摘要：下面是的源碼如其名，是一個隊列操作，將要變更的統一插入隊列，待一一處理。只有控制下的事件周期，會執行切換狀態，保證批量操作能被截獲并插入堆棧。也就是源碼中的。他在初始被賦值為，也就是以下兩個對象，在源碼被稱作為。本文參考源碼版本為。

不知道什么時候開始，很多人開始認為setState是異步操作，所謂的異步操作，就是我們在執行了setState之后，立即通過this.state.xxx不能拿到更新之后的值。這樣的認知其實有一種先入為主的意識，也許是受到很多不知名博主的不科學言論導致的錯誤認知，也有可能是日常開發過程中積累的經驗。畢竟大部分開發寫setState這樣的方法，都是在組件的生命周期（如componentDidMount、componentWillMount）中，或者react的事件處理機制中，這種教科書式的寫代碼方式，基本不會碰到有數據異常。

雖然官方文檔對setState這種同步行為語焉不詳，但是我們可以發現某些情況下，setState是真的可以同步獲取數據的。通過本文我們可以了解react這方面的工作原理，對于我們的思考開發方案，解決疑難問題，避免不必要的錯誤，也許會有不少幫助。

我們先來說結論：

在React中，如果是由React引發的事件處理（比如通過onClick引發的事件處理，componentWillMount等生命周期），調用setState不會同步更新this.state；除此之外的setState調用會同步執行this.state。所謂“除此之外”，指的是繞過React通過addEventListener直接添加的事件處理函數，還有通過setTimeout/setInterval產生的異步調用。

不想看長篇大論的同學，到這里就可以結束了。想了解原理的同學請繼續參觀。。

用過angular框架的同學也許記得angular的代碼模式中有一個$timeout這樣的調用方法，和setTimeout功能基本一致，但是setTimeout卻不能實時觸發UI的更新。這是因為$timeout比setTimeout添加了對UI更新（臟檢查）的處理，在延時結束后立即調用更新方法更新UI的渲染。同樣的道理，我們必須使用react指定的方式更新state才能同步UI的渲染，因為react控制下的事件會同步處理UI的更新。而直接使用this.state.xxx = xxx這樣的方式僅僅改變了數據，沒有改變UI，這就不是React倡導的reactive programing了。

實際上，在react的源碼中我們會發現，大部分react控制下的事件或生命周期，會調用batchedUpdates（查看如下代碼）。這個方法會觸發component渲染的狀態isBatchingUpdates。同樣的，react的事件監聽機制會觸發batchedUpdates方法，同樣會將isBatchingUpdates狀態置為true。

// 更新狀態
batchingStrategy.batchedUpdates(method, component);

在組件渲染狀態isBatchingUpdates中，任何的setState都不會觸發更新，而是進入隊列。除此之外，通過setTimeout/setInterval產生的異步調用是可以同步更新state的。這樣的講解比較抽象，我們可以直接根據以下源碼開始理解。

下面我們來看下setState在源碼中的定義：

/**
 * 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");
  }
};

根據源碼中的注釋，有這么一句話。

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不能確保實時更新state，官方從來沒有說過setState是一種異步操作，但也沒有否認，只是告訴我們什么時候會觸發同步操作，什么時候是異步操作。所以我們工作中千萬不要被一些民間偏方蒙蔽雙眼，多看看源代碼，發現原理的同時，還可以發現很多好玩的東西，開源庫的好處就是在于我們能在源碼中發現真理。

我們在源碼的這段注釋里也能看到setState的一些有趣玩法，比如

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

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

回到正題，源碼中setState執行了this.updater.enqueueSetState方法和this.updater.enqueueCallback方法 ，暫且不論enqueueCallback，我們關注下enqueueSetState的作用。

下面是enqueueSetState的源碼：

/**
   * 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，是一個隊列操作，將要變更的state統一插入隊列，待一一處理。隊列數據_pengdingStateQueue會掛載在一個組件對象上internalInstance，對于internalInstance想要了解下的同學，可以參考下react源碼中的ReactInstanceMap這個概念。

隊列操作完成之后，就開始真正的更新操作了。

更新方法enqueueUpdate的源碼如下：

/**
 * 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);
}

第一次執行setState的時候，可以進入if語句，遇到里面的return語句，終止執行。如果不是正處于創建或更新組件階段，則處理update事務。

第二次執行setState的時候，進入不了if語句，將組件放入dirtyComponents。如果正在創建或更新組件,則暫且先不處理update,只是將組件放在dirtyComponents數組中。

enqueueUpdate包含了React避免重復render的邏輯。參考源碼中batchedUpdates的調用情況，mountComponent和updateComponent方法在執行的最開始，會調用到batchedUpdates進行批處理更新，這些是react實例的生命周期，此時會將isBatchingUpdates設置為true，也就是將狀態標記為現在正處于更新階段了。之后React以事務的方式處理組件update，事務處理完后會調用wrapper.close(), 而TRANSACTION_WRAPPERS中包含了RESET_BATCHED_UPDATES這個wrapper，故最終會調用RESET_BATCHED_UPDATES.close(), 它最終會將isBatchingUpdates設置為false。

聽不懂？聽不懂沒關系。。我們會一句句剖析。

enqueueUpdate和batchingStrategy的概念我們放一起考慮。

batchingStrategy簡單直譯叫做批量處理策略。這個是React處理批量state操作時的精髓，源碼如下：

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源碼中所述，每次執行更新前，會預先判斷isBatchingUpdates是否處理批量更新狀態，如我們常見的周期諸如componentWillMount、componentDidMount，都是處于isBatchingUpdates的批量更新狀態，此時執行的setState操作，不會進入if語句執行update，而是進入dirtyComponents的堆棧中。

這就是文章開頭所說的栗子，為什么setTimeout執行的setState會同步更新state，而react生命周期中執行的setState只能異步更新的原因。只有react控制下的事件周期，會執行batchedUpdates切換isBatchingUpdates狀態，保證批量操作能被截獲并插入堆棧。其他事件都和同步執行update方法無異。

執行batchedUpdates之后，會立即將isBatchingUpdates賦值為true，表明此時即將進入更新狀態，所有之后的setState進入隊列等待。

這里我們以普通的setTimeout為例，執行一次更新。業務代如下：

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

執行時isBatchingUpdates默認是false，所以當我們執行到batchedUpdates這一步的時候，源碼中alreadyBatchingUpdates被賦值為false，我們會跳過if進入else條件，執行下一階段transaction.perform。

perform為我們執行了UI更新的第一步預操作。這里我們會執行一系列更新初始化操作和更新狀態的關閉。該方法做了try-catch控制，大量數據操作有可能引發錯誤exception，perform方法在這里對錯誤做了截獲控制。

/**
 * 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;
}

源碼中執行了一些錯誤的預判，最終我們真正執行的是closeAll方法。關于state的數據更新，從close開始。

/**
 * 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;
}

在介紹close之前，我們先了解下兩個對象。也就是源碼中的this.transactionWrappers。他在初始被賦值為[FLUSH_BATCHED_UPDATES, RESET_BATCHED_UPDATES]，也就是以下兩個對象，在源碼被稱作為wrapper。

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

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

源碼中我們看到closeAll執行了一次for循環，并執行了每個wrapper的close方法。

RESET_BATCHED_UPDATES的close方法很簡單，把isBatchingUpdates更新中這個狀態做了一個close的操作，也就是賦值為false，表明本次批量更新已結束。

FLUSH_BATCHED_UPDATES的close方法執行的是flushBatchedUpdates方法。

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);
    }
  }
};

我們暫且不論asap是什么，可以看到flushBatchedUpdates做的是對dirtyComponents的批量處理操作，對于隊列中的每個component執行perform更新。這些更新都會執行真正的更新方法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中的核心處理是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);
  }
}

在這里我們終于又看到了我們熟悉的_pendingStateQueue，還記得這是什么嗎？是的，這就是state的更新隊列，performUpdateIfNecessary做了隊列的特殊判斷，避免導致錯誤更新。

接下來的這段代碼是updateComponent，源碼內容比較長，但是我們可以看到很多熟知的生命周期方法的身影，比如說componentWillReceiveProps和shouldComponentUpdate，做了component的更新判斷。

這部分方法統一歸屬于ReactCompositeComponentMixin模塊，有興趣了解整個生命周期的同學可以參考下源碼中的該模塊源碼，這里我們不再擴展，會繼續講解state的更新過程。

/**
 * 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;
  }
}

跳過除了state的其他源碼部分，我們可以看到該方法中仍然嵌套了一段對state的更新方法，這個方法就是state更新的終點_processPendingState。

為什么對state中的同一屬性做多次setState處理，不會得到多次更新？比如

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

那是因為源碼中的多個nextState的更新，只做了一次assign操作，如下源碼請查看：

_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;
}

有人說，React抽象來說，就是一個公式
UI=f(state).

的確如此，一個簡單的setState執行過程，內部暗藏了這么深的玄機，經歷多個模塊的處理，經歷多個錯誤處理機制以及對數據邊界的判斷，保證了一次更新的正常進行。同時我們也發現了為什么setState的操作不能簡單的說作是一個異步操作，大家應該在文章中已經找到了答案。

對其他react深層的理解，感興趣的同學可以多多參考下源碼。本文參考react源碼版本為15.0.1。