React16源码: React中event事件触发的源码实现

event 事件触发过程

1 ）概述

• 在之前事件绑定时，绑定的是两个方法
  • 一个是 dispatchInteractiveEvent
  • 另外一个 dispatchEvent
• 其实它们调用的方法都是差不多的，一开始会有一点小的区别

2 ）源码

定位到 packages/react-dom/src/events/ReactDOMEventListener.js#L165

进入 trapCapturedEvent

export function trapCapturedEvent(
  topLevelType: DOMTopLevelEventType,
  element: Document | Element,
) {
  if (!element) {
    return null;
  }
  // 注意这里，根据是否是 Interactive 类型的事件，调用的不同的回调，最终赋值给 dispatch
  const dispatch = isInteractiveTopLevelEventType(topLevelType)
    ? dispatchInteractiveEvent
    : dispatchEvent;

  addEventCaptureListener(
    element,
    getRawEventName(topLevelType),
    // Check if interactive and wrap in interactiveUpdates
    // 这边的topleveltype呢，是我们在进行 dom 的事件绑定的时候已经通过 bind 给它绑定好了
    // 在绑定事件的时候，就已经确定了一个值, 比如说是onChange这类的toplevel的事件名称
    dispatch.bind(null, topLevelType),
  );
}

2.1 先看 dispatchInteractiveEvent

定位到 packages/react-dom/src/events/ReactDOMEventListener.js#L184

// packages/react-dom/src/events/ReactDOMEventListener.js#L184
// nativeEvent，就是我们事件触发的时候，我们的domm的事件体系会给我们一个event对象
// 可以通过它来进入默认行为之类的这么一个事件对象
function dispatchInteractiveEvent(topLevelType, nativeEvent) {
  interactiveUpdates(dispatchEvent, topLevelType, nativeEvent);
}

// packages/events/ReactGenericBatching.js#L55
export function interactiveUpdates(fn, a, b) {
  return _interactiveUpdatesImpl(fn, a, b);
}

// packages/events/ReactGenericBatching.js#L23
let _interactiveUpdatesImpl = function(fn, a, b) {
  return fn(a, b);
};

• 可见 dispatchInteractiveEvent 最终还是要调用 dispatchEvent, 只是多包了一层

2.2 进入 dispatchEvent

定位到 packages/react-dom/src/events/ReactDOMEventListener.js#L188

export function dispatchEvent(
  topLevelType: DOMTopLevelEventType,
  nativeEvent: AnyNativeEvent,
) {
  if (!_enabled) {
    return;
  }

  // 注意这里
  const nativeEventTarget = getEventTarget(nativeEvent);
  let targetInst = getClosestInstanceFromNode(nativeEventTarget);
  // 存在 符合条件的，并且没有被挂载
  if (
    targetInst !== null &&
    typeof targetInst.tag === 'number' &&
    !isFiberMounted(targetInst)
  ) {
    // If we get an event (ex: img onload) before committing that
    // component's mount, ignore it for now (that is, treat it as if it was an
    // event on a non-React tree). We might also consider queueing events and
    // dispatching them after the mount.
    targetInst = null; // 这个 target 置空
  }

  // 这里只是一个对象，用于携带信息
  const bookKeeping = getTopLevelCallbackBookKeeping(
    topLevelType,
    nativeEvent,
    targetInst,
  );

  try {
    // Event queue being processed in the same cycle allows
    // `preventDefault`.
    batchedUpdates(handleTopLevel, bookKeeping);
  } finally {
    releaseTopLevelCallbackBookKeeping(bookKeeping);
  }
}

• 首先它要获取 nativeEventTarget，这个 target 就是我们 event 对象上面的 target

• 它是出于对各种系统的一个兼容调用的一个方法来进行一个 Polyfill

• 进入这个 getEventTarget 方法

  import {TEXT_NODE} from '../shared/HTMLNodeType';
  
  /**
   * Gets the target node from a native browser event by accounting for
   * inconsistencies in browser DOM APIs.
   *
   * @param {object} nativeEvent Native browser event.
   * @return {DOMEventTarget} Target node.
   */
  function getEventTarget(nativeEvent) {
    // 这个是对 IE9 的兼容
    // Fallback to nativeEvent.srcElement for IE9
    // https://github.com/facebook/react/issues/12506
    let target = nativeEvent.target || nativeEvent.srcElement || window;
  
    // Normalize SVG <use> element events #4963
    if (target.correspondingUseElement) {
      target = target.correspondingUseElement;
    }
  
    // Safari may fire events on text nodes (Node.TEXT_NODE is 3).
    // @see http://www.quirksmode.org/js/events_properties.html
    return target.nodeType === TEXT_NODE ? target.parentNode : target;
  }

  • 这个方法主要是为了兼容浏览器API获取 target 对象, 最终根据是否是 TEXT_NODE 返回 target.parentNode 或 target

• 进入 getClosestInstanceFromNode

  export function getClosestInstanceFromNode(node) {
    // 存在，直接 return
    if (node[internalInstanceKey]) {
      return node[internalInstanceKey]; // 这个就是 初始化dom节点的时候，在node上插入这么一个key, 来指定对应的 fiber 对象
    }
  
    // 不存在，一直找 parentNode
    while (!node[internalInstanceKey]) {
      if (node.parentNode) {
        node = node.parentNode;
      } else {
        // Top of the tree. This node must not be part of a React tree (or is
        // unmounted, potentially).
        return null;
      }
    }
    // 这个 inst 就是一个 fiber 对象， 找到是 HostComponent 或 HostText 类型的
    let inst = node[internalInstanceKey];
    if (inst.tag === HostComponent || inst.tag === HostText) {
      // In Fiber, this will always be the deepest root.
      return inst;
    }
  
    return null;
  }

• 进入 isFiberMounted

  const MOUNTING = 1;
  const MOUNTED = 2;
  const UNMOUNTED = 3;
  
  function isFiberMountedImpl(fiber: Fiber): number {
    let node = fiber;
    // 不存在，说明是即将插入或没有插入的节点
    if (!fiber.alternate) {
      // If there is no alternate, this might be a new tree that isn't inserted
      // yet. If it is, then it will have a pending insertion effect on it.
      // 这种是即将要插入的
      if ((node.effectTag & Placement) !== NoEffect) {
        return MOUNTING;
      }
      // 如果当前不是，向上找父节点，也是即将插入的
      while (node.return) {
        node = node.return;
        if ((node.effectTag & Placement) !== NoEffect) {
          return MOUNTING;
        }
      }
    } else {
      // 存在 alternate, 继续向上找
      while (node.return) {
        node = node.return;
      }
    }
    // 如果上级存在，并且是 HostRoot 说明已经被挂载了
    if (node.tag === HostRoot) {
      // TODO: Check if this was a nested HostRoot when used with
      // renderContainerIntoSubtree.
      return MOUNTED;
    }
    // If we didn't hit the root, that means that we're in an disconnected tree
    // that has been unmounted.
    // 其他情况都是未挂载的
    return UNMOUNTED;
  }
  
  export function isFiberMounted(fiber: Fiber): boolean {
    return isFiberMountedImpl(fiber) === MOUNTED;
  }

• 进入 getTopLevelCallbackBookKeeping

  // packages/react-dom/src/events/ReactDOMEventListener.js#L50
  const CALLBACK_BOOKKEEPING_POOL_SIZE = 10;
  const callbackBookkeepingPool = [];
  
  // Used to store ancestor hierarchy in top level callback
  function getTopLevelCallbackBookKeeping(
    topLevelType,
    nativeEvent,
    targetInst,
  ): {
    topLevelType: ?DOMTopLevelEventType,
    nativeEvent: ?AnyNativeEvent,
    targetInst: Fiber | null,
    ancestors: Array<Fiber>,
  } {
    if (callbackBookkeepingPool.length) {
      const instance = callbackBookkeepingPool.pop();
      instance.topLevelType = topLevelType;
      instance.nativeEvent = nativeEvent;
      instance.targetInst = targetInst;
      return instance;
    }
    return {
      topLevelType,
      nativeEvent,
      targetInst,
      ancestors: [],
    };
  }

  • 这个函数就是一缓存机制
  • 最终我们在执行完整个react事件之后，会把这个对象再归还到这个pool里面来进行一个存储
  • 让浏览器的js运行环境不去删除这个对象，也让我们在后期不需要去重新创建一个对象，
  • 以减少这个对象声明和对象垃圾回收的一个性能开销

• 进入 batchedUpdates

  // packages/events/ReactGenericBatching.js#L29
  let _batchedUpdatesImpl = function(fn, bookkeeping) {
    return fn(bookkeeping);
  };
  let isBatching = false;
  export function batchedUpdates(fn, bookkeeping) {
    if (isBatching) {
      // If we are currently inside another batch, we need to wait until it
      // fully completes before restoring state.
      return fn(bookkeeping);
    }
    isBatching = true;
    try {
      return _batchedUpdatesImpl(fn, bookkeeping);
    } finally {
      // Here we wait until all updates have propagated, which is important
      // when using controlled components within layers:
      // https://github.com/facebook/react/issues/1698
      // Then we restore state of any controlled component.
      isBatching = false;
      // 下面的代码其实就跟 input 控制输入 有关的
      // 我们知道在 react 当中我们通过 value 给 input 标签上面去绑定了值，直接在外部输入的值
      // 如果没有 onChange 事件来处理这个 state，它这个值输不进去的，这就是 input的控制输入 的一个概念
      // 其控制就是在这里实现的
      const controlledComponentsHavePendingUpdates = needsStateRestore();
      if (controlledComponentsHavePendingUpdates) {
        // If a controlled event was fired, we may need to restore the state of
        // the DOM node back to the controlled value. This is necessary when React
        // bails out of the update without touching the DOM.
        _flushInteractiveUpdatesImpl();
        restoreStateIfNeeded();
      }
    }
  }
  
  // packages/events/ReactControlledComponent.js#L58
  export function needsStateRestore(): boolean {
    return restoreTarget !== null || restoreQueue !== null;
  }
  
  export function restoreStateIfNeeded() {
    if (!restoreTarget) {
      return;
    }
    const target = restoreTarget;
    const queuedTargets = restoreQueue;
    restoreTarget = null;
    restoreQueue = null;
  
    restoreStateOfTarget(target);
    if (queuedTargets) {
      for (let i = 0; i < queuedTargets.length; i++) {
        restoreStateOfTarget(queuedTargets[i]);
      }
    }
  }

• batchedUpdates 里面调用的方法是 handleTopLevel

  function handleTopLevel(bookKeeping) {
    let targetInst = bookKeeping.targetInst;
  
    // Loop through the hierarchy, in case there's any nested components.
    // It's important that we build the array of ancestors before calling any
    // event handlers, because event handlers can modify the DOM, leading to
    // inconsistencies with ReactMount's node cache. See #1105.
    let ancestor = targetInst;
    do {
      // 如果不存在，就push，并且跳出循环
      if (!ancestor) {
        bookKeeping.ancestors.push(ancestor);
        break;
      }
      // 如果存在了，找到 Root 挂载节点
      const root = findRootContainerNode(ancestor);
      // 不存在挂载节点，则跳出
      if (!root) {
        break;
      }
      // 存在挂载节点
      bookKeeping.ancestors.push(ancestor);
      // 这个方法之前已经分析了，找到存储的 fiber 对象
      // 在这里, 这个时候传入了是这个root，对于大部分情况来讲, HostRoot 已经没有上级的节点，会是处于react的一个应用当中
      // 也可能是会有这种情况的, 比如通过一些比较 hack 的一些方式, 我们在react应用里面再去渲染了一个新的react应用
      // 这种方法也可能是存在的，所以在这边就尝试了这么去做
      // 因为它们两个如果确实出现这种情况，那么它们的root节点是不一样的
      // 事件正常来讲是要冒泡到最外层的那个root树的最顶上的
      // 所以这种情况需要去调用这个方法，去把所有的 ancestor 给找到，并且推到这个 bookKeeping.ancestors 里面
      ancestor = getClosestInstanceFromNode(root); 
    } while (ancestor);
  
    // 对于每一个 concest，去获取它的 targetInst 对大部分情况, 其实就是我们这个 targetInst 
    // 就是我们这个事件触发的那一个节点 event.target 对应的那个fiber对象
    for (let i = 0; i < bookKeeping.ancestors.length; i++) {
      targetInst = bookKeeping.ancestors[i];
      runExtractedEventsInBatch(
        bookKeeping.topLevelType,
        targetInst,
        bookKeeping.nativeEvent,
        getEventTarget(bookKeeping.nativeEvent),
      );
    }
  }

  • 进入 findRootContainerNode

    function findRootContainerNode(inst) {
      // TODO: It may be a good idea to cache this to prevent unnecessary DOM
      // traversal, but caching is difficult to do correctly without using a
      // mutation observer to listen for all DOM changes.
      // 向上找
      while (inst.return) {
        inst = inst.return;
      }
      // 如果不是 HostRoot 则失败
      if (inst.tag !== HostRoot) {
        // This can happen if we're in a detached tree.
        return null;
      }
      // 找到 react 应用挂载的 dom节点
      return inst.stateNode.containerInfo;
    }

  • 进入 runExtractedEventsInBatch

    export function runExtractedEventsInBatch(
      topLevelType: TopLevelType,
      targetInst: null | Fiber,
      nativeEvent: AnyNativeEvent,
      nativeEventTarget: EventTarget,
    ) {
      // 获取所有事件
      const events = extractEvents(
        topLevelType,
        targetInst,
        nativeEvent,
        nativeEventTarget,
      );
      runEventsInBatch(events);
    }
    
    // 生成事件对象
    function extractEvents(
      topLevelType: TopLevelType,
      targetInst: null | Fiber,
      nativeEvent: AnyNativeEvent,
      nativeEventTarget: EventTarget,
    ): Array<ReactSyntheticEvent> | ReactSyntheticEvent | null {
      let events = null;
      // 调用每一个plugin, 在内部，调用 possiblePlugin.extractEvents
      for (let i = 0; i < plugins.length; i++) {
        // Not every plugin in the ordering may be loaded at runtime.
        const possiblePlugin: PluginModule<AnyNativeEvent> = plugins[i];
        if (possiblePlugin) {
          const extractedEvents = possiblePlugin.extractEvents(
            topLevelType,
            targetInst,
            nativeEvent,
            nativeEventTarget,
          );
          // 如果存在，则插入到对象中
          if (extractedEvents) {
            events = accumulateInto(events, extractedEvents);
          }
        }
      }
      return events;
    }
    
    let eventQueue: ?(Array<ReactSyntheticEvent> | ReactSyntheticEvent) = null;
    export function runEventsInBatch(
      events: Array<ReactSyntheticEvent> | ReactSyntheticEvent | null,
    ) {
      if (events !== null) {
        eventQueue = accumulateInto(eventQueue, events);
      }
    
      // Set `eventQueue` to null before processing it so that we can tell if more
      // events get enqueued while processing.
      const processingEventQueue = eventQueue;
      eventQueue = null;
    
      if (!processingEventQueue) {
        return;
      }
      // 对这个 processingEventQueue, 可能是数组，也可能只有一个event的这个内容
      // 对它调用了这个 executeDispatchesAndReleaseTopLevel 方法
      forEachAccumulated(processingEventQueue, executeDispatchesAndReleaseTopLevel);
      invariant(
        !eventQueue,
        'processEventQueue(): Additional events were enqueued while processing ' +
          'an event queue. Support for this has not yet been implemented.',
      );
      // This would be a good time to rethrow if any of the event handlers threw.
      rethrowCaughtError();
    }

    • 进入 accumulateInto

      // 把两个值(数组)合并，形成一个数组
      function accumulateInto<T>(
        current: ?(Array<T> | T),
        next: T | Array<T>,
      ): T | Array<T> {
        invariant(
          next != null,
          'accumulateInto(...): Accumulated items must not be null or undefined.',
        );
      
        if (current == null) {
          return next;
        }
      
        // Both are not empty. Warning: Never call x.concat(y) when you are not
        // certain that x is an Array (x could be a string with concat method).
        // 合并两个数组
        if (Array.isArray(current)) {
          if (Array.isArray(next)) {
            current.push.apply(current, next);
            return current;
          }
          current.push(next);
          return current;
        }
      
        if (Array.isArray(next)) {
          // A bit too dangerous to mutate `next`.
          return [current].concat(next);
        }
      
        return [current, next];
      }

    • 进入 forEachAccumulated

      // 对于传进来的一个数组，判断它是否是一个数组
      // 如果是数组就会去对每一项调用这个 callback
      // 如果它不是一个数组，就直接调用这个callback传入这个值就可以了
      function forEachAccumulated<T>(
        arr: ?(Array<T> | T),
        cb: (elem: T) => void,
        scope: ?any,
      ) {
        if (Array.isArray(arr)) {
          arr.forEach(cb, scope);
        } else if (arr) {
          cb.call(scope, arr);
        }
      }

    • 进入 executeDispatchesAndReleaseTopLevel

      const executeDispatchesAndReleaseTopLevel = function(e) {
        return executeDispatchesAndRelease(e);
      };
      
      const executeDispatchesAndRelease = function(event: ReactSyntheticEvent) {
        if (event) {
          executeDispatchesInOrder(event);
      
          if (!event.isPersistent()) {
            event.constructor.release(event);
          }
        }
      };
      
      // packages/events/EventPluginUtils.js#L76
      // 最后真正调用这个事件的地方，其实它里面整个过程会非常的复杂，有各种各样的函数的嵌套调用
      // 其实里面可能有将近一半的函数都是工具类型的函数，注意阅读代码时，别被绕进去
      export function executeDispatchesInOrder(event) {
        // 这边获取了 dispatchListeners 以及 dispatchInstances 这两个数据
        // 这两个数据都来自 event 对象，上面会挂载了一个叫 _dispatchListeners 和 _dispatchInstances
        // 这两个东西都是数组，并且是一一对应的关系
        const dispatchListeners = event._dispatchListeners;
        const dispatchInstances = event._dispatchInstances;
        // 忽略
        if (__DEV__) {
          validateEventDispatches(event);
        }
        // 然后，它去判断一下是否是一个数组, 如果是一个数组对它进行一个遍历
        if (Array.isArray(dispatchListeners)) {
          for (let i = 0; i < dispatchListeners.length; i++) {
            // 并且判断一下 event 是否已经 isPropagationStopped，就是我们已经停止冒泡了
            // 如果是的话，我们就直接 break
            if (event.isPropagationStopped()) {
              break;
            }
            // Listeners and Instances are two parallel arrays that are always in sync.
            executeDispatch(event, dispatchListeners[i], dispatchInstances[i]);
          }
        // 不是数组，但是存在
        } else if (dispatchListeners) {
          // 调用这个方法
          executeDispatch(event, dispatchListeners, dispatchInstances);
        }
        // 重置
        event._dispatchListeners = null;
        event._dispatchInstances = null;
      }

      • 进入 executeDispatch

        function executeDispatch(event, listener, inst) {
          const type = event.type || 'unknown-event';
          event.currentTarget = getNodeFromInstance(inst); // 从 inst 上获取 node 节点
          invokeGuardedCallbackAndCatchFirstError(type, listener, undefined, event); // 这里回调最终被触发调用
          event.currentTarget = null;
        }

        • 进入 invokeGuardedCallbackAndCatchFirstError

          // packages/shared/ReactErrorUtils.js#L67
          export function invokeGuardedCallbackAndCatchFirstError<
            A,
            B,
            C,
            D,
            E,
            F,
            Context,
          >(
            name: string | null,
            func: (a: A, b: B, c: C, d: D, e: E, f: F) => void,
            context: Context,
            a: A,
            b: B,
            c: C,
            d: D,
            e: E,
            f: F,
          ): void {
            // 这个函数之前遇到过，目前不再展开
            invokeGuardedCallback.apply(this, arguments);
            if (hasError) {
              const error = clearCaughtError();
              if (!hasRethrowError) {
                hasRethrowError = true;
                rethrowError = error;
              }
            }
          }

• 到这里为止，这边的 listener 就已经被调用了

• 我们真正的每一个节点上面，如果有绑定这个事件，它就会调用它的一个回调

• 这就是事件触发的整个流程, 非常的繁琐，有各种各样的方法，嵌套的调用