1 还是先声明一个组件，从最开始一步步来寻源；
class App extends Component {
//只在组件重新加载的时候执行一次
constructor(props) {
super(props);
//..
}
//other methods
}
//ReactBaseClasses.js中如下：这里就是setState函数的来源；
//super其实就是下面这个函数
function ReactComponent(props, context, updater) {
this.props = props;
this.context = context;
this.refs = emptyObject;
// We initialize the default updater but the real one gets injected by the
// renderer.
this.updater = updater || ReactNoopUpdateQueue;
}
ReactComponent.prototype.setState = function (partialState, callback) {
this.updater.enqueueSetState(this, partialState);
if (callback) {
this.updater.enqueueCallback(this, callback, 'setState');
}
};
所以主要来看是否传入了updater参数，也就是说何时进行 new 组件；具体的updater参数是怎么传递进来的，以及是那个对象，参见
react源码分析系列文章下面的react中context updater到底是如何传递的
这里直接说结果，updater对象其实就是ReactUpdateQueue.js 中暴漏出的ReactUpdateQueue对象；
2 既然找到了setState之后执行的动作，我们在一步步深入进去
class Root extends React.Component {
constructor(props) {
super(props);
this.state = {
count: 0
};
}
componentDidMount() {
let me = this;
me.setState({
count: me.state.count + 1
});
console.log(me.state.count); // 打印出0
me.setState({
count: me.state.count + 1
});
console.log(me.state.count); // 打印出0
setTimeout(function(){
me.setState({
count: me.state.count + 1
});
console.log(me.state.count); // 打印出2
}, 0);
setTimeout(function(){
me.setState({
count: me.state.count + 1
});
console.log(me.state.count); // 打印出3
}, 0);
}
render() {
return (
<h1>{this.state.count}</h1>
)
}
}
ReactComponent.prototype.setState = function (partialState, callback) {
this.updater.enqueueSetState(this, partialState);
if (callback) {
this.updater.enqueueCallback(this, callback, 'setState');
}
};
ReactUpdateQueue.js
var ReactUpdates = require('./ReactUpdates');
function enqueueUpdate(internalInstance) {
ReactUpdates.enqueueUpdate(internalInstance);
};
function getInternalInstanceReadyForUpdate(publicInstance, callerName) {
//在ReactCompositeComponent.js中有这样一行代码，这就是其来源；
// Store a reference from the instance back to the internal representation
//ReactInstanceMap.set(inst, this);
var internalInstance = ReactInstanceMap.get(publicInstance);
//返回的是在ReactCompositeComponent.js中construct函数返回的对象；ReactInstance实例对象并不是简单的new 我们写的组件的实例对象，而是经过instantiateReactComponent.js中ReactCompositeComponentWrapper函数包装的对象；详见 创建React组件方式以及源码分析.md
return internalInstance;
};
var ReactUpdateQueue = {
//。。。。省略其他代码
enqueueCallback: function (publicInstance, callback, callerName) {
ReactUpdateQueue.validateCallback(callback, callerName);
var internalInstance = getInternalInstanceReadyForUpdate(publicInstance);
if (!internalInstance) {
return null;
}
//这里将callback放入组件实例的_pendingCallbacks数组中；
if (internalInstance._pendingCallbacks) {
internalInstance._pendingCallbacks.push(callback);
} else {
internalInstance._pendingCallbacks = [callback];
}
// TODO: The callback here is ignored when setState is called from
// componentWillMount. Either fix it or disallow doing so completely in
// favor of getInitialState. Alternatively, we can disallow
// componentWillMount during server-side rendering.
enqueueUpdate(internalInstance);
},
enqueueSetState: function (publicInstance, partialState) {
var internalInstance = getInternalInstanceReadyForUpdate(publicInstance, 'setState');
if (!internalInstance) {
return;
}
//这里，初始化queue变量，同时初始化 internalInstance._pendingStateQueue = [ ] ;
//对于 || 的短路运算还是要多梳理下
//queue数组（模拟队列）中存放着setState放进来的对象；
var queue = internalInstance._pendingStateQueue || (internalInstance._pendingStateQueue = []);
//这里将partialState放入queue数组中，也就是internalInstance._pendingStateQueue 数组中，此时，每次setState的partialState,都放进了React组件实例对象上的_pendingStateQueue属性中，成为一个数组；
queue.push(partialState);
enqueueUpdate(internalInstance);
},
};
module.exports = ReactUpdateQueue;
可以看到enqueueSetState enqueueCallback 最后都会执行enqueueUpdate；
function enqueueUpdate(internalInstance) {
ReactUpdates.enqueueUpdate(internalInstance);
}
ReactUpdates.js
var dirtyComponents = [];
var updateBatchNumber = 0;
var asapCallbackQueue = CallbackQueue.getPooled();
var asapEnqueued = false;
//这里声明batchingStrategy为null,后期通过注册给其赋值；
var batchingStrategy = null;
//这里的component参数是js中ReactCompositeComponentWrapper函数包装的后的React组件实例对象；
function enqueueUpdate(component) {
ensureInjected();
//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
if (!batchingStrategy.isBatchingUpdates) {
//batchedUpdates就是ReactDefaultBatchingStrategy.js中声明的
batchingStrategy.batchedUpdates(enqueueUpdate, component);
return;
}
//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
dirtyComponents.push(component);
if (component._updateBatchNumber == null) {
component._updateBatchNumber = updateBatchNumber + 1;
}
};
//enqueueUpdate包含了React避免重复render的逻辑。mountComponent和updateComponent方法在执行的最开始，会调用到batchedUpdates进行批处理更新，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。
ReactDefaultBatchingStrategy.js（如何注册的可以看下https://github.com/jimwmg/JiM…，React中context和updater是如何传递的 和 React中的注册机制）
ReactDefaultBatchingStrategy.js
//RESET_BATCHED_UPDATES用来管理isBatchingUpdates的状态
var RESET_BATCHED_UPDATES = {
initialize: emptyFunction,
close: function () {
// 事务批更新处理结束时，将isBatchingUpdates设为了false
ReactDefaultBatchingStrategy.isBatchingUpdates = false;
}
};
//FLUSH_BATCHED_UPDATES会在一个transaction的close阶段运行runBatchedUpdates，从而执行update。
//因为close的执行顺序是FLUSH_BATCHED_UPDATES.close ==> 然后RESET_BATCHED_UPDATES.close
var FLUSH_BATCHED_UPDATES = {
initialize: emptyFunction,
close: ReactUpdates.flushBatchedUpdates.bind(ReactUpdates)
};
var TRANSACTION_WRAPPERS = [FLUSH_BATCHED_UPDATES, RESET_BATCHED_UPDATES];
function ReactDefaultBatchingStrategyTransaction() {
this.reinitializeTransaction();
}
_assign(ReactDefaultBatchingStrategyTransaction.prototype, Transaction, {
getTransactionWrappers: function () {
return TRANSACTION_WRAPPERS;
}
});
//这个transition就是下面ReactDefaultBatchingStrategy对象中使用的transaction变量
var transaction = new ReactDefaultBatchingStrategyTransaction();
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;
// 批处理最开始时，将isBatchingUpdates设为true，表明正在更新
ReactDefaultBatchingStrategy.isBatchingUpdates = true;
// The code is written this way to avoid extra allocations
if (alreadyBatchingUpdates) {
return callback(a, b, c, d, e);
} else {
//transition在上面已经声明； // 以事务的方式处理updates，后面详细分析transaction
return transaction.perform(callback, null, a, b, c, d, e);
}
}
};
module.exports = ReactDefaultBatchingStrategy;
接下来我们看下React中的事物处理机制到底是如何运行的；
Transaction.js
var _prodInvariant = require('./reactProdInvariant');
var invariant = require('fbjs/lib/invariant');
var OBSERVED_ERROR = {};
var TransactionImpl = {
reinitializeTransaction: function () {
//getTransactionWrappers这个函数ReactDefaultBatchingStrategy.js中声明的，上面有；返回一个数组；
this.transactionWrappers = this.getTransactionWrappers();
if (this.wrapperInitData) {
this.wrapperInitData.length = 0;
} else {
this.wrapperInitData = [];
}
this._isInTransaction = false;
},
_isInTransaction: false,
getTransactionWrappers: null,
isInTransaction: function () {
return !!this._isInTransaction;
},
perform: function (method, scope, a, b, c, d, e, f) {
var errorThrown;
var ret;
try {
this._isInTransaction = true;
errorThrown = true;
//var TRANSACTION_WRAPPERS = [FLUSH_BATCHED_UPDATES, RESET_BATCHED_UPDATES];
//1 这里会先执行所有的TRANSACTION_WRAPPERS中成员的initialize方法，上面声明的其都是emptyFunction
this.initializeAll(0);
//2 这里其实还是执行的 enqueueUpdate 函数
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.
//3 执行TRANSACTION_WRAPPERS对象中成员的所有close方法；
this.closeAll(0);
}
} finally {
this._isInTransaction = false;
}
}
return ret;
},
initializeAll: function (startIndex) {
var transactionWrappers = this.transactionWrappers;
for (var i = startIndex; i < transactionWrappers.length; i++) {
var wrapper = transactionWrappers[i];
try {
this.wrapperInitData[i] = OBSERVED_ERROR;
this.wrapperInitData[i] = wrapper.initialize ? wrapper.initialize.call(this) : null;
} finally {
if (this.wrapperInitData[i] === OBSERVED_ERROR) {
try {
this.initializeAll(i + 1);
} catch (err) {}
}
}
}
},
closeAll: function (startIndex) {
var transactionWrappers = this.transactionWrappers;
for (var i = startIndex; i < transactionWrappers.length; i++) {
var wrapper = transactionWrappers[i];
var initData = this.wrapperInitData[i];
var errorThrown;
try {
errorThrown = true;
if (initData !== OBSERVED_ERROR && wrapper.close) {
wrapper.close.call(this, initData);
}
errorThrown = false;
} finally {
if (errorThrown) {
try {
this.closeAll(i + 1);
} catch (e) {}
}
}
}
this.wrapperInitData.length = 0;
}
};
module.exports = TransactionImpl
//3 执行TRANSACTION_WRAPPERS对象中成员的所有close方法；
var FLUSH_BATCHED_UPDATES = {
initialize: emptyFunction,
close: ReactUpdates.flushBatchedUpdates.bind(ReactUpdates)
};
接着会执行ReactUpdates.js中的flushBatchedUpdates方法
ReactUpdates.js中
var flushBatchedUpdates = function () {
while (dirtyComponents.length || asapEnqueued) {
if (dirtyComponents.length) {
var transaction = ReactUpdatesFlushTransaction.getPooled();
//这里执行runBatchedUpdates函数；
transaction.perform(runBatchedUpdates, null, transaction);
ReactUpdatesFlushTransaction.release(transaction);
}
if (asapEnqueued) {
asapEnqueued = false;
var queue = asapCallbackQueue;
asapCallbackQueue = CallbackQueue.getPooled();
queue.notifyAll();
CallbackQueue.release(queue);
}
}
};
function runBatchedUpdates(transaction) {
var len = transaction.dirtyComponentsLength;
dirtyComponents.sort(mountOrderComparator);
updateBatchNumber++;
for (var i = 0; i < len; i++) {
var component = dirtyComponents[i];
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.type.isReactTopLevelWrapper) {
namedComponent = component._renderedComponent;
}
markerName = 'React update: ' + namedComponent.getName();
console.time(markerName);
}
//这里才是真正的开始更新组件
ReactReconciler.performUpdateIfNecessary(component, transaction.reconcileTransaction, updateBatchNumber);
if (markerName) {
console.timeEnd(markerName);
}
if (callbacks) {
for (var j = 0; j < callbacks.length; j++) {
transaction.callbackQueue.enqueue(callbacks[j], component.getPublicInstance());
}
}
}
}
ReactReconciler.js中
performUpdateIfNecessary: function (internalInstance, transaction, updateBatchNumber) {
if (internalInstance._updateBatchNumber !== updateBatchNumber) {
// The component's enqueued batch number should always be the current
// batch or the following one.
return;
}
//这里执行React组件实例对象的更新；internalInstance上的performUpdateIfNecessary在ReactCompositeComponent.js中的；
internalInstance.performUpdateIfNecessary(transaction);
if (process.env.NODE_ENV !== 'production') {
if (internalInstance._debugID !== 0) {
ReactInstrumentation.debugTool.onUpdateComponent(internalInstance._debugID);
}
}
}
ReactCompositeComponent.js
performUpdateIfNecessary: function (transaction) {
if (this._pendingElement != null) {
// receiveComponent会最终调用到updateComponent，从而刷新View
ReactReconciler.receiveComponent(this, this._pendingElement, transaction, this._context);
} else if (this._pendingStateQueue !== null || this._pendingForceUpdate) {
// 执行updateComponent，从而刷新View。
this.updateComponent(transaction, this._currentElement, this._currentElement, this._context, this._context);
} else {
this._updateBatchNumber = null;
}
},
//执行更新React组件的props. state。context函数
updateComponent: function (transaction, prevParentElement, nextParentElement, prevUnmaskedContext, nextUnmaskedContext) {
var inst = this._instance;
var willReceive = false;
var nextContext;
// Determine if the context has changed or not
if (this._context === nextUnmaskedContext) {
nextContext = inst.context;
} else {
nextContext = this._processContext(nextUnmaskedContext);
willReceive = true;
}
var prevProps = prevParentElement.props;
var nextProps = nextParentElement.props;
// Not a simple state update but a props update
if (prevParentElement !== nextParentElement) {
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) {
if (process.env.NODE_ENV !== 'production') {
measureLifeCyclePerf(function () {
return inst.componentWillReceiveProps(nextProps, nextContext);
}, this._debugID, 'componentWillReceiveProps');
} else {
inst.componentWillReceiveProps(nextProps, nextContext);
}
}
//这里可以知道为什么setState可以接受函数，主要就是_processPendingState函数；
//这里仅仅是将每次setState放入到_pendingStateQueue队列中的值，合并到nextState,并没有真正的更新state的值；真正更新组件的state的值是在下面；
var nextState = this._processPendingState(nextProps, nextContext);
var shouldUpdate = true;
if (!this._pendingForceUpdate) {
if (inst.shouldComponentUpdate) {
if (process.env.NODE_ENV !== 'production') {
shouldUpdate = measureLifeCyclePerf(function () {
return inst.shouldComponentUpdate(nextProps, nextState, nextContext);
}, this._debugID, 'shouldComponentUpdate');
} else {
shouldUpdate = inst.shouldComponentUpdate(nextProps, nextState, nextContext);
}
} else {
if (this._compositeType === CompositeTypes.PureClass) {
shouldUpdate = !shallowEqual(prevProps, nextProps) || !shallowEqual(inst.state, nextState);
}
}
}
this._updateBatchNumber = null;
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.
//诺：在这里更新组件的state. props 等值；
this._currentElement = nextParentElement;
this._context = nextUnmaskedContext;
inst.props = nextProps;
inst.state = nextState;
inst.context = nextContext;
}
},
_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];
//如果是setState的参数是一个函数，那么该函数接受三个参数，分别是state props context
_assign(nextState, typeof partial === 'function' ? partial.call(inst, nextState, props, context) : partial);
}
return nextState;
},
this.state的更新会在_processPendingState执行完执行。所以两次setState取到的都是this.state.count最初的值0，这就解释了之前的现象。其实，这也是React为了解决这种前后state依赖但是state又没及时更新的一种方案，因此在使用时大家要根据实际情况来判断该用哪种方式传参。来看个小例子直观感受下
handleClickOnLikeButton () {
this.setState({ count: 0 }) // => this.state.count 还是 undefined
this.setState({ count: this.state.count + 1}) // => undefined + 1 = NaN
this.setState({ count: this.state.count + 2}) // => NaN + 2 = NaN
}
//....VS ....
handleClickOnLikeButton () {
this.setState((prevState) => {
return { count: 0 }
})
this.setState((prevState) => {
return { count: prevState.count + 1 } // 上一个 setState 的返回是 count 为 0，当前返回 1
})
this.setState((prevState) => {
return { count: prevState.count + 2 } // 上一个 setState 的返回是 count 为 1，当前返回 3
})
// 最后的结果是 this.state.count 为 3
}
...
setState流程还是很复杂的，设计也很精巧，避免了重复无谓的刷新组件。它的主要流程如下
enqueueSetState将state放入队列中，并调用enqueueUpdate处理要更新的Component
如果组件当前正处于update事务中，则先将Component存入dirtyComponent中。否则调用batchedUpdates处理。
batchedUpdates发起一次transaction.perform()事务
开始执行事务初始化，运行，结束三个阶段
初始化：事务初始化阶段没有注册方法，故无方法要执行
运行：执行setSate时传入的callback方法，一般不会传callback参数
结束：更新isBatchingUpdates为false，并执行FLUSH_BATCHED_UPDATES这个wrapper中的close方法
FLUSH_BATCHED_UPDATES在close阶段，会循环遍历所有的dirtyComponents，调用updateComponent刷新组件，并执行它的pendingCallbacks, 也就是setState中设置的callback。
参考：
React中setState实现机制
React中setState
