React setState

不知道什么时候开始，很多人开始认为setState是异步操作，所谓的异步操作，就是我们在执行了setState之后，立即通过this.state.xxx不能拿到更新之后的值。这样的认知其实有一种先入为主的意识，也许是受到很多不知名博主的不科学言论导致的错误认知，也有可能是日常开发过程中积累的经验。毕竟大部分开发写setState这样的方法，都是在组件的生命周期（如componentDidMount、componentWillMount）中，或者react的事件处理机制中，这种教科书式的写代码方式，基本不会碰到有数据异常。

虽然官方文档对setState这种同步行为语焉不详，但是我们可以发现某些情况下，setState是真的可以同步获取数据的。通过本文我们可以了解react这方面的工作原理，对于我们的思考开发方案，解决疑难问题，避免不必要的错误，也许会有不少帮助。

我们先来说结论：

在React中，如果是由React引发的事件处理（比如通过onClick引发的事件处理，componentWillMount等生命周期），调用setState不会同步更新this.state；除此之外的setState调用会同步执行this.state。所谓“除此之外”，指的是绕过React通过addEventListener直接添加的事件处理函数，还有通过setTimeout/setInterval产生的异步调用。

不想看长篇大论的同学，到这里就可以结束了。想了解原理的同学请继续参观。。

用过angular框架的同学也许记得angular的代码模式中有一个$timeout这样的调用方法，和setTimeout功能基本一致，但是setTimeout却不能实时触发UI的更新。这是因为$timeout比setTimeout添加了对UI更新（脏检查）的处理，在延时结束后立即调用更新方法更新UI的渲染。同样的道理，我们必须使用react指定的方式更新state才能同步UI的渲染，因为react控制下的事件会同步处理UI的更新。而直接使用this.state.xxx = xxx这样的方式仅仅改变了数据，没有改变UI，这就不是React倡导的reactive programing了。

实际上，在react的源码中我们会发现，大部分react控制下的事件或生命周期，会调用batchedUpdates（查看如下代码）。这个方法会触发component渲染的状态isBatchingUpdates。同样的，react的事件监听机制会触发batchedUpdates方法，同样会将isBatchingUpdates状态置为true。

// 更新状态
batchingStrategy.batchedUpdates(method, component);

在组件渲染状态isBatchingUpdates中，任何的setState都不会触发更新，而是进入队列。除此之外，通过setTimeout/setInterval产生的异步调用是可以同步更新state的。这样的讲解比较抽象，我们可以直接根据以下源码开始理解。

setState

下面我们来看下setState在源码中的定义：

/**
 * Sets a subset of the state. Always use this to mutate
 * state. You should treat `this.state` as immutable.
 *
 * There is no guarantee that `this.state` will be immediately updated, so
 * accessing `this.state` after calling this method may return the old value.
 *
 * There is no guarantee that calls to `setState` will run synchronously,
 * as they may eventually be batched together.  You can provide an optional
 * callback that will be executed when the call to setState is actually
 * completed.
 *
 * When a function is provided to setState, it will be called at some point in
 * the future (not synchronously). It will be called with the up to date
 * component arguments (state, props, context). These values can be different
 * from this.* because your function may be called after receiveProps but before
 * shouldComponentUpdate, and this new state, props, and context will not yet be
 * assigned to this.
 *
 * @param {object|function} partialState Next partial state or function to
 *        produce next partial state to be merged with current state.
 * @param {?function} callback Called after state is updated.
 * @final
 * @protected
 */
ReactComponent.prototype.setState = function (partialState, callback) {
  this.updater.enqueueSetState(this, partialState);
  if (callback) {
    this.updater.enqueueCallback(this, callback, 'setState');
  }
};

根据源码中的注释，有这么一句话。

There is no guarantee that this.state will be immediately updated, so accessing this.state after calling this method may return the old value.

大概意思就是setState不能确保实时更新state，官方从来没有说过setState是一种异步操作，但也没有否认，只是告诉我们什么时候会触发同步操作，什么时候是异步操作。所以我们工作中千万不要被一些民间偏方蒙蔽双眼，多看看源代码，发现原理的同时，还可以发现很多好玩的东西，开源库的好处就是在于我们能在源码中发现真理。

我们在源码的这段注释里也能看到setState的一些有趣玩法，比如

// 在回调中操作更新后的state
this.setState({
    count: 1
}, function () {
    console.log('# next State', this.state);
});

// 以非对象的形式操作
this.setState((state, props, context) => {
    return {
        count: state.count + 1
    }
});

回到正题，源码中setState执行了this.updater.enqueueSetState方法和this.updater.enqueueCallback方法 ，暂且不论enqueueCallback，我们关注下enqueueSetState的作用。

enqueueSetState

下面是enqueueSetState的源码：

/**
   * Sets a subset of the state. This only exists because _pendingState is
   * internal. This provides a merging strategy that is not available to deep
   * properties which is confusing. TODO: Expose pendingState or don't use it
   * during the merge.
   *
   * @param {ReactClass} publicInstance The instance that should rerender.
   * @param {object} partialState Next partial state to be merged with state.
   * @internal
   */
  enqueueSetState: function (publicInstance, partialState) {
    var internalInstance = getInternalInstanceReadyForUpdate(publicInstance, 'setState');

    if (!internalInstance) {
      return;
    }

    var queue = internalInstance._pendingStateQueue || (internalInstance._pendingStateQueue = []);
    queue.push(partialState);

    enqueueUpdate(internalInstance);
}

enqueueSetState如其名，是一个队列操作，将要变更的state统一插入队列，待一一处理。队列数据_pengdingStateQueue会挂载在一个组件对象上internalInstance，对于internalInstance想要了解下的同学，可以参考下react源码中的ReactInstanceMap这个概念。

队列操作完成之后，就开始真正的更新操作了。

enqueueUpdate

更新方法enqueueUpdate的源码如下：

/**
 * Mark a component as needing a rerender, adding an optional callback to a
 * list of functions which will be executed once the rerender occurs.
 */
function enqueueUpdate(component) {
  ensureInjected();

  // Various parts of our code (such as ReactCompositeComponent's
  // _renderValidatedComponent) assume that calls to render aren't nested;
  // verify that that's the case. (This is called by each top-level update
  // function, like setProps, setState, forceUpdate, etc.; creation and
  // destruction of top-level components is guarded in ReactMount.)

  if (!batchingStrategy.isBatchingUpdates) {
    batchingStrategy.batchedUpdates(enqueueUpdate, component);
    return;
  }

  dirtyComponents.push(component);
}

第一次执行setState的时候，可以进入if语句，遇到里面的return语句，终止执行。如果不是正处于创建或更新组件阶段，则处理update事务。

第二次执行setState的时候，进入不了if语句，将组件放入dirtyComponents。如果正在创建或更新组件,则暂且先不处理update,只是将组件放在dirtyComponents数组中。

enqueueUpdate包含了React避免重复render的逻辑。参考源码中batchedUpdates的调用情况，mountComponent和updateComponent方法在执行的最开始，会调用到batchedUpdates进行批处理更新，这些是react实例的生命周期，此时会将isBatchingUpdates设置为true，也就是将状态标记为现在正处于更新阶段了。之后React以事务的方式处理组件update，事务处理完后会调用wrapper.close(), 而TRANSACTION_WRAPPERS中包含了RESET_BATCHED_UPDATES这个wrapper，故最终会调用RESET_BATCHED_UPDATES.close(), 它最终会将isBatchingUpdates设置为false。

听不懂？听不懂没关系。。我们会一句句剖析。

enqueueUpdate和batchingStrategy的概念我们放一起考虑。

batchingStrategy简单直译叫做批量处理策略。这个是React处理批量state操作时的精髓，源码如下：

var ReactDefaultBatchingStrategy = {
  isBatchingUpdates: false,

  /**
   * Call the provided function in a context within which calls to `setState`
   * and friends are batched such that components aren't updated unnecessarily.
   */
  batchedUpdates: function (callback, a, b, c, d, e) {
    var alreadyBatchingUpdates = ReactDefaultBatchingStrategy.isBatchingUpdates;

    ReactDefaultBatchingStrategy.isBatchingUpdates = true;

    // The code is written this way to avoid extra allocations
    if (alreadyBatchingUpdates) {
      callback(a, b, c, d, e);
    } else {
      transaction.perform(callback, null, a, b, c, d, e);
    }
  }
};

如enqueueUpdate源码中所述，每次执行更新前，会预先判断isBatchingUpdates是否处理批量更新状态，如我们常见的周期诸如componentWillMount、componentDidMount，都是处于isBatchingUpdates的批量更新状态，此时执行的setState操作，不会进入if语句执行update，而是进入dirtyComponents的堆栈中。

这就是文章开头所说的栗子，为什么setTimeout执行的setState会同步更新state，而react生命周期中执行的setState只能异步更新的原因。只有react控制下的事件周期，会执行batchedUpdates切换isBatchingUpdates状态，保证批量操作能被截获并插入堆栈。其他事件都和同步执行update方法无异。

执行batchedUpdates之后，会立即将isBatchingUpdates赋值为true，表明此时即将进入更新状态，所有之后的setState进入队列等待。

这里我们以普通的setTimeout为例，执行一次更新。业务代如下：

setTimeout(function () {
    this.setState({
        count: this.state.count + 1
    });
}, 0);

执行时isBatchingUpdates默认是false，所以当我们执行到batchedUpdates这一步的时候，源码中alreadyBatchingUpdates被赋值为false，我们会跳过if进入else条件，执行下一阶段transaction.perform。

transaction.perform

perform为我们执行了UI更新的第一步预操作。这里我们会执行一系列更新初始化操作和更新状态的关闭。该方法做了try-catch控制，大量数据操作有可能引发错误exception，perform方法在这里对错误做了截获控制。

/**
 * Executes the function within a safety window. Use this for the top level
 * methods that result in large amounts of computation/mutations that would
 * need to be safety checked. The optional arguments helps prevent the need
 * to bind in many cases.
 *
 * @param {function} method Member of scope to call.
 * @param {Object} scope Scope to invoke from.
 * @param {Object?=} a Argument to pass to the method.
 * @param {Object?=} b Argument to pass to the method.
 * @param {Object?=} c Argument to pass to the method.
 * @param {Object?=} d Argument to pass to the method.
 * @param {Object?=} e Argument to pass to the method.
 * @param {Object?=} f Argument to pass to the method.
 *
 * @return {*} Return value from `method`.
 */
perform: function (method, scope, a, b, c, d, e, f) {
  !!this.isInTransaction() ? "development" !== 'production' ? invariant(false, 'Transaction.perform(...): Cannot initialize a transaction when there ' + 'is already an outstanding transaction.') : invariant(false) : void 0;
  var errorThrown;
  var ret;
  try {
    this._isInTransaction = true;
    // Catching errors makes debugging more difficult, so we start with
    // errorThrown set to true before setting it to false after calling
    // close -- if it's still set to true in the finally block, it means
    // one of these calls threw.
    errorThrown = true;
    this.initializeAll(0);
    ret = method.call(scope, a, b, c, d, e, f);
    errorThrown = false;
  } finally {
    try {
      if (errorThrown) {
        // If `method` throws, prefer to show that stack trace over any thrown
        // by invoking `closeAll`.
        try {
          this.closeAll(0);
        } catch (err) {}
      } else {
        // Since `method` didn't throw, we don't want to silence the exception
        // here.
        this.closeAll(0);
      }
    } finally {
      this._isInTransaction = false;
    }
  }
  return ret;
}

源码中执行了一些错误的预判，最终我们真正执行的是closeAll方法。关于state的数据更新，从close开始。

close

/**
 * Invokes each of `this.transactionWrappers.close[i]` functions, passing into
 * them the respective return values of `this.transactionWrappers.init[i]`
 * (`close`rs that correspond to initializers that failed will not be
 * invoked).
 */
closeAll: function (startIndex) {
  !this.isInTransaction() ? "development" !== 'production' ? invariant(false, 'Transaction.closeAll(): Cannot close transaction when none are open.') : invariant(false) : void 0;
  var transactionWrappers = this.transactionWrappers;
  for (var i = startIndex; i < transactionWrappers.length; i++) {
    var wrapper = transactionWrappers[i];
    var initData = this.wrapperInitData[i];
    var errorThrown;
    try {
      // Catching errors makes debugging more difficult, so we start with
      // errorThrown set to true before setting it to false after calling
      // close -- if it's still set to true in the finally block, it means
      // wrapper.close threw.
      errorThrown = true;
      if (initData !== Transaction.OBSERVED_ERROR && wrapper.close) {
        wrapper.close.call(this, initData);
      }
      errorThrown = false;
    } finally {
      if (errorThrown) {
        // The closer for wrapper i threw an error; close the remaining
        // wrappers but silence any exceptions from them to ensure that the
        // first error is the one to bubble up.
        try {
          this.closeAll(i + 1);
        } catch (e) {}
      }
    }
  }
  this.wrapperInitData.length = 0;
}

在介绍close之前，我们先了解下两个对象。也就是源码中的this.transactionWrappers。他在初始被赋值为[FLUSH_BATCHED_UPDATES, RESET_BATCHED_UPDATES]，也就是以下两个对象，在源码被称作为wrapper。

var RESET_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: function () {
    ReactDefaultBatchingStrategy.isBatchingUpdates = false;
  }
};

var FLUSH_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: ReactUpdates.flushBatchedUpdates.bind(ReactUpdates)
};

源码中我们看到closeAll执行了一次for循环，并执行了每个wrapper的close方法。

RESET_BATCHED_UPDATES的close方法很简单，把isBatchingUpdates更新中这个状态做了一个close的操作，也就是赋值为false，表明本次批量更新已结束。

FLUSH_BATCHED_UPDATES的close方法执行的是flushBatchedUpdates方法。

flushBatchedUpdates

var flushBatchedUpdates = function () {
  // ReactUpdatesFlushTransaction's wrappers will clear the dirtyComponents
  // array and perform any updates enqueued by mount-ready handlers (i.e.,
  // componentDidUpdate) but we need to check here too in order to catch
  // updates enqueued by setState callbacks and asap calls.
  while (dirtyComponents.length || asapEnqueued) {
    if (dirtyComponents.length) {
      var transaction = ReactUpdatesFlushTransaction.getPooled();
      transaction.perform(runBatchedUpdates, null, transaction);
      ReactUpdatesFlushTransaction.release(transaction);
    }

    if (asapEnqueued) {
      asapEnqueued = false;
      var queue = asapCallbackQueue;
      asapCallbackQueue = CallbackQueue.getPooled();
      queue.notifyAll();
      CallbackQueue.release(queue);
    }
  }
};

我们暂且不论asap是什么，可以看到flushBatchedUpdates做的是对dirtyComponents的批量处理操作，对于队列中的每个component执行perform更新。这些更新都会执行真正的更新方法runBatchedUpdates。

function runBatchedUpdates(transaction) {
  var len = transaction.dirtyComponentsLength;
  !(len === dirtyComponents.length) ? "development" !== 'production' ? invariant(false, 'Expected flush transaction\'s stored dirty-components length (%s) to ' + 'match dirty-components array length (%s).', len, dirtyComponents.length) : invariant(false) : void 0;

  // Since reconciling a component higher in the owner hierarchy usually (not
  // always -- see shouldComponentUpdate()) will reconcile children, reconcile
  // them before their children by sorting the array.
  dirtyComponents.sort(mountOrderComparator);

  for (var i = 0; i < len; i++) {
    // If a component is unmounted before pending changes apply, it will still
    // be here, but we assume that it has cleared its _pendingCallbacks and
    // that performUpdateIfNecessary is a noop.
    var component = dirtyComponents[i];

    // If performUpdateIfNecessary happens to enqueue any new updates, we
    // shouldn't execute the callbacks until the next render happens, so
    // stash the callbacks first
    var callbacks = component._pendingCallbacks;
    component._pendingCallbacks = null;

    var markerName;
    if (ReactFeatureFlags.logTopLevelRenders) {
      var namedComponent = component;
      // Duck type TopLevelWrapper. This is probably always true.
      if (component._currentElement.props === component._renderedComponent._currentElement) {
        namedComponent = component._renderedComponent;
      }
      markerName = 'React update: ' + namedComponent.getName();
      console.time(markerName);
    }

    ReactReconciler.performUpdateIfNecessary(component, transaction.reconcileTransaction);

    if (markerName) {
      console.timeEnd(markerName);
    }

    if (callbacks) {
      for (var j = 0; j < callbacks.length; j++) {
        transaction.callbackQueue.enqueue(callbacks[j], component.getPublicInstance());
      }
    }
  }
}

runBatchedUpdates中的核心处理是ReactReconciler.performUpdateIfNecessary。

/**
   * If any of `_pendingElement`, `_pendingStateQueue`, or `_pendingForceUpdate`
   * is set, update the component.
   *
   * @param {ReactReconcileTransaction} transaction
   * @internal
   */
performUpdateIfNecessary: function (transaction) {
  if (this._pendingElement != null) {
    ReactReconciler.receiveComponent(this, this._pendingElement, transaction, this._context);
  }

  if (this._pendingStateQueue !== null || this._pendingForceUpdate) {
    this.updateComponent(transaction, this._currentElement, this._currentElement, this._context, this._context);
  }
}

在这里我们终于又看到了我们熟悉的_pendingStateQueue，还记得这是什么吗？是的，这就是state的更新队列，performUpdateIfNecessary做了队列的特殊判断，避免导致错误更新。

接下来的这段代码是updateComponent，源码内容比较长，但是我们可以看到很多熟知的生命周期方法的身影，比如说componentWillReceiveProps和shouldComponentUpdate，做了component的更新判断。

这部分方法统一归属于ReactCompositeComponentMixin模块，有兴趣了解整个生命周期的同学可以参考下源码中的该模块源码，这里我们不再扩展，会继续讲解state的更新过程。

updateComponent

/**
 * Perform an update to a mounted component. The componentWillReceiveProps and
 * shouldComponentUpdate methods are called, then (assuming the update isn't
 * skipped) the remaining update lifecycle methods are called and the DOM
 * representation is updated.
 *
 * By default, this implements React's rendering and reconciliation algorithm.
 * Sophisticated clients may wish to override this.
 *
 * @param {ReactReconcileTransaction} transaction
 * @param {ReactElement} prevParentElement
 * @param {ReactElement} nextParentElement
 * @internal
 * @overridable
 */
updateComponent: function (transaction, prevParentElement, nextParentElement, prevUnmaskedContext, nextUnmaskedContext) {
  var inst = this._instance;
  var willReceive = false;
  var nextContext;
  var nextProps;

  // Determine if the context has changed or not
  if (this._context === nextUnmaskedContext) {
    nextContext = inst.context;
  } else {
    nextContext = this._processContext(nextUnmaskedContext);
    willReceive = true;
  }

  // Distinguish between a props update versus a simple state update
  if (prevParentElement === nextParentElement) {
    // Skip checking prop types again -- we don't read inst.props to avoid
    // warning for DOM component props in this upgrade
    nextProps = nextParentElement.props;
  } else {
    nextProps = this._processProps(nextParentElement.props);
    willReceive = true;
  }

  // An update here will schedule an update but immediately set
  // _pendingStateQueue which will ensure that any state updates gets
  // immediately reconciled instead of waiting for the next batch.
  if (willReceive && inst.componentWillReceiveProps) {
    inst.componentWillReceiveProps(nextProps, nextContext);
  }

  var nextState = this._processPendingState(nextProps, nextContext);

  var shouldUpdate = this._pendingForceUpdate || !inst.shouldComponentUpdate || inst.shouldComponentUpdate(nextProps, nextState, nextContext);

  if ("development" !== 'production') {
    "development" !== 'production' ? warning(shouldUpdate !== undefined, '%s.shouldComponentUpdate(): Returned undefined instead of a ' + 'boolean value. Make sure to return true or false.', this.getName() || 'ReactCompositeComponent') : void 0;
  }

  if (shouldUpdate) {
    this._pendingForceUpdate = false;
    // Will set `this.props`, `this.state` and `this.context`.
    this._performComponentUpdate(nextParentElement, nextProps, nextState, nextContext, transaction, nextUnmaskedContext);
  } else {
    // If it's determined that a component should not update, we still want
    // to set props and state but we shortcut the rest of the update.
    this._currentElement = nextParentElement;
    this._context = nextUnmaskedContext;
    inst.props = nextProps;
    inst.state = nextState;
    inst.context = nextContext;
  }
}

跳过除了state的其他源码部分，我们可以看到该方法中仍然嵌套了一段对state的更新方法，这个方法就是state更新的终点_processPendingState。

_processPendingState

为什么对state中的同一属性做多次setState处理，不会得到多次更新？比如

this.setState({ count: count++ });
this.set

那是因为源码中的多个nextState的更新，只做了一次assign操作，如下源码请查看：

_processPendingState: function (props, context) {
  var inst = this._instance;
  var queue = this._pendingStateQueue;
  var replace = this._pendingReplaceState;
  this._pendingReplaceState = false;
  this._pendingStateQueue = null;

  if (!queue) {
    return inst.state;
  }

  if (replace && queue.length === 1) {
    return queue[0];
  }

  var nextState = _assign({}, replace ? queue[0] : inst.state);
  for (var i = replace ? 1 : 0; i < queue.length; i++) {
    var partial = queue[i];
    _assign(nextState, typeof partial === 'function' ? partial.call(inst, nextState, props, context) : partial);
  }

  return nextState;
}

有人说，React抽象来说，就是一个公式
UI=f(state).

的确如此，一个简单的setState执行过程，内部暗藏了这么深的玄机，经历多个模块的处理，经历多个错误处理机制以及对数据边界的判断，保证了一次更新的正常进行。同时我们也发现了为什么setState的操作不能简单的说作是一个异步操作，大家应该在文章中已经找到了答案。

对其他react深层的理解，感兴趣的同学可以多多参考下源码。本文参考react源码版本为15.0.1。