Flutter引擎源碼解讀-Flutter是如何在iOS上運行起來的

時間 2020-05-09

標籤 flutter 引擎源碼解讀如何 ios 運行起來欄目 iOS 简体版

原文原文鏈接

摘要

本文主要是針對 Flutter 在 iOS 上是如何運行起來的源碼進行串聯，總結大體的運行流程。c++

涉及到的關鍵類有如下幾個：web

FlutterViewController
FlutterView
FlutterEngine
DartIsolate

FlutterViewController

Flutter 嵌入原生應用必須有個載體，從這個點入手，在 Flutter Engine 源碼中的 API 的入口點是 FlutterViewController，對其頭文件源碼作精簡，大體以下shell

@interface FlutterViewController : UIViewController <FlutterTextureRegistry, FlutterPluginRegistry>

- (instancetype)initWithEngine:(FlutterEngine*)engine
                       nibName:(nullable NSString*)nibName
                        bundle:(nullable NSBundle*)nibBundle NS_DESIGNATED_INITIALIZER;

- (instancetype)initWithProject:(nullable FlutterDartProject*)project
                        nibName:(nullable NSString*)nibName
                         bundle:(nullable NSBundle*)nibBundle NS_DESIGNATED_INITIALIZER;

- (void)handleStatusBarTouches:(UIEvent*)event;

- (void)setFlutterViewDidRenderCallback:(void (^)(void))callback;

- (NSString*)lookupKeyForAsset:(NSString*)asset;

- (NSString*)lookupKeyForAsset:(NSString*)asset fromPackage:(NSString*)package;

- (void)setInitialRoute:(NSString*)route;

- (void)popRoute;

- (void)pushRoute:(NSString*)route;

- (id<FlutterPluginRegistry>)pluginRegistry;

@property(nonatomic, readonly, getter=isDisplayingFlutterUI) BOOL displayingFlutterUI;

@property(strong, nonatomic) UIView* splashScreenView;

- (BOOL)loadDefaultSplashScreenView;

@property(nonatomic, getter=isViewOpaque) BOOL viewOpaque;

@property(weak, nonatomic, readonly) FlutterEngine* engine;

@property(nonatomic, readonly) NSObject<FlutterBinaryMessenger>* binaryMessenger;

@end
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FlutterViewController 的構造函數

FlutterViewController 有兩個構造函數，本質上是同樣的，第一個構造函數是谷歌爲了在存在多個 FlutterViewController 的場景下爲了讓使用者能複用 FlutterEngine 而開放的。json

- (instancetype)initWithEngine:(FlutterEngine*)engine
                       nibName:(nullable NSString*)nibName
                        bundle:(nullable NSBundle*)nibBundle {
  NSAssert(engine != nil, @"Engine is required");
  self = [super initWithNibName:nibName bundle:nibBundle];
  if (self) {
    _viewOpaque = YES;
    _engine.reset([engine retain]);
    _engineNeedsLaunch = NO;
    _flutterView.reset([[FlutterView alloc] initWithDelegate:_engine opaque:self.isViewOpaque]);
    _weakFactory = std::make_unique<fml::WeakPtrFactory<FlutterViewController>>(self);
    _ongoingTouches = [[NSMutableSet alloc] init];

    [self performCommonViewControllerInitialization];
    [engine setViewController:self];
  }

  return self;
}

- (instancetype)initWithProject:(nullable FlutterDartProject*)project
                        nibName:(nullable NSString*)nibName
                         bundle:(nullable NSBundle*)nibBundle {
  self = [super initWithNibName:nibName bundle:nibBundle];
  if (self) {
    _viewOpaque = YES;
    _weakFactory = std::make_unique<fml::WeakPtrFactory<FlutterViewController>>(self);
    _engine.reset([[FlutterEngine alloc] initWithName:@"io.flutter"
                                              project:project
                               allowHeadlessExecution:NO]);
    _flutterView.reset([[FlutterView alloc] initWithDelegate:_engine opaque:self.isViewOpaque]);
    [_engine.get() createShell:nil libraryURI:nil];
    _engineNeedsLaunch = YES;
    _ongoingTouches = [[NSMutableSet alloc] init];
    [self loadDefaultSplashScreenView];
    [self performCommonViewControllerInitialization];
  }

  return self;
}

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在構造函數中主要作了這麼幾件事情：api

初始化或者替換當前的 FlutterEngineapp
初始化 FlutterViewless
初始化正在發生的手勢集合async
加載閃屏頁，傳入 FlutterEngine 的構造函數沒有這項，應該是考慮了多 FlutterViewController 的場景下很差頻繁加載閃屏頁ide
設置 UIInterfaceOrientationMask 和 UIStatusBarStyle函數
添加一系列的通知，包括 Application 的生命週期，鍵盤事件，Accessibility的事件等
將 FlutterViewController 設置給 FlutterEngine

第二個構造函數中還多了這行代碼，第一個構造函數把這個調用延後了而已

[_engine.get() createShell:nil libraryURI:nil];
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FlutterViewController 的 loadView

在 loadView 函數中，設置了 FlutterViewController 的 view，並判斷是否須要加載閃屏頁，能夠經過重寫 splashScreenView 的 get 方法返回 nil 的方式完全不加載閃屏頁

- (void)loadView {
  self.view = _flutterView.get();
  self.view.multipleTouchEnabled = YES;
  self.view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;

  [self installSplashScreenViewIfNecessary];
}
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FlutterViewController 對 Navigator 的操做

FlutterViewController 提供了三個接口容許咱們在原生端對 dart 的 Navigator 直接進行操做

- (void)setInitialRoute:(NSString*)route {
  [[_engine.get() navigationChannel] invokeMethod:@"setInitialRoute" arguments:route];
}

- (void)popRoute {
  [[_engine.get() navigationChannel] invokeMethod:@"popRoute" arguments:nil];
}

- (void)pushRoute:(NSString*)route {
  [[_engine.get() navigationChannel] invokeMethod:@"pushRoute" arguments:route];
}
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setInitialRoute

setInitialRoute 在 iOS 端經過 navigationChannel 來告訴 dart 具體的 initialRoute，這個過程略微特殊，並不會在 dart 端直接接收 channel 信息，而是在引擎層面作了處理，web_ui 不在本文的解析範疇，這裏直接洗跟原生相關的點

setInitialRoute 設置流程以下：

DispatchPlatformMessage -> HandleNavigationPlatformMessage -> initial_route_

void Engine::DispatchPlatformMessage(fml::RefPtr<PlatformMessage> message) {
  if (message->channel() == kLifecycleChannel) {
    if (HandleLifecyclePlatformMessage(message.get()))
      return;
  } else if (message->channel() == kLocalizationChannel) {
    if (HandleLocalizationPlatformMessage(message.get()))
      return;
  } else if (message->channel() == kSettingsChannel) {
    HandleSettingsPlatformMessage(message.get());
    return;
  }

  if (runtime_controller_->IsRootIsolateRunning() &&
      runtime_controller_->DispatchPlatformMessage(std::move(message))) {
    return;
  }

  // If there's no runtime_, we may still need to set the initial route.
  if (message->channel() == kNavigationChannel) {
    HandleNavigationPlatformMessage(std::move(message));
    return;
  }

  FML_DLOG(WARNING) << "Dropping platform message on channel: "
                    << message->channel();
}
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bool Engine::HandleNavigationPlatformMessage(
    fml::RefPtr<PlatformMessage> message) {
  const auto& data = message->data();

  rapidjson::Document document;
  document.Parse(reinterpret_cast<const char*>(data.data()), data.size());
  if (document.HasParseError() || !document.IsObject())
    return false;
  auto root = document.GetObject();
  auto method = root.FindMember("method");
  if (method->value != "setInitialRoute")
    return false;
  auto route = root.FindMember("args");
  initial_route_ = std::move(route->value.GetString());
  return true;
}
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setInitialRoute 最終在 HandleNavigationPlatformMessage 函數中直接被賦值給 initial_route_。

setInitialRoute 讀取流程以下：

Window.defaultRouteName -> DefaultRouteName -> Engine::DefaultRouteName -> initial_route_

能夠看到，關鍵字 native，這是 dart 爲了方便綁定 C/C++ 導出方法而添加的關鍵字，對應的 key 是 Window_defaultRouteName

class Window {
  String get defaultRouteName => _defaultRouteName();
  String _defaultRouteName() native 'Window_defaultRouteName';
}
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能夠找到在引擎層的 flutter 命名空間下，有下面這個函數，註冊了對應的導出函數，這裏對應的是 DefaultRouteName

void Window::RegisterNatives(tonic::DartLibraryNatives* natives) {
  natives->Register({
      {"Window_defaultRouteName", DefaultRouteName, 1, true},
      {"Window_scheduleFrame", ScheduleFrame, 1, true},
      {"Window_sendPlatformMessage", _SendPlatformMessage, 4, true},
      {"Window_respondToPlatformMessage", _RespondToPlatformMessage, 3, true},
      {"Window_render", Render, 2, true},
      {"Window_updateSemantics", UpdateSemantics, 2, true},
      {"Window_setIsolateDebugName", SetIsolateDebugName, 2, true},
      {"Window_reportUnhandledException", ReportUnhandledException, 2, true},
      {"Window_setNeedsReportTimings", SetNeedsReportTimings, 2, true},
  });
}
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void DefaultRouteName(Dart_NativeArguments args) {
  std::string routeName =
      UIDartState::Current()->window()->client()->DefaultRouteName();
  Dart_SetReturnValue(args, tonic::StdStringToDart(routeName));
}
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再往下就是到 engine.cc 文件下面的函數，讀取 initial_route_ 的值

std::string Engine::DefaultRouteName() {
  if (!initial_route_.empty()) {
    return initial_route_;
  }
  return "/";
}
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至此，完成了在原生端設置 defaultRouteName，在 dart 端獲取該值的流程。

pushRoute and popRoute

實現方式主要仍是經過引擎內置的 navigationChannel 通知 dart 端，對應的在 dart 端 SystemChannels 類中，存在對應的 channel

static const MethodChannel navigation = MethodChannel(
      'flutter/navigation',
      JSONMethodCodec(),
  );
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最終處理 pushRoute 和 popRoute 的邏輯在 WidgetsBinding 類中，主要是如下幾個函數

Future<dynamic> _handleNavigationInvocation(MethodCall methodCall) {
    switch (methodCall.method) {
      case 'popRoute':
        return handlePopRoute();
      case 'pushRoute':
        return handlePushRoute(methodCall.arguments as String);
    }
    return Future<dynamic>.value();
  }

  Future<void> handlePushRoute(String route) async {
    for (final WidgetsBindingObserver observer in List<WidgetsBindingObserver>.from(_observers)) {
      if (await observer.didPushRoute(route))
        return;
    }
  }

  Future<void> handlePopRoute() async {
    for (final WidgetsBindingObserver observer in List<WidgetsBindingObserver>.from(_observers)) {
      if (await observer.didPopRoute())
        return;
    }
    SystemNavigator.pop();
  }
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這段代碼表示只有調用的方法返回 true 時才中斷，每一個 handle 函數具體的處理邏輯是經過某個 WidgetsBindingObserver 來實現了，繼續跟進找到以下代碼

class _WidgetsAppState extends State<WidgetsApp> with WidgetsBindingObserver {

  @override
  Future<bool> didPopRoute() async {
    assert(mounted);
    final NavigatorState navigator = _navigator?.currentState;
    if (navigator == null)
      return false;
    return await navigator.maybePop();
  }

  @override
  Future<bool> didPushRoute(String route) async {
    assert(mounted);
    final NavigatorState navigator = _navigator?.currentState;
    if (navigator == null)
      return false;
    navigator.pushNamed(route);
    return true;
  }
}
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handlePopRoute 函數中，若是沒有任何一個 observer 返回 true，則最終調用 SystemNavigator.pop(); 來退出應用程序

class SystemNavigator {
  static Future<void> pop({bool animated}) async {
    await SystemChannels.platform.invokeMethod<void>('SystemNavigator.pop', animated);
  }
}
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FlutterView

FlutterView 並無太多功能，主要是兩點：

初始化時傳入 FlutterViewEngineDelegate
建立 flutter::IOSSurface

@protocol FlutterViewEngineDelegate <NSObject>

- (flutter::Rasterizer::Screenshot)takeScreenshot:(flutter::Rasterizer::ScreenshotType)type
                                  asBase64Encoded:(BOOL)base64Encode;

- (flutter::FlutterPlatformViewsController*)platformViewsController;

@end

@interface FlutterView : UIView

- (instancetype)initWithDelegate:(id<FlutterViewEngineDelegate>)delegate
                          opaque:(BOOL)opaque NS_DESIGNATED_INITIALIZER;
- (std::unique_ptr<flutter::IOSSurface>)createSurface:
    (std::shared_ptr<flutter::IOSGLContext>)context;

@end
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takeScreenshot:asBase64Encoded: 應該是 FlutterView 渲染的數據源，具體參考 drawLayer:inContext: 的源碼

@implementation FlutterView
- (void)drawLayer:(CALayer*)layer inContext:(CGContextRef)context {
  if (layer != self.layer || context == nullptr) {
    return;
  }

  auto screenshot = [_delegate takeScreenshot:flutter::Rasterizer::ScreenshotType::UncompressedImage
                              asBase64Encoded:NO];

  if (!screenshot.data || screenshot.data->isEmpty() || screenshot.frame_size.isEmpty()) {
    return;
  }

  NSData* data = [NSData dataWithBytes:const_cast<void*>(screenshot.data->data())
                                length:screenshot.data->size()];

  fml::CFRef<CGDataProviderRef> image_data_provider(
      CGDataProviderCreateWithCFData(reinterpret_cast<CFDataRef>(data)));

  fml::CFRef<CGColorSpaceRef> colorspace(CGColorSpaceCreateDeviceRGB());

  fml::CFRef<CGImageRef> image(CGImageCreate(
      screenshot.frame_size.width(),      // size_t width
      screenshot.frame_size.height(),     // size_t height
      8,                                  // size_t bitsPerComponent
      32,                                 // size_t bitsPerPixel,
      4 * screenshot.frame_size.width(),  // size_t bytesPerRow
      colorspace,                         // CGColorSpaceRef space
      static_cast<CGBitmapInfo>(kCGImageAlphaPremultipliedLast |
                                kCGBitmapByteOrder32Big),  // CGBitmapInfo bitmapInfo
      image_data_provider,                                 // CGDataProviderRef provider
      nullptr,                                             // const CGFloat* decode
      false,                                               // bool shouldInterpolate
      kCGRenderingIntentDefault                            // CGColorRenderingIntent intent
      ));

  const CGRect frame_rect =
      CGRectMake(0.0, 0.0, screenshot.frame_size.width(), screenshot.frame_size.height());

  CGContextSaveGState(context);
  CGContextTranslateCTM(context, 0.0, CGBitmapContextGetHeight(context));
  CGContextScaleCTM(context, 1.0, -1.0);
  CGContextDrawImage(context, frame_rect, image);
  CGContextRestoreGState(context);
}
@end
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後面咱們會看到 FlutterViewEngineDelegate 其實是被 FlutterEngine 實現了。

這裏不對 IOSSurface 作過多解析，其是創建在三種 layer 之上的，能夠在編譯期選擇使用何種渲染方式

若是是模擬器，則使用正常的 CALayer
使用 Metal 渲染的情形則使用 CAMetalLayer
使用 OpenGL 渲染的情形則使用 CAEAGLLayer

+ (Class)layerClass {
#if TARGET_IPHONE_SIMULATOR
  return [CALayer class];
#else // TARGET_IPHONE_SIMULATOR
#if FLUTTER_SHELL_ENABLE_METAL
  return [CAMetalLayer class];
#else // FLUTTER_SHELL_ENABLE_METAL
  return [CAEAGLLayer class];
#endif // FLUTTER_SHELL_ENABLE_METAL
#endif // TARGET_IPHONE_SIMULATOR
}
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在 createSurface 函數中主要是分別建立三種對應的 IOSSurface

CALayer -> IOSSurfaceSoftware CAEAGLLayer -> IOSSurfaceGL CAMetalLayer -> IOSSurfaceMetal

再往下的渲染實際上就要交給 FlutterEngine 自身了。

FlutterEngine

FlutterEngine 對外暴露的接口不算多，主要就這麼幾點

構造函數，initWithName:project:allowHeadlessExecution，allowHeadlessExecution容許初始化引擎時不強依賴FlutterViewController`
啓動引擎，runWithEntrypoint:libraryURI: 可傳入自定義的 entrypoint
釋放資源，destroyContext
語義樹是否創建，ensureSemanticsEnabled，關於語義樹文檔比較少，大概是殘疾人模式下須要用到的東西
FlutterViewController 的 get/set
最後是一堆內置的 channel

咱們主要關心引擎的構造、啓動、釋放和 FlutterViewController 就差很少了，FlutterTextureRegistry, FlutterPluginRegistry 不在本文關注範圍內

@interface FlutterEngine : NSObject <FlutterTextureRegistry, FlutterPluginRegistry>

- (instancetype)initWithName:(NSString*)labelPrefix
                     project:(nullable FlutterDartProject*)project
      allowHeadlessExecution:(BOOL)allowHeadlessExecution NS_DESIGNATED_INITIALIZER;

- (BOOL)runWithEntrypoint:(nullable NSString*)entrypoint libraryURI:(nullable NSString*)uri;

- (void)destroyContext;

- (void)ensureSemanticsEnabled;

@property(nonatomic, weak) FlutterViewController* viewController;

@property(nonatomic, readonly, nullable) FlutterMethodChannel* localizationChannel;

@property(nonatomic, readonly) FlutterMethodChannel* navigationChannel;

@property(nonatomic, readonly) FlutterMethodChannel* platformChannel;

@property(nonatomic, readonly) FlutterMethodChannel* textInputChannel;

@property(nonatomic, readonly) FlutterBasicMessageChannel* lifecycleChannel;

@property(nonatomic, readonly) FlutterBasicMessageChannel* systemChannel;

@property(nonatomic, readonly) FlutterBasicMessageChannel* settingsChannel;

@property(nonatomic, readonly) NSObject<FlutterBinaryMessenger>* binaryMessenger;

@property(nonatomic, readonly, copy, nullable) NSString* isolateId;

@end
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FlutterEngine 的構造

FlutterEngine在構造時，要關注的有幾下兩點：

FlutterDartProject 初始化
FlutterPlatformViewsController 的初始化

- (instancetype)initWithName:(NSString*)labelPrefix
                     project:(FlutterDartProject*)project
      allowHeadlessExecution:(BOOL)allowHeadlessExecution {
  self = [super init];
  NSAssert(self, @"Super init cannot be nil");
  NSAssert(labelPrefix, @"labelPrefix is required");

  _allowHeadlessExecution = allowHeadlessExecution;
  _labelPrefix = [labelPrefix copy];

  _weakFactory = std::make_unique<fml::WeakPtrFactory<FlutterEngine>>(self);

  if (project == nil)
    _dartProject.reset([[FlutterDartProject alloc] init]);
  else
    _dartProject.reset([project retain]);

  _pluginPublications = [NSMutableDictionary new];
  _platformViewsController.reset(new flutter::FlutterPlatformViewsController());

  _binaryMessenger = [[FlutterBinaryMessengerRelay alloc] initWithParent:self];

  NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
  [center addObserver:self
             selector:@selector(onMemoryWarning:)
                 name:UIApplicationDidReceiveMemoryWarningNotification
               object:nil];

  return self;
}
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FlutterEngine 的啓動

FlutterEngine 層面，須要關注如下一些類：

FlutterDartProject
flutter::ThreadHost
flutter::Shell
FlutterObservatoryPublisher
FlutterPlatformViewsController

FlutterEngine 的啓動，主要是兩個事情

createShell
launchEngine

- (BOOL)runWithEntrypoint:(NSString*)entrypoint libraryURI:(NSString*)libraryURI {
  if ([self createShell:entrypoint libraryURI:libraryURI]) {
    [self launchEngine:entrypoint libraryURI:libraryURI];
  }

  return _shell != nullptr;
}
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createShell

createShell 的源碼比較多，作了下精簡，主要是如下幾點：

初始化 MessageLoop
初始化 ThreadHost
設置 on_create_platform_view 回調
設置 on_create_rasterizer 回調
初始化 flutter::TaskRunners，若是開啓 embedded_views_preview 則使用當前線程的 TaskRunner 做爲 gpu 線程的 TaskRunner
建立 shell，最終是啓動 Isolate
建立 FlutterPlatformViewsController
建立 FlutterObservatoryPublisher
設置 PlatformView channels

- (BOOL)createShell:(NSString*)entrypoint libraryURI:(NSString*)libraryURI {

  // ……

  fml::MessageLoop::EnsureInitializedForCurrentThread();

  _threadHost = {threadLabel.UTF8String, flutter::ThreadHost::Type::UI |
                                         flutter::ThreadHost::Type::GPU |
                                         flutter::ThreadHost::Type::IO};

  flutter::Shell::CreateCallback<flutter::PlatformView> on_create_platform_view =
      [](flutter::Shell& shell) {
        return std::make_unique<flutter::PlatformViewIOS>(shell, shell.GetTaskRunners());
      };

  flutter::Shell::CreateCallback<flutter::Rasterizer> on_create_rasterizer =
      [](flutter::Shell& shell) {
        return std::make_unique<flutter::Rasterizer>(shell, shell.GetTaskRunners());
      };

  if (flutter::IsIosEmbeddedViewsPreviewEnabled()) {
    flutter::TaskRunners task_runners(threadLabel.UTF8String,                          // label
                                      fml::MessageLoop::GetCurrent().GetTaskRunner(),  // platform
                                      fml::MessageLoop::GetCurrent().GetTaskRunner(),  // gpu
                                      _threadHost.ui_thread->GetTaskRunner(),          // ui
                                      _threadHost.io_thread->GetTaskRunner()           // io
    );
    // Create the shell. This is a blocking operation.
    _shell = flutter::Shell::Create(std::move(task_runners),  // task runners
                                    std::move(settings),      // settings
                                    on_create_platform_view,  // platform view creation
                                    on_create_rasterizer      // rasterzier creation
    );
  } else {
    flutter::TaskRunners task_runners(threadLabel.UTF8String,                          // label
                                      fml::MessageLoop::GetCurrent().GetTaskRunner(),  // platform
                                      _threadHost.gpu_thread->GetTaskRunner(),         // gpu
                                      _threadHost.ui_thread->GetTaskRunner(),          // ui
                                      _threadHost.io_thread->GetTaskRunner()           // io
    );
    // Create the shell. This is a blocking operation.
    _shell = flutter::Shell::Create(std::move(task_runners),  // task runners
                                    std::move(settings),      // settings
                                    on_create_platform_view,  // platform view creation
                                    on_create_rasterizer      // rasterzier creation
    );
  }

  if (_shell != nullptr) {
    [self setupChannels];
    if (!_platformViewsController) {
      _platformViewsController.reset(new flutter::FlutterPlatformViewsController());
    }
    _publisher.reset([[FlutterObservatoryPublisher alloc] init]);
    [self maybeSetupPlatformViewChannels];
  }

  return _shell != nullptr;
}
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這裏能夠看到會啓動四個 TaskRunner，分別爲 platform,gpu, ui,io，但實際上並不必定對應四個線程。

launchEngine

launchEngine 實際上仍是在 shell 上進行的操做

- (void)launchEngine:(NSString*)entrypoint libraryURI:(NSString*)libraryOrNil {
  // Launch the Dart application with the inferred run configuration.
  self.shell.RunEngine([_dartProject.get() runConfigurationForEntrypoint:entrypoint
                                                            libraryOrNil:libraryOrNil]);
}
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void Shell::RunEngine(RunConfiguration run_configuration) {
  RunEngine(std::move(run_configuration), nullptr);
}

void Shell::RunEngine(RunConfiguration run_configuration,
                      std::function<void(Engine::RunStatus)> result_callback) {
  auto result = [platform_runner = task_runners_.GetPlatformTaskRunner(),
                 result_callback](Engine::RunStatus run_result) {
    if (!result_callback) {
      return;
    }
    platform_runner->PostTask(
        [result_callback, run_result]() { result_callback(run_result); });
  };
  FML_DCHECK(is_setup_);
  FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());

  if (!weak_engine_) {
    result(Engine::RunStatus::Failure);
  }
  fml::TaskRunner::RunNowOrPostTask(
      task_runners_.GetUITaskRunner(),
      fml::MakeCopyable(
          [run_configuration = std::move(run_configuration),
           weak_engine = weak_engine_, result]() mutable {
            if (!weak_engine) {
              FML_LOG(ERROR)
                  << "Could not launch engine with configuration - no engine.";
              result(Engine::RunStatus::Failure);
              return;
            }
            auto run_result = weak_engine->Run(std::move(run_configuration));
            if (run_result == flutter::Engine::RunStatus::Failure) {
              FML_LOG(ERROR) << "Could not launch engine with configuration.";
            }
            result(run_result);
          }));
}
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再跟下去，最終會到[shell > common > engine.cc] 裏面的 run 函數，最核心的是這行代碼 PrepareAndLaunchIsolate，最終整個流程跑下來就是爲了啓動 Isolate

Engine::RunStatus Engine::Run(RunConfiguration configuration) {
  if (!configuration.IsValid()) {
    FML_LOG(ERROR) << "Engine run configuration was invalid.";
    return RunStatus::Failure;
  }

  auto isolate_launch_status =
      PrepareAndLaunchIsolate(std::move(configuration));
  if (isolate_launch_status == Engine::RunStatus::Failure) {
    FML_LOG(ERROR) << "Engine not prepare and launch isolate.";
    return isolate_launch_status;
  } else if (isolate_launch_status ==
             Engine::RunStatus::FailureAlreadyRunning) {
    return isolate_launch_status;
  }

  std::shared_ptr<DartIsolate> isolate =
      runtime_controller_->GetRootIsolate().lock();

  bool isolate_running =
      isolate && isolate->GetPhase() == DartIsolate::Phase::Running;

  if (isolate_running) {
    tonic::DartState::Scope scope(isolate.get());

    if (settings_.root_isolate_create_callback) {
      settings_.root_isolate_create_callback();
    }

    if (settings_.root_isolate_shutdown_callback) {
      isolate->AddIsolateShutdownCallback(
          settings_.root_isolate_shutdown_callback);
    }

    std::string service_id = isolate->GetServiceId();
    fml::RefPtr<PlatformMessage> service_id_message =
        fml::MakeRefCounted<flutter::PlatformMessage>(
            kIsolateChannel,
            std::vector<uint8_t>(service_id.begin(), service_id.end()),
            nullptr);
    HandlePlatformMessage(service_id_message);
  }

  return isolate_running ? Engine::RunStatus::Success
                         : Engine::RunStatus::Failure;
}
複製代碼

DartIsolate

對 PrepareAndLaunchIsolate 函數作下精簡，剩下兩個點

PrepareIsolate
RunFromLibrary

Engine::RunStatus Engine::PrepareAndLaunchIsolate(RunConfiguration configuration) {
  // ……

  if (!isolate_configuration->PrepareIsolate(*isolate)) {
    return RunStatus::Failure;
  }

  if (!isolate->RunFromLibrary(configuration.GetEntrypointLibrary(),
                               configuration.GetEntrypoint(),
                               settings_.dart_entrypoint_args)) {
    return RunStatus::Failure;
  }

  return RunStatus::Success;
}
複製代碼

主要看看 RunFromLibrary 作了什麼

查找 entrypoint
調用 entrypoint 的函數，InvokeMainEntrypoint

bool DartIsolate::RunFromLibrary(const std::string& library_name,
                                 const std::string& entrypoint_name,
                                 const std::vector<std::string>& args,
                                 fml::closure on_run) {
  tonic::DartState::Scope scope(this);

  auto user_entrypoint_function =
      Dart_GetField(Dart_LookupLibrary(tonic::ToDart(library_name.c_str())),
                    tonic::ToDart(entrypoint_name.c_str()));

  auto entrypoint_args = tonic::ToDart(args);

  if (!InvokeMainEntrypoint(user_entrypoint_function, entrypoint_args)) {
    return false;
  }
  phase_ = Phase::Running;
  if (on_run) {
    on_run();
  }
  return true;
}
複製代碼

再看看 InvokeMainEntrypoint 作了什麼，源碼作了精簡，主要就這兩步，咱們能夠在 dart 端找到對應的函數

_getStartMainIsolateFunction
_runMainZoned

static bool InvokeMainEntrypoint(Dart_Handle user_entrypoint_function, Dart_Handle args) {

  Dart_Handle start_main_isolate_function =
      tonic::DartInvokeField(Dart_LookupLibrary(tonic::ToDart("dart:isolate")),
                             "_getStartMainIsolateFunction", {});

  if (tonic::LogIfError(tonic::DartInvokeField(
          Dart_LookupLibrary(tonic::ToDart("dart:ui")), "_runMainZoned",
          {start_main_isolate_function, user_entrypoint_function, args}))) {
    FML_LOG(ERROR) << "Could not invoke the main entrypoint.";
    return false;
  }

  return true;
}
複製代碼

再往下就是 tonic 庫，這是 fuchsia 下的庫。

總結

Flutter 運行於 iOS 之上，從源碼層面看，有如下幾點收穫：

複用了現有的三類 CALayer 來繪製界面，drawLayer 時會調用 takeScreenshot 來獲取 Flutter 界面的光柵圖
在原生端不會創建對應的語義樹，須要額外生成
Flutter 自身會起一個徹底獨立的線程環境來運行，咱們須要關注的是四個 TaskRunner，Platform TaskRunner 不必定是獨立的線程
Platform TaskRunner，原生端跟 Flutter 的全部交互都會在 Platform TaskRunner 進行處理
dart 端能夠經過 native 關鍵字調用 C/C++ 的函數，獲取基本類型的數據返回值，性能比 channel 要好
FlutterViewController 將全部的手勢交互相關的都轉發給 FlutterEngine