iOS.ReactNative-2-bridge-and-react-native-app-execution

Bridge and React Native App Execution

基于0.18.1 

Async batched bridge used to communicate with the JavaScript application.

分析Objective-C和JavaScript的通信机制。

Bridge承担以下工作(或者提供接口):

A: 执行JavaScript代码

- (void)enqueueJSCall:(NSString *)moduleDotMethod args:(NSArray *)args;

B: 管理"bridge module"

- (id)moduleForName:(NSString *)moduleName;
- (id)moduleForClass:(Class)moduleClass;

C: 创建 JavaScript 执行器

- (void)initModules
{
    ......
    _javaScriptExecutor = [self moduleForClass:self.executorClass];

  

1. 寻找起点-RCTRootView

在React-Native Based的工程中, 我们看到在AppDelegate.m文件中有以下代码:

  RCTRootView *rootView = [[RCTRootView alloc] initWithBundleURL:jsCodeLocation
                                                      moduleName:@"AwesomeProject"
                                                   launchOptions:launchOptions];

  self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
  UIViewController *rootViewController = [[UIViewController alloc] init];
  rootViewController.view = rootView;

之所以可以使用JS来进行iOS App开发，RCTRootView类是可以进行探索其原因的起点。

2. RCTBridge

接下来浏览类RCTRootView源码，RCTRootView是UIView的子类，并且很简单。其中有一个属性:

@property (nonatomic, strong, readonly) RCTBridge *bridge;

通读类RCTBridge的代码，只有很少的200～300行。但是发现类RCTBatchedBridge继承自类RCTBridge。

类RCTBatchedBridge是Bridge模块的一个私有类，只被在类RCTBridge中被使用。这样设计使得接口和繁复的实现

分离。

3. RCTBatchedBridge

TODO: Rewrite this section to match the modification in version 0.18.1

观察类RCTBatchedBridge的initiailizer，发现对接口的'initJS'的调用。在这里终于和JS '发生关系'。

 - (instancetype)initWithParentBridge:(RCTBridge *)bridge
 {
      // 省略多行代码
      // ...... 
      // ......
      /**
       * Initialize and register bridge modules *before* adding the display link
       * so we don't have threading issues
       */
       [self registerModules];  // A

      /**
       * Start the application script
       */
       [self initJS];  // B

       }
     return self;
}

在查看initJS方法的代码之前，我们先来关注比较重要的方法 registerModules。

3.0 Module是什么东西?

Module 在React Native中实际上是 可以被 JavaScript 代码调用的模块, 实现了接口RCTBridgeModule的类。

Module 包含有 Native类型, JavaScript源码类型。

A): Native类型:

由Objective-C来实现相应的功能，并将接口提供给JavaScript代码调用。

B): JavaScript源码类型:

也就是用JavaScript写的React Native App。这个类型的Module由 RCTSourceCode类 来代表。

3.1 registerModules 方法

TODO: Rewrite this section to match the modification in version 0.18.1

- (void)registerModules
{
  RCTAssertMainThread();

  // Register passed-in module instances
  NSMutableDictionary *preregisteredModules = [[NSMutableDictionary alloc] init];
  for (id<RCTBridgeModule> module in self.moduleProvider ? self.moduleProvider() : nil) {  // A
    preregisteredModules[RCTBridgeModuleNameForClass([module class])] = module;
  }

  // Instantiate modules
  _moduleDataByID = [[NSMutableArray alloc] init];
  NSMutableDictionary *modulesByName = [preregisteredModules mutableCopy];    
  for (Class moduleClass in RCTGetModuleClasses()) {   // B
     NSString *moduleName = RCTBridgeModuleNameForClass(moduleClass);

     // Check if module instance has already been registered for this name
     id<RCTBridgeModule> module = modulesByName[moduleName];

     if (module) {
       // Preregistered instances takes precedence, no questions asked
       if (!preregisteredModules[moduleName]) {
         // It's OK to have a name collision as long as the second instance is nil
         RCTAssert([[moduleClass alloc] init] == nil,
                   @"Attempted to register RCTBridgeModule class %@ for the name "
                   "'%@', but name was already registered by class %@", moduleClass,
                   moduleName, [modulesByName[moduleName] class]);
       }
       if ([module class] != moduleClass) {
         RCTLogInfo(@"RCTBridgeModule of class %@ with name '%@' was encountered "
                    "in the project, but name was already registered by class %@."
                    "That's fine if it's intentional - just letting you know.",
                    moduleClass, moduleName, [modulesByName[moduleName] class]);
       }
     } else {
       // Module name hasn't been used before, so go ahead and instantiate
       module = [[moduleClass alloc] init];
     }
     if (module) {
       modulesByName[moduleName] = module;
     }
  }

  // Store modules
  _modulesByName = [[RCTModuleMap alloc] initWithDictionary:modulesByName];

  /**
   * The executor is a bridge module, wait for it to be created and set it before
   * any other module has access to the bridge
   */
  _javaScriptExecutor = _modulesByName[RCTBridgeModuleNameForClass(self.executorClass)]; // C
  RCTLatestExecutor = _javaScriptExecutor;

  [_javaScriptExecutor setUp];

  // Set bridge
  for (id<RCTBridgeModule> module in _modulesByName.allValues) {  // D
    if ([module respondsToSelector:@selector(setBridge:)]) {
      module.bridge = self;
    }

    RCTModuleData *moduleData = [[RCTModuleData alloc] initWithExecutor:_javaScriptExecutor
                                                                  uid:@(_moduleDataByID.count)
                                                             instance:module];
    [_moduleDataByID addObject:moduleData];

    if ([module conformsToProtocol:@protocol(RCTFrameUpdateObserver)]) {
      [_frameUpdateObservers addObject:moduleData];
    }
  }
  // E
  [[NSNotificationCenter defaultCenter] postNotificationName:RCTDidCreateNativeModules
                                                      object:self];
}

A): A部分用来注册外部传递进来的Module。由于RCTBridge创建RCTBatchedBridge对象时，传入的参数导致 

属性 self.moduleProvider 的值为nil，故我们先跳过这部分，直接跳到B部分。

B): B部分的循环是将加载的ModuleClass进行注册，注册到成员变量 '_modulesByName'中。

(其中的 RCTGetModuleClasses()和 RCTBridgeModuleNameForClass()

参见 iOS.ReactNative-3-about-viewmanager-uimanager-and-bridgemodule 中的说明)

C): 从'_modulesByName'中获取javaScriptExecutor，JS Executor是关键所在，JS Executor是执行JS代码的。

在 initJS 方法中会用到。

D): 为ModuleObject(模块对象/模块实例, 或者简称: 模块)设置bridge以及ModuleData(模块元数据)，最后将实现接口RCTFrameUpdateObserver

的模块对象添加到'_frameUpdateObservers' 中。

E): 发送通知, NativeModules已创建完毕。TODO: 该通知的观察者做了哪些工作? 

3.2 initJS 方法

TODO: Rewrite this section to match the modification in version 0.18.1

- (void)initJS
{
  RCTAssertMainThread();

  // Inject module data into JS context
  NSMutableDictionary *config = [[NSMutableDictionary alloc] init];  // A
  for (RCTModuleData *moduleData in _moduleDataByID) {
    config[moduleData.name] = moduleData.config;
  }
  NSString *configJSON = RCTJSONStringify(@{
    @"remoteModuleConfig": config,
  }, NULL);
  [_javaScriptExecutor injectJSONText:configJSON
                  asGlobalObjectNamed:@"__fbBatchedBridgeConfig"
                             callback:^(NSError *error) {
    if (error) {
      [[RCTRedBox sharedInstance] showError:error];
    }
  }];

  NSURL *bundleURL = _parentBridge.bundleURL;
  if (_javaScriptExecutor == nil) {

    /**
     * HACK (tadeu): If it failed to connect to the debugger, set loading to NO
     * so we can attempt to reload again.
     */
    _loading = NO;

  } else if (!bundleURL) {

    // Allow testing without a script
    dispatch_async(dispatch_get_main_queue(), ^{
      _loading = NO;
      [[NSNotificationCenter defaultCenter] postNotificationName:RCTJavaScriptDidLoadNotification
                                                          object:_parentBridge
                                                        userInfo:@{ @"bridge": self }];
    });
  } else {   40 
    RCTProfileBeginEvent();
    RCTPerformanceLoggerStart(RCTPLScriptDownload);
    RCTJavaScriptLoader *loader = [[RCTJavaScriptLoader alloc] initWithBridge:self]; // B
    [loader loadBundleAtURL:bundleURL onComplete:^(NSError *error, NSString *script) {
      RCTPerformanceLoggerEnd(RCTPLScriptDownload);
      RCTProfileEndEvent(@"JavaScript download", @"init,download", @[]);

      _loading = NO;
      if (!self.isValid) {
        return;
      }

      static BOOL shouldDismiss = NO;
      if (shouldDismiss) {
        [[RCTRedBox sharedInstance] dismiss];
      }
      static dispatch_once_t onceToken;
      dispatch_once(&onceToken, ^{
        shouldDismiss = YES;
      });

      RCTSourceCode *sourceCodeModule = self.modules[RCTBridgeModuleNameForClass([RCTSourceCode class])];
      sourceCodeModule.scriptURL = bundleURL;
      sourceCodeModule.scriptText = script;
      if (error) {

        NSArray *stack = [error userInfo][@"stack"];
        if (stack) {
          [[RCTRedBox sharedInstance] showErrorMessage:[error localizedDescription]
                                             withStack:stack];
        } else {
          [[RCTRedBox sharedInstance] showErrorMessage:[error localizedDescription]
                                           withDetails:[error localizedFailureReason]];
        }

        NSDictionary *userInfo = @{@"bridge": self, @"error": error};
        [[NSNotificationCenter defaultCenter] postNotificationName:RCTJavaScriptDidFailToLoadNotification
                                                            object:_parentBridge
                                                          userInfo:userInfo];

      } else {

        [self enqueueApplicationScript:script url:bundleURL onComplete:^(NSError *loadError) { // C

          if (loadError) {
            [[RCTRedBox sharedInstance] showError:loadError];
            return;
          }

          /**
           * Register the display link to start sending js calls after everything
           * is setup
           */
          NSRunLoop *targetRunLoop = [_javaScriptExecutor isKindOfClass:[RCTContextExecutor class]] ? [NSRunLoop currentRunLoop] : [NSRunLoop mainRunLoop];  
          [_jsDisplayLink addToRunLoop:targetRunLoop forMode:NSRunLoopCommonModes];  // D
　　　　　　// E
          [[NSNotificationCenter defaultCenter] postNotificationName:RCTJavaScriptDidLoadNotification
                                                              object:_parentBridge
                                                            userInfo:@{ @"bridge": self }];
        }];
      }
    }];
  }
}

A): 将每个ModuleObject 元数据中的config 注册到 JS Executor中。(config 参见 5. RCTModuleData)

B): 拉取JS Bundler, 可以将JS Bundler看作JS代码的'包'。 

C): 执行Application JS code。(参见 4. JS Executor) 

D): 将'_jsDisplayLink'添加到runloop中。'_jsDisplayLink'周期性的触发的工作是什么?

E): 发送通知 RCTJavaScriptDidLoadNotification。该通知的观察者进行了哪些处理?

到此，焦点会集中到JS Executor上面，接下来进行JS Executor的代码阅读。

3.3 invalidate 方法

TODO: Rewrite this section to match the modification in version 0.18.1

3.4 React Native App源码的执行

3.4.1 执行步骤

1: RCTBatchedBridge类的init方法中调用start方法来启动React Native App

- (instancetype)initWithParentBridge:(RCTBridge *)bridge
{
    .......
    
    [self start]; // 1
  }
  return self;
}

2: 在start方法的最后，模块初始化完毕，并且(React Native App的)源码加载完毕后执行源码。

模块的初始化包含两个部分:

A) JavaScript 模块的初始化

B) Native 模块的初始化。方法 initModules 完成了 Native模块的初始化。

- (void)start
{
     ......
      // Synchronously initialize all native modules that cannot be loaded lazily
  [self initModules]; 
    
    ......
  dispatch_group_notify(initModulesAndLoadSource, dispatch_get_main_queue(), ^{
    RCTBatchedBridge *strongSelf = weakSelf;
    if (sourceCode && strongSelf.loading) {
      dispatch_async(bridgeQueue, ^{
        [weakSelf executeSourceCode:sourceCode]; // 2
      });
    }
  });

3: executeSourceCode方法调用方法enqueueApplicationScript来执行(React Native App的)JavaScript源码。

- (void)executeSourceCode:(NSData *)sourceCode
{
  ......

  RCTSourceCode *sourceCodeModule = [self moduleForClass:[RCTSourceCode class]];
  sourceCodeModule.scriptURL = self.bundleURL;
  sourceCodeModule.scriptData = sourceCode;

  [self enqueueApplicationScript:sourceCode url:self.bundleURL onComplete:^(NSError *loadError) { // 3
    ......

    // Register the display link to start sending js calls after everything is setup
  NSRunLoop *targetRunLoop = [_javaScriptExecutor isKindOfClass:[RCTJSCExecutor class]] ? [NSRunLoop currentRunLoop] : [NSRunLoop mainRunLoop];
    [_jsDisplayLink addToRunLoop:targetRunLoop forMode:NSRunLoopCommonModes];

    // Perform the state update and notification on the main thread, so we can't run into
    // timing issues with RCTRootView
    dispatch_async(dispatch_get_main_queue(), ^{
      [self didFinishLoading];
      [[NSNotificationCenter defaultCenter]
       postNotificationName:RCTJavaScriptDidLoadNotification
       object:_parentBridge userInfo:@{@"bridge": self}];
    });
  }];

}

4: 方法enqueueApplicationScript 最终依赖RCTJSCExecutor类型的实例 _javaScriptExecutor

来执行(React Native App的)JavaScript源码。 

- (void)enqueueApplicationScript:(NSData *)script
                             url:(NSURL *)url
                      onComplete:(RCTJavaScriptCompleteBlock)onComplete
{

  [_javaScriptExecutor executeApplicationScript:script sourceURL:url onComplete:^(NSError *scriptLoadError) {  // 4

   .......

    [_javaScriptExecutor flushedQueue:^(id json, NSError *error)
     {

       [self handleBuffer:json batchEnded:YES];

       onComplete(error);
     }];
  }];
}

下一节来走读 类 RCTJSCExecutor 和 接口RCTJavaScriptExecutor。

4. JS Executor

TODO: Rewrite this section to match the modification in version 0.18.1

4.0 接口RCTJavaScriptExecutor

接口RCTJavaScriptExecutor定义了JS Executor需要实现的接口。在React中提供了两个JS Executor的实现,

在React/Executors Group中：RCTWebViewExecutor、RCTContextExecutor。

WebSocket中也有一个实现: RCTWebSocketExecutor。

下面是接口RCTJavaScriptExecutor的方法声明:

typedef void (^RCTJavaScriptCompleteBlock)(NSError *error);
typedef void (^RCTJavaScriptCallback)(id json, NSError *error);

/**
 * Abstracts away a JavaScript execution context - we may be running code in a
 * web view (for debugging purposes), or may be running code in a `JSContext`.
 */
@protocol RCTJavaScriptExecutor <RCTInvalidating, RCTBridgeModule>

/**
 * Used to set up the executor after the bridge has been fully initialized.
 * Do any expensive setup in this method instead of `-init`.
 */
- (void)setUp;

/**
 * Executes given method with arguments on JS thread and calls the given callback
 * with JSValue and JSContext as a result of the JS module call.
 */
- (void)executeJSCall:(NSString *)name
               method:(NSString *)method
            arguments:(NSArray *)arguments
             callback:(RCTJavaScriptCallback)onComplete;

/**
 * Runs an application script, and notifies of the script load being complete via `onComplete`.
 */
- (void)executeApplicationScript:(NSString *)script
                       sourceURL:(NSURL *)sourceURL
                      onComplete:(RCTJavaScriptCompleteBlock)onComplete;

// 将由script表示的JavaScript脚本代表的object以objectName注册为全局变量。  
- (void)injectJSONText:(NSString *)script
   asGlobalObjectNamed:(NSString *)objectName
              callback:(RCTJavaScriptCompleteBlock)onComplete;

/**
 * Enqueue a block to run in the executors JS thread. Fallback to `dispatch_async`
 * on the main queue if the executor doesn't own a thread.
 */
- (void)executeBlockOnJavaScriptQueue:(dispatch_block_t)block;

@optional

/**
 * Special case for Timers + ContextExecutor - instead of the default
 *   if jsthread then call else dispatch call on jsthread
 * ensure the call is made async on the jsthread
 */
- (void)executeAsyncBlockOnJavaScriptQueue:(dispatch_block_t)block;

@end

4.1 RCTJSCExecutor类

RCTJSCExecutor实现了接口RCTJavaScriptExecutor: 

/**
 * Uses a JavaScriptCore context as the execution engine.
 */
@interface RCTJSCExecutor : NSObject <RCTJavaScriptExecutor>

5. RCTModuleData

RCTModuleData实例保存关于RCTBridgeModule实例的数据，这些数据包含:  "bridge module"模块的类(1),

"bridge module"模块在Javascript中名字(2), "bridge module"模块导出到JavaScript中的 Method(3), 

"bridge module"模块实例(4), the module method dispatch queue(5), "bridge module"模块的配置信息(6)。

@property (nonatomic, strong, readonly) Class moduleClass; // 1
@property (nonatomic, copy, readonly) NSString *name;  // 2

  

@property (nonatomic, copy, readonly) NSArray<id<RCTBridgeMethod>> *methods; // 3

@property (nonatomic, strong, readonly) id<RCTBridgeModule> instance;  // 4

@property (nonatomic, strong, readonly) dispatch_queue_t methodQueue; // 5

@property (nonatomic, copy, readonly) NSArray *config; // 6

 RCTModuleData的方法instance 会创建"bridge module"模块实例:

- (id<RCTBridgeModule>)instance
{
  [_instanceLock lock];
  if (!_setupComplete) {
    if (!_instance) {
      _instance = [_moduleClass new];
    }
    // Bridge must be set before methodQueue is set up, as methodQueue
    // initialization requires it (View Managers get their queue by calling
    // self.bridge.uiManager.methodQueue)
    [self setBridgeForInstance];
    [self setUpMethodQueue];
    [_bridge registerModuleForFrameUpdates:_instance withModuleData:self];
    _setupComplete = YES;
  }
  [_instanceLock unlock];
  return _instance;
}

6. RCTJavaScriptLoader

从本地文件系统或者远程Server加载JavaScript。

+ (void)loadBundleAtURL:(NSURL *)scriptURL onComplete:(RCTSourceLoadBlock)onComplete

该接口实现使用了 NSURLSessionDataTask, React Native需要 iOS 7.0+ 的系统。

  

7. RCTSourceCode

RCTSourceCode抽象JavaScript源码数据, 包含属性:

scriptData 和 scriptURL

@interface RCTSourceCode : NSObject <RCTBridgeModule> // E

@property (nonatomic, copy) NSData *scriptData;
@property (nonatomic, copy) NSURL *scriptURL;

@end

E: 关于 RCTBridgeModule接口 参见 iOS.ReactNative-3-about-viewmanager-uimanager-and-bridgemodule

---

Reference

1. React-Native: RCTBridge.m/h