@ static void OnPlatformMessage(const FlutterPlatformMessage* message, FlutterEngi
   _macOSCompositor = std::make_unique<flutter::FlutterCompositor>(
       _viewProvider, _platformViewController, _renderer.device);
   _macOSCompositor->SetPresentCallback([weakSelf](bool has_flutter_content) {
+    // TODO(dkwingsmt): The compositor only supports single-view for now. As
+    // more classes are gradually converted to multi-view, it should get the
+    // view ID from somewhere.
+    uint64_t viewId = kFlutterDefaultViewId;
     if (has_flutter_content) {
-      return [weakSelf.renderer present] == YES;
+      return [weakSelf.renderer present:viewId] == YES;
     } else {
-      [weakSelf.renderer presentWithoutContent];
+      [weakSelf.renderer presentWithoutContent:viewId];
       return true;
     }
   });

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterRenderer.h

@@ -30,30 +30,25 @@
  */
 - (nullable instancetype)initWithFlutterEngine:(nonnull FlutterEngine*)flutterEngine;
 
-/**
- * Sets the FlutterView to render to.
- */
-- (void)setFlutterView:(nullable FlutterView*)view;
-
 /**
  * Creates a FlutterRendererConfig that renders using the appropriate backend.
  */
 - (FlutterRendererConfig)createRendererConfig;
 
 /**
- * Called by the engine when the context's buffers should be swapped.
+ * Called by the engine when the given view's buffers should be swapped.
  */
-- (BOOL)present;
+- (BOOL)present:(uint64_t)viewId;
 
 /**
  * Tells the renderer that there is no Flutter content available for this frame.
  */
-- (void)presentWithoutContent;
+- (void)presentWithoutContent:(uint64_t)viewId;
 
 /**
- * Creates a Metal texture for the given size.
+ * Creates a Metal texture for the given view with the given size.
  */
-- (FlutterMetalTexture)createTextureForSize:(CGSize)size;
+- (FlutterMetalTexture)createTextureForView:(uint64_t)viewId size:(CGSize)size;
 
 /**
  * Populates the texture registry with the provided metalTexture.

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterRenderer.mm

@@ -6,19 +6,29 @@
 
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine_Internal.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterExternalTexture.h"
-#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterView.h"
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterViewController_Internal.h"
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterViewEngineProvider.h"
 #include "flutter/shell/platform/embedder/embedder.h"
 
 #pragma mark - Static callbacks that require the engine.
 
-static FlutterMetalTexture OnGetNextDrawable(FlutterEngine* engine,
-                                             const FlutterFrameInfo* frameInfo) {
+static FlutterMetalTexture OnGetNextDrawableForDefaultView(FlutterEngine* engine,
+                                                           const FlutterFrameInfo* frameInfo) {
+  // TODO(dkwingsmt): This callback only supports single-view, therefore it only
+  // operates on the default view. To support multi-view, we need a new callback
+  // that also receives a view ID, or pass the ID via FlutterFrameInfo.
+  uint64_t viewId = kFlutterDefaultViewId;
   CGSize size = CGSizeMake(frameInfo->size.width, frameInfo->size.height);
-  return [engine.renderer createTextureForSize:size];
+  return [engine.renderer createTextureForView:viewId size:size];
 }
 
-static bool OnPresentDrawable(FlutterEngine* engine, const FlutterMetalTexture* texture) {
-  return [engine.renderer present];
+static bool OnPresentDrawableOfDefaultView(FlutterEngine* engine,
+                                           const FlutterMetalTexture* texture) {
+  // TODO(dkwingsmt): This callback only supports single-view, therefore it only
+  // operates on the default view. To support multi-view, we need a new callback
+  // that also receives a view ID.
+  uint64_t viewId = kFlutterDefaultViewId;
+  return [engine.renderer present:viewId];
 }
 
 static bool OnAcquireExternalTexture(FlutterEngine* engine,
@@ -33,7 +43,7 @@ static bool OnAcquireExternalTexture(FlutterEngine* engine,
 #pragma mark - FlutterRenderer implementation
 
 @implementation FlutterRenderer {
-  FlutterView* _flutterView;
+  FlutterViewEngineProvider* _viewProvider;
 
   FlutterDarwinContextMetalSkia* _darwinMetalContext;
 }
@@ -41,6 +51,7 @@ static bool OnAcquireExternalTexture(FlutterEngine* engine,
 - (instancetype)initWithFlutterEngine:(nonnull FlutterEngine*)flutterEngine {
   self = [super initWithDelegate:self engine:flutterEngine];
   if (self) {
+    _viewProvider = [[FlutterViewEngineProvider alloc] initWithEngine:flutterEngine];
     _device = MTLCreateSystemDefaultDevice();
     if (!_device) {
       NSLog(@"Could not acquire Metal device.");
@@ -59,10 +70,6 @@ static bool OnAcquireExternalTexture(FlutterEngine* engine,
   return self;
 }
 
-- (void)setFlutterView:(FlutterView*)view {
-  _flutterView = view;
-}
-
 - (FlutterRendererConfig)createRendererConfig {
   FlutterRendererConfig config = {
       .type = FlutterRendererType::kMetal,
@@ -70,9 +77,9 @@ static bool OnAcquireExternalTexture(FlutterEngine* engine,
       .metal.device = (__bridge FlutterMetalDeviceHandle)_device,
       .metal.present_command_queue = (__bridge FlutterMetalCommandQueueHandle)_commandQueue,
       .metal.get_next_drawable_callback =
-          reinterpret_cast<FlutterMetalTextureCallback>(OnGetNextDrawable),
+          reinterpret_cast<FlutterMetalTextureCallback>(OnGetNextDrawableForDefaultView),
       .metal.present_drawable_callback =
-          reinterpret_cast<FlutterMetalPresentCallback>(OnPresentDrawable),
+          reinterpret_cast<FlutterMetalPresentCallback>(OnPresentDrawableOfDefaultView),
       .metal.external_texture_frame_callback =
           reinterpret_cast<FlutterMetalTextureFrameCallback>(OnAcquireExternalTexture),
   };
@@ -81,9 +88,15 @@ static bool OnAcquireExternalTexture(FlutterEngine* engine,
 
 #pragma mark - Embedder callback implementations.
 
-- (FlutterMetalTexture)createTextureForSize:(CGSize)size {
+- (FlutterMetalTexture)createTextureForView:(uint64_t)viewId size:(CGSize)size {
+  FlutterView* view = [_viewProvider getView:viewId];
+  NSAssert(view != nil, @"Can't create texture on a non-existent view 0x%llx.", viewId);
+  if (view == nil) {
+    // FlutterMetalTexture has texture `null`, therefore is discarded.
+    return FlutterMetalTexture{};
+  }
   FlutterMetalRenderBackingStore* backingStore =
-      (FlutterMetalRenderBackingStore*)[_flutterView backingStoreForSize:size];
+      (FlutterMetalRenderBackingStore*)[view backingStoreForSize:size];
   id<MTLTexture> texture = backingStore.texture;
   FlutterMetalTexture embedderTexture;
   embedderTexture.struct_size = sizeof(FlutterMetalTexture);
@@ -92,19 +105,21 @@ static bool OnAcquireExternalTexture(FlutterEngine* engine,
   return embedderTexture;
 }
 
-- (BOOL)present {
-  if (!_flutterView) {
+- (BOOL)present:(uint64_t)viewId {
+  FlutterView* view = [_viewProvider getView:viewId];
+  if (view == nil) {
     return NO;
   }
-  [_flutterView present];
+  [view present];
   return YES;
 }
 
-- (void)presentWithoutContent {
-  if (!_flutterView) {
+- (void)presentWithoutContent:(uint64_t)viewId {
+  FlutterView* view = [_viewProvider getView:viewId];
+  if (view == nil) {
     return;
   }
-  [_flutterView presentWithoutContent];
+  [view presentWithoutContent];
 }
 
 #pragma mark - FlutterTextureRegistrar methods.

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterRendererTest.mm

@@ -9,6 +9,7 @@
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine_Internal.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterRenderer.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterView.h"
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterViewController_Internal.h"
 #include "flutter/shell/platform/embedder/embedder.h"
 #include "flutter/shell/platform/embedder/test_utils/proc_table_replacement.h"
 #include "flutter/testing/testing.h"
@@ -24,21 +25,29 @@ FlutterEngine* CreateTestEngine() {
              ICUDataPath:[fixtures stringByAppendingString:@"/icudtl.dat"]];
   return [[FlutterEngine alloc] initWithName:@"test" project:project allowHeadlessExecution:true];
 }
+
+void SetEngineDefaultView(FlutterEngine* engine, id flutterView) {
+  id mockFlutterViewController = OCMClassMock([FlutterViewController class]);
+  OCMStub([mockFlutterViewController flutterView]).andReturn(flutterView);
+  [engine setViewController:mockFlutterViewController];
+}
+
 }  // namespace
 
 TEST(FlutterRenderer, PresentDelegatesToFlutterView) {
   FlutterEngine* engine = CreateTestEngine();
   FlutterRenderer* renderer = [[FlutterRenderer alloc] initWithFlutterEngine:engine];
   id mockFlutterView = OCMClassMock([FlutterView class]);
+  SetEngineDefaultView(engine, mockFlutterView);
   [(FlutterView*)[mockFlutterView expect] present];
-  [renderer setFlutterView:mockFlutterView];
-  [renderer present];
+  [renderer present:kFlutterDefaultViewId];
 }
 
 TEST(FlutterRenderer, TextureReturnedByFlutterView) {
   FlutterEngine* engine = CreateTestEngine();
   FlutterRenderer* renderer = [[FlutterRenderer alloc] initWithFlutterEngine:engine];
   id mockFlutterView = OCMClassMock([FlutterView class]);
+  SetEngineDefaultView(engine, mockFlutterView);
   FlutterFrameInfo frameInfo;
   frameInfo.struct_size = sizeof(FlutterFrameInfo);
   FlutterUIntSize dimensions;
@@ -47,8 +56,7 @@ TEST(FlutterRenderer, TextureReturnedByFlutterView) {
   frameInfo.size = dimensions;
   CGSize size = CGSizeMake(dimensions.width, dimensions.height);
   [[mockFlutterView expect] backingStoreForSize:size];
-  [renderer setFlutterView:mockFlutterView];
-  [renderer createTextureForSize:size];
+  [renderer createTextureForView:kFlutterDefaultViewId size:size];
 }
 
 }  // namespace flutter::testing

engine/src/flutter/shell/platform/embedder/embedder.h

@@ -448,7 +448,9 @@ typedef struct {
 /// This information is passed to the embedder when requesting a frame buffer
 /// object.
 ///
-/// See: \ref FlutterOpenGLRendererConfig.fbo_with_frame_info_callback.
+/// See: \ref FlutterOpenGLRendererConfig.fbo_with_frame_info_callback,
+/// \ref FlutterMetalRendererConfig.get_next_drawable_callback,
+/// and \ref FlutterVulkanRendererConfig.get_next_image_callback.
 typedef struct {
   /// The size of this struct. Must be sizeof(FlutterFrameInfo).
   size_t struct_size;
@@ -633,6 +635,8 @@ typedef struct {
   int64_t texture_id;
   /// Handle to the MTLTexture that is owned by the embedder. Engine will render
   /// the frame into this texture.
+  ///
+  /// A NULL texture is considered invalid.
   FlutterMetalTextureHandle texture;
   /// A baton that is not interpreted by the engine in any way. It will be given
   /// back to the embedder in the destruction callback below. Embedder resources