RN 47 初始化 Bridge 过程

- (instancetype)initWithDelegate:(id<RCTBridgeDelegate>)delegate
                       bundleURL:(NSURL *)bundleURL
                  moduleProvider:(RCTBridgeModuleListProvider)block
                   launchOptions:(NSDictionary *)launchOptions
{
  if (self = [super init]) {
    _delegate = delegate;
    _bundleURL = bundleURL;
    _moduleProvider = block;
    _launchOptions = [launchOptions copy];

    [self setUp];
  }
  return self;
}

其中 delegate 是 nil。主要的过程在 setUp 里。

setUp 过程

  Class bridgeClass = self.bridgeClass;
  self.batchedBridge = [[bridgeClass alloc] initWithParentBridge:self];
  [self.batchedBridge start];

主要是找到 Bridge 对应的类，然后初始化，然后调用 Bridge 的 start 函数。

找到 Bridge 对应的类

- (Class)bridgeClass
{
  // In order to facilitate switching between bridges with only build
  // file changes, this uses reflection to check which bridges are
  // available.  This is a short-term hack until RCTBatchedBridge is
  // removed.

  Class batchedBridgeClass = objc_lookUpClass("RCTBatchedBridge");
  Class cxxBridgeClass = objc_lookUpClass("RCTCxxBridge");

  Class implClass = nil;

  if ([self.delegate respondsToSelector:@selector(shouldBridgeUseCxxBridge:)]) {
    if ([self.delegate shouldBridgeUseCxxBridge:self]) {
      implClass = cxxBridgeClass;
    } else {
      implClass = batchedBridgeClass;
    }
  } else if (cxxBridgeClass != nil) {
    implClass = cxxBridgeClass;
  } else if (batchedBridgeClass != nil) {
    implClass = batchedBridgeClass;
  }

  RCTAssert(implClass != nil, @"No bridge implementation is available, giving up.");
  return implClass;
 }

可以简化为看有没有 RCTCxxBridge 这个类，然后看下有没有 RCTBatchedBridge 这个类。最终结果是 RCTCxxBridge 这个类。
因为 RCTCxxBridge 是 C++ 文件，所以要加入 -lc++ 的编译选项。

初始化 RCTCxxBridge

_pendingCalls = [NSMutableArray new];
_displayLink = [RCTDisplayLink new];

[RCTBridge setCurrentBridge:self];  

初始化了一个数组，存储的是 dispatch_block_t。
创建了一个 RCTDisplayLink，即一个定时器，定时调用 _jsThreadUpdate 函数。
设置 RCTBridge 的 RCTCurrentBridgeInstance 是刚刚生成的实例。

调用 start 函数

开启一个 JS 线程，命名为 com.facebook.react.JavaScript。

  _jsThread = [[NSThread alloc] initWithTarget:self
                                      selector:@selector(runJSRunLoop)
                                        object:nil];
  _jsThread.name = RCTJSThreadName;
  _jsThread.qualityOfService = NSOperationQualityOfServiceUserInteractive;
  [_jsThread start];

初始化不能被延迟初始化的 Native 模块。

  [self _initModulesWithDispatchGroup:prepareBridge];

.....
加载 JS Bundle 中的代码

[self loadSource:^(NSError *error, NSData *source, int64_t sourceLength) {

执行代码

[strongSelf executeSourceCode:sourceCode sync:NO];