前言
- 适合有一定 React 项目经验阅读,默认对 React 的常用 api 较为熟悉
- 研究 React 源码是结合网上的一些分析文章+自己看代码理解
- 最开始看是因为项目中遇到性能问题,网上没有相关资料,所以想找到具体影响的点
- 以下的代码解析以 15.4.1 版本为基础,且去除了开发环境的warning,为了区分,保留的注释都为英文,新增的注释为中文,尽量保持原注释
- 文中有部分自己的演绎、理解、猜测,如有误烦请指出
基础概念
ReactElement
- 数据类,只包含
props refs key
等 - 由
React.creatElement(ReactElement.js)
创建,React.createClass
中render
中返回的实际也是个ReactElement
ReactComponent
- 控制类,包含组件状态,操作方法等
- 包括字符组件、原生 DOM 组件、自定义组件(和空组件)
- 在挂载组件(
mountComponent
)的时候,会调用到instantiateReactComponent
方法,利用工厂模式,通过不同的输入返回不同的component
代码(instantiateReactComponent.js):
function instantiateReactComponent(node,shouldHaveDebugID) {
var instance;
if (node === null || node === false) {
instance = ReactEmptyComponent.create(instantiateReactComponent);
} else if (typeof node === 'object') {
var element = node;
// Special case string values
if (typeof element.type === 'string') {
instance = ReactHostComponent.createInternalComponent(element);
} else if (isInternalComponentType(element.type)) {
// This is temporarily available for custom components that are not string
// representation,we can drop this code path.
} else {
instance = new ReactCompositeComponentWrapper(element);
}
} else if (typeof node === 'string' || typeof node === 'number') {
instance = ReactHostComponent.createInstanceForText(node);
} else {
}
// These two fields are used by the DOM and ART diffing algorithms
// respectively. Instead of using expandos on components,we should be
// storing the state needed by the diffing algorithms elsewhere.
instance._mountIndex = 0;
instance._mountImage = null;
return instance;
}
ReactDOMTextComponent
只关心文本,ReactDOMComponent
会稍微简单一些,ReactCompositeComponent
需要关心的最多,包括得到原生DOM
的渲染内容
ReactClass
这个比较特殊,对比 ES5 写法: var MyComponent = React.createClass({})
,ES6写法:class MyComponent extends React.Component
,为什么用createClass
却得到了Component
呢?通过源码来看,这两个 api 的实现几乎是一样的,也可以看到,ES6 的写法简洁的多,不用那些getInitialState
等特定 api,React 在之后的版本也会抛弃createClass
这个 api。并且,在此 api 中,React 进行了autobind
。
ReactClass.js:
var ReactClass = { createClass: function (spec) { // ensure that Constructor.name !== 'Constructor' var Constructor = identity(function (props,context,updater) { // Wire up auto-binding if (this.__reactAutoBindPairs.length) { bindAutoBindMethods(this); }
this.props = props;
this.context = context;
this.refs = emptyObject;
this.updater = updater || ReactNoopUpdateQueue;
this.state = null;
// ReactClasses doesn't have constructors. Instead,they use the
// getInitialState and componentWillMount methods for initialization.
var initialState = this.getInitialState ? this.getInitialState() : null;
this.state = initialState;
});
Constructor.prototype = new ReactClassComponent();
Constructor.prototype.constructor = Constructor;
Constructor.prototype.__reactAutoBindPairs = [];
injectedMixins.forEach(mixSpecIntoComponent.bind(null,Constructor));
mixSpecIntoComponent(Constructor,spec);
// Initialize the defaultProps property after all mixins have been merged.
if (Constructor.getDefaultProps) { Constructor.defaultProps = Constructor.getDefaultProps(); } // Reduce time spent doing lookups by setting these on the prototype. for (var methodName in ReactClassInterface) { if (!Constructor.prototype[methodName]) { Constructor.prototype[methodName] = null; } } return Constructor;
}
}
var ReactClassComponent = function () {};
_assign(ReactClassComponent.prototype,ReactComponent.prototype,ReactClassMixin);
ReactComponent.js:
function ReactComponent(props,updater) {
this.props = props;
this.context = context;
this.refs = emptyObject;
this.updater = updater || ReactNoopUpdateQueue;
}
ReactComponent.prototype.isReactComponent = {};
ReactComponent.prototype.setState = function (partialState,callback) {
this.updater.enqueueSetState(this,partialState);
if (callback) {
this.updater.enqueueCallback(this,callback,'setState');
}
};
ReactComponent.prototype.forceUpdate = function (callback) {
this.updater.enqueueForceUpdate(this);
if (callback) {
this.updater.enqueueCallback(this,'forceUpdate');
}
};
对象池
- 开辟空间是需要一定代价的
- 如果引用释放而进入 gc,gc 会比较消耗性能和时间,如果内存抖动(大量的对象被创建又在短时间内马上被释放)而频繁 gc 则会影响用户体验
- 既然创建和销毁对象是很耗时的,所以要尽可能减少创建和销毁对象的次数
- 使用时候申请(getPooled)和释放(release)成对出现,使用一个对象后一定要释放还给池子(释放时候要对内部变量置空方便下次使用)
代码(PooledClass.js):
// 只展示部分
var oneArgumentPooler = function (copyFieldsFrom) {
var Klass = this;
if (Klass.instancePool.length) {
var instance = Klass.instancePool.pop();
Klass.call(instance,copyFieldsFrom);
return instance;
} else {
return new Klass(copyFieldsFrom);
}
};
var standardReleaser = function (instance) {
var Klass = this;
if (Klass.instancePool.length < Klass.poolSize) {
Klass.instancePool.push(instance);
}
};
var DEFAULT_POOL_SIZE = 10;
var DEFAULT_POOLER = oneArgumentPooler;
var addPoolingTo = function (CopyConstructor,pooler) {
// Casting as any so that flow ignores the actual implementation and trusts
// it to match the type we declared
var NewKlass = CopyConstructor;
NewKlass.instancePool = [];
NewKlass.getPooled = pooler || DEFAULT_POOLER;
if (!NewKlass.poolSize) {
NewKlass.poolSize = DEFAULT_POOL_SIZE;
}
NewKlass.release = standardReleaser;
return NewKlass;
};
var PooledClass = {
addPoolingTo: addPoolingTo,oneArgumentPooler: oneArgumentPooler,twoArgumentPooler: twoArgumentPooler,threeArgumentPooler: threeArgumentPooler,fourArgumentPooler: fourArgumentPooler,fiveArgumentPooler: fiveArgumentPooler
};
module.exports = PooledClass;
使用例子(ReactUpdate.js):
var transaction = ReactUpdatesFlushTransaction.getPooled();
destructor: function () { this.dirtyComponentsLength = null; CallbackQueue.release(this.callbackQueue); this.callbackQueue = null; ReactUpdates.ReactReconcileTransaction.release(this.reconcileTransaction); this.reconcileTransaction = null; } ReactUpdatesFlushTransaction.release(transaction);
- 可以看到,如果短时间内生成了大量的对象占满了池子,后续的对象是不能复用只能新建的
- 对比连接池、线程池:完成任务后并不销毁,而是可以复用去执行其他任务
事务机制
- React 通过事务机制来完成一些特定操作,比如
merge state,update component
示意图(Transaction.js):
代码(Transaction.js):
var TransactionImpl = {
perform: function (method,scope,a,b,c,d,e,f) {
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,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;
},// 执行所有 wrapper 中的 initialize 方法
initializeAll: function (startIndex) {
},// 执行所有 wrapper 中的 close 方法
closeAll: function (startIndex) {
}
};
module.exports = TransactionImpl;
可以看到和后端的事务是有差异的(有点类似AOP),虽然都叫transaction
,并没有commit
,而是自动执行,初始方法没有提供rollback
,有二次封装提供的(ReactReconcileTransaction.js
)
下文会提到事务机制的具体使用场景
事件分发
框图(ReactBrowserEventEmitter.js)
组件上声明的事件最终绑定到了 document 上,而不是 React 组件对应的 DOM 节点,这样简化了 DOM 原生事件,减少了内存开销
以队列的方式,从触发事件的组件向父组件回溯,调用相应 callback,也就是 React 自身实现了一套事件冒泡机制,虽然 React对合成事件封装了
stopPropagation
,但是并不能阻止自己手动绑定的原生事件的冒泡,所以项目中要避免手动绑定原生事件使用对象池来管理合成事件对象的创建和销毁,好处在上文中有描述
ReactEventListener
:负责事件注册和事件分发ReactEventEmitter
:负责事件执行EventPluginHub
:负责事件的存储,具体存储在listenerBankPlugin: 根据不同的事件类型,构造不同的合成事件,可以连接原生事件和组件
当事件触发时,会调用
ReactEventListener.dispatchEvent
,进行分发:找到具体的ReactComponent
,然后向上遍历父组件,实现冒泡代码较多,就不具体分析了,这种统一收集然后分发的思路,可以用在具体项目中
生命周期
整体流程:
- 主要讲述
mount
和update
,里面也有很多相类似的操作 componentWillMount
,render
,componentDidMount
都是在mountComponent
中被调用
分析 ReactCompositeComponent.js 中的mountComponent,发现输出是return {?string} Rendered markup to be inserted into the DOM.
mountComponent: function (transaction,hostParent,hostContainerInfo,context) {
var _this = this;
this._context = context;
this._mountOrder = nextMountID++;
this._hostParent = hostParent;
this._hostContainerInfo = hostContainerInfo;
var publicProps = this._currentElement.props;
var publicContext = this._processContext(context);
var Component = this._currentElement.type;
var updateQueue = transaction.getUpdateQueue();
// Initialize the public class
var doConstruct = shouldConstruct(Component);
// 最终会调用 new Component()
var inst = this._constructComponent(doConstruct,publicProps,publicContext,updateQueue);
var renderedElement;
// Support functional components
if (!doConstruct && (inst == null || inst.render == null)) {
renderedElement = inst;
inst = new StatelessComponent(Component);
this._compositeType = CompositeTypes.StatelessFunctional;
} else {
// 大家经常在用户端用到的 PureComponent,会对 state 进行浅比较然后决定是否执行 render
if (isPureComponent(Component)) {
this._compositeType = CompositeTypes.PureClass;
} else {
this._compositeType = CompositeTypes.ImpureClass;
}
}
// These should be set up in the constructor,but as a convenience for
// simpler class abstractions,we set them up after the fact.
inst.props = publicProps;
inst.context = publicContext;
inst.refs = emptyObject;
inst.updater = updateQueue;
this._instance = inst;
// Store a reference from the instance back to the internal representation
// 以 element 为 key,存在了 Map 中,之后会用到
ReactInstanceMap.set(inst,this);
var initialState = inst.state;
if (initialState === undefined) {
inst.state = initialState = null;
}
this._pendingStateQueue = null;
this._pendingReplaceState = false;
this._pendingForceUpdate = false;
var markup;
if (inst.unstable_handleError) {
markup = this.performInitialMountWithErrorHandling(renderedElement,transaction,context);
} else {
markup = this.performInitialMount(renderedElement,context);
}
if (inst.componentDidMount) {
transaction.getReactMountReady().enqueue(inst.componentDidMount,inst);
}
return markup;
}
function shouldConstruct(Component) {
return !!(Component.prototype && Component.prototype.isReactComponent);
}
可以看到,mountComponet 先做实例对象的初始化(props,state 等),然后调用performInitialMount挂载(performInitialMountWithErrorHandling最终也会调用performInitialMount,只是多了错误处理),然后调用componentDidMount
transaction.getReactMountReady()会得到CallbackQueue,所以只是加入到队列中,后续执行
我们来看performInitialMount(依然在 ReactCompositeComponent.js 中)
performInitialMount: function (renderedElement,context) {
var inst = this._instance;
var debugID = 0;
if (inst.componentWillMount) {
inst.componentWillMount();
// When mounting,calls to `setState` by `componentWillMount` will set
// `this._pendingStateQueue` without triggering a re-render.
if (this._pendingStateQueue) {
inst.state = this._processPendingState(inst.props,inst.context);
}
}
// If not a stateless component,we now render
// 返回 ReactElement,这也就是上文说的 render 返回 ReactElement
if (renderedElement === undefined) {
renderedElement = this._renderValidatedComponent();
}
var nodeType = ReactNodeTypes.getType(renderedElement);
this._renderedNodeType = nodeType;
var child = this._instantiateReactComponent(renderedElement,nodeType !== ReactNodeTypes.EMPTY);
this._renderedComponent = child;
var markup = ReactReconciler.mountComponent(child,this._processChildContext(context),debugID);
return markup;
}
performInitialMount
中先调用componentWillMount
,这个过程中merge state
,然后调用_renderValidatedComponent
(最终会调用inst.render()
)返回ReactElement
,然后调用_instantiateReactComponent
由ReactElement
创建ReactComponent
,最后进行递归渲染。挂载之后,可以通过
setState
来更新(机制较为复杂,后文会单独分析),此过程通过调用updateComponent
来完成更新。我们来看updateComponent
(依然在 ReactCompositeComponent.js 中)
updateComponent: function (transaction,prevParentElement,nextParentElement,prevUnmaskedContext,nextUnmaskedContext) {
var inst = this._instance;
var willReceive = false;
var nextContext;
// context 相关,React 建议少用 context
// 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) {
inst.componentWillReceiveProps(nextProps,nextContext);
}
var nextState = this._processPendingState(nextProps,nextContext);
var shouldUpdate = true;
if (!this._pendingForceUpdate) {
if (inst.shouldComponentUpdate) {
shouldUpdate = inst.shouldComponentUpdate(nextProps,nextState,nextContext);
} else {
if (this._compositeType === CompositeTypes.PureClass) {
// 这里,就是上文提到的,PureComponent 里的浅比较
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,nextContext,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;
}
}
updateComponent
中,先调用componentWillReceiveProps
,然后merge state
,然后调用shouldComponentUpdate
判断是否需要更新,可以看到,如果组件内部没有自定义,且用的是PureComponent
,会对state
进行浅比较,设置shouldUpdate
,最终调用_performComponentUpdate
来进行更新。而在_performComponentUpdate
中,会先调用componentWillUpdate
,然后调用updateRenderedComponent
进行更新,最后调用componentDidUpdate
(过程较简单,就不列代码了)。下面看一下updateRenderedComponent
的更新机制(依然在ReactCompositeComponent.js
中)
_updateRenderedComponent: function (transaction,context) {
var prevComponentInstance = this._renderedComponent;
var prevRenderedElement = prevComponentInstance._currentElement;
var nextRenderedElement = this._renderValidatedComponent();
var debugID = 0;
if (shouldUpdateReactComponent(prevRenderedElement,nextRenderedElement)) {
ReactReconciler.receiveComponent(prevComponentInstance,nextRenderedElement,this._processChildContext(context));
} else {
var oldHostNode = ReactReconciler.getHostNode(prevComponentInstance);
ReactReconciler.unmountComponent(prevComponentInstance,false);
var nodeType = ReactNodeTypes.getType(nextRenderedElement);
this._renderedNodeType = nodeType;
var child = this._instantiateReactComponent(nextRenderedElement,nodeType !== ReactNodeTypes.EMPTY);
this._renderedComponent = child;
var nextMarkup = ReactReconciler.mountComponent(child,this._hostParent,this._hostContainerInfo,debugID);
this._replaceNodeWithMarkup(oldHostNode,nextMarkup,prevComponentInstance);
}
},
可以看到,如果需要更新,则调用ReactReconciler.receiveComponent
,会递归更新子组件,否则直接卸载然后挂载。所以,重点是在shouldUpdateReactComponent的判断,React 为了简化 diff,所以有一个假设:在组件层级、type、key 不变的时候,才进行比较更新,否则先 unmount 然后重新 mount。来看shouldUpdateReactComponent(shouldUpdateReactComponent.js) :
function shouldUpdateReactComponent(prevElement,nextElement) {
var prevEmpty = prevElement === null || prevElement === false;
var nextEmpty = nextElement === null || nextElement === false;
if (prevEmpty || nextEmpty) {
return prevEmpty === nextEmpty;
}
var prevType = typeof prevElement;
var nextType = typeof nextElement;
// 如果前后两次都为文本元素,则更新
if (prevType === 'string' || prevType === 'number') {
return nextType === 'string' || nextType === 'number';
} else {
// 如果为 ReactDomComponent 或 ReactCompositeComponent,则需要层级 type 和 key 相同,才进行 update(层级在递归中保证相同)
return nextType === 'object' && prevElement.type === nextElement.type && prevElement.key === nextElement.key;
}
}
接下来是重头戏:setState
,上文中已经提到了此 api 为:
ReactComponent.prototype.setState = function (partialState,'setState');
}
};
可以看到这里只是简单的调用enqueueSetState放入队列中,而我们知道,不可能这么简单的。来看enqueueSetState(ReactUpdateQueue.js中),this.updater会在 mount 时候赋值为updateQueue
enqueueSetState: function (publicInstance,partialState) {
var internalInstance = getInternalInstanceReadyForUpdate(publicInstance,'setState');
if (!internalInstance) {
return;
}
// 获取队列,如果为空则创建
var queue = internalInstance._pendingStateQueue || (internalInstance._pendingStateQueue = []);
// 将待 merge 的 state 放入队列
queue.push(partialState);
// 将待更新的组件放入队列
enqueueUpdate(internalInstance);
},function getInternalInstanceReadyForUpdate(publicInstance,callerName) {
// 上文提到的以 element 为 key 存入 map,这里可以取到 component
var internalInstance = ReactInstanceMap.get(publicInstance);
if (!internalInstance) {
return null;
}
return internalInstance;
}
再来看enqueueUpdate(ReactUpdates.js):
function enqueueUpdate(component) {
if (!batchingStrategy.isBatchingUpdates) {
batchingStrategy.batchedUpdates(enqueueUpdate,component);
return;
}
dirtyComponents.push(component);
if (component._updateBatchNumber == null) {
component._updateBatchNumber = updateBatchNumber + 1;
}
}
- 可以看到,如果不处于
isBatchingUpdates
时,则调用batchingStrategy.batchedUpdates
,如果处于的话,则将component
放入dirtyComponents
中等待以后处理。这样保证了避免重复render
,因为mountComponent
和updateComponent
执行的开始,会将isBatchingUpdates
设置为true,之后以事务的方式处理,包括最后时候将isBatchingUpdates
置为false。 - 大家一定对
batchingStrategy
和dirtyComponents
的定义,batchingStrategy
由ReactUpdates.injection
注入,而dirtyComponents
是定义在ReactUpdates.js
中,也就是说二者都为全局的 - 综上,在特定生命周期
getInitialState,componentWillMount,render,componentWillUpdate
中调用setState
,并不会引起updateComponent
(componentDidMount、componentDidUpdate 中会)。来看batchedUpdates
(ReactDefaultBatchingStrategy.js):
batchedUpdates: function (callback,e) {
var alreadyBatchingUpdates = ReactDefaultBatchingStrategy.isBatchingUpdates;
ReactDefaultBatchingStrategy.isBatchingUpdates = true;
// The code is written this way to avoid extra allocations
if (alreadyBatchingUpdates) {
return callback(a,e);
} else {
// 注意这里,上一个代码块中可以看到,当 isBatchingUpdates 为 false 时,callback 为 enqueueUpdate 自身
// 所以即以事务的方式处理
return transaction.perform(callback,null,e);
}
}
var transaction = new ReactDefaultBatchingStrategyTransaction();
- 可以看到,当以事务的方式调用进入
enqueueUpdate
时,isBatchingUpdates
已经为true,所以执行dirtyComponents.push(component);
。 - 注意到callbakc其实就是自身
enqueueUpdate
,当isBatchingUpdates
为false时,也用transaction.perform
调用enqueueUpdate
,使得结果一样 - 详细介绍事务 transaction 的应用,上文中提到过,事务可以利用wrapper封装,开始和结束时会调用所有 wrapper 的相应方法,来看这两个wrapper:
RESET_BATCHED_UPDATES
FLUSH_BATCHED_UPDATES
(ReactDefaultBatchingStrategy.js):
var RESET_BATCHED_UPDATES = {
initialize: emptyFunction,close: function () {
ReactDefaultBatchingStrategy.isBatchingUpdates = false;
}
};
var FLUSH_BATCHED_UPDATES = {
initialize: emptyFunction,close: ReactUpdates.flushBatchedUpdates.bind(ReactUpdates)
};
// flushBatchedUpdates 在 ReactUpdates.js 中
var flushBatchedUpdates = function () {
// ReactUpdatesFlushTransaction's wrappers will clear the dirtyComponents
// asapEnqueued 为提前执行回调,暂不分析
while (dirtyComponents.length || asapEnqueued) {
if (dirtyComponents.length) {
var transaction = ReactUpdatesFlushTransaction.getPooled();
transaction.perform(,transaction);
ReactUpdatesFlushTransaction.release(transaction);
}
if (asapEnqueued) {
}
}
};
- 但是,仔细看上面的过程,把组件放入 dirtyComponents 后,事务结束马上就执行 close方法进行了处理了,和之前理解的流程好像不太一致?这时候再回头看mountComponent和updateComponent,它们的参数:@param {ReactReconcileTransaction} transaction,也就是说整个过程都在ReactReconcileTransaction事务中(事件回调同理),自然在其中的生命周期调用setState不用引起重复 render,只会将 state 放入队列和将组件放入 dirtyComponents 中,然后在结束后统一处理
- ReactReconcileTransaction中 initialize 用于清空回调队列;close 用于触发回调函数componentDidMount、componentDidUpdate 执行
- 我开始一直比较疑惑的是ReactDefaultBatchingStrategy.batchedUpdates中的ReactDefaultBatchingStrategyTransaction和ReactReconcileTransaction到底是什么关系?我试图找出两个transaction 中 wrapper 是否有 merge 的情况,发现没有。目前大概的理解和结论是这样的:整个生命周期就是一个 transaction,即对应ReactDefaultBatchingStrategy.batchedUpdates,而ReactReconcileTransaction粒度较小,负责单个组件(所以也能看到,前者直接new,而后者利用了对象池)。通过各自 wrapper 可以看到,前者([FLUSH_BATCHED_UPDATES,RESET_BATCHED_UPDATES])负责了全部组件更新 和 callback,后者([SELECTION_RESTORATION,EVENT_SUPPRESSION,ON_DOM_READY_QUEUEING)负责了各自组件自身的问题,如 focus 等。
- 例证:ReactDom 中调用render(插入过程),实际最终调用了 ReactMount的_renderNewRootComponent,其中执行了ReactUpdates.batchedUpdates(batchedMountComponentIntoNode,componentInstance,container,shouldReuseMarkup,context);(注意出现了batchedUpdates),而batchedMountComponentIntoNode中调用了
ReactUpdates.ReactReconcileTransaction.getPooled
,这样,嵌套关系就联系起来了 - 例证:
ReactEventListener
的dispatchEvent
,会调用ReactUpdates.batchedUpdates(handleTopLevelImpl,bookKeeping);
和上述同理 - 熟悉 React 生命周期的同学一定对父子组件各生命周期的执行顺序很清晰(比如
componentWillMount
是从父到子),以上述的理论,是如何保证的么?上文中可以看到,FLUSH_BATCHED_UPDATES
的close
方法利调用了runBatchedUpdates
,来看这个方法(ReactUpdates.js):
function runBatchedUpdates(transaction) {
var len = transaction.dirtyComponentsLength;
// reconcile them before their children by sorting the array.
dirtyComponents.sort(mountOrderComparator);
// Any updates enqueued while reconciling must be performed after this entire
// batch. Otherwise,if dirtyComponents is [A,B] where A has children B and
// C,B could update twice in a single batch if C's render enqueues an update
// to B (since B would have already updated,we should skip it,and the only
// way we can know to do so is by checking the batch counter).
updateBatchNumber++;
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 was 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;
ReactReconciler.performUpdateIfNecessary(component,transaction.reconcileTransaction,updateBatchNumber);
if (callbacks) {
for (var j = 0; j < callbacks.length; j++) {
transaction.callbackQueue.enqueue(callbacks[j],component.getPublicInstance());
}
}
}
}
function mountOrderComparator(c1,c2) {
return c1._mountOrder - c2._mountOrder;
}
flushBatchedUpdates
在事务ReactUpdatesFlushTransaction
中,此事务是对ReactReconcileTransaction
和CallbackQueue
的封装,结束时置空dirtyComponents
并通知回调performUpdateIfNecessary
最终会调用updateComponent
,进行更新
diff 算法
- 传统对于树的 diff 算法,时间复杂度要达到 o(n^3),这对于用户端显然是不能接受的。而 React基于几个基础假设,将时间复杂度优化为 o(n)
- 假设(策略)
- Web UI 中 DOM 节点跨层级的移动操作特别少,可以忽略不计
- 拥有相同类的两个组件将会生成相似的树形结构,拥有不同类的两个组件将会生成不同的树形结构
- 对于同一层级的一组子节点,它们可以通过唯一 id进行区分
场景
- tree diff:只对比同层级节点(注意前文中所有代码中,都是只比较prevRenderedElement和nextRenderedElement)
- component diff: 如果类型相同则继续比较,如果类型不同则直接卸载再挂载,即上文中提到的shouldUpdateReactComponent(虽然当两个
component 是不同类型但结构相似时,React diff 会影响性能,但正如 React 官方博客所言:不同类型的component 是很少存在相似 DOM tree 的机会,因此为这种极端情况而做太多比较是不值得的) - element diff: 当一组节点处于同一层级时,React 对于每个节点提供了三种操作,分别为INSERT_MARKUP(插入)、MOVE_EXISTING(移动)、 REMOVE_NODE(删除)
上文的代码中,除了关心 type,还关心 key,这也是 diff 算法的关键,如图
首先对新集合的节点进行循环遍历,for (name in nextChildren),如果存在相同节点,则进行操作,是否移动是通过比较
child._mountIndex < lastIndex
,符合则进行节点移动操作(即在老集合中的位置和 lastIndex 比较),lastIndex 表示访问过的节点在老集合中最右的位置(即最大的位置)。这是一种顺序优化手段,lastIndex 一直在更新,表示访问过的节点在老集合中最右的位置,如果新集合中当前访问的节点比 lastIndex大,说明当前访问节点在老集合中就比上一个节点位置靠后,则该节点不会影响其他节点的位置,因此不用添加到差异队列中,即不执行移动操作,只有当访问的节点比lastIndex 小时,才需要进行移动操作。来看具体过程:- 从新集合中取得 B,判断老集合中存在相同节点 B,通过对比节点位置判断是否进行移动操作,B 在老集合中的位置 B._mountIndex = 1,此时 lastIndex = 0,不满足 child._mountIndex < lastIndex 的条件,因此不对 B 进行移动操作;更新 lastIndex = Math.max(prevChild._mountIndex,lastIndex),其中
prevChild._mountIndex 表示 B 在老集合中的位置,则 lastIndex = 1,并将 B的位置更新为新集合中的位置prevChild._mountIndex = nextIndex,此时新集合中 B._mountIndex = 0,nextIndex++ 进入下一个节点的判断 - 从新集合中取得 A,判断老集合中存在相同节点 A,通过对比节点位置判断是否进行移动操作,A 在老集合中的位置 A._mountIndex = 0,此时 lastIndex = 1,满足 child._mountIndex < lastIndex的条件,因此对 A 进行移动操作 enqueueMove(this,child._mountIndex,toIndex),其中 toIndex 其实就是nextIndex,表示 A 需要移动到的位置;更新 lastIndex =Math.max(prevChild._mountIndex,lastIndex),则 lastIndex = 1,并将 A的位置更新为新集合中的位置 prevChild._mountIndex = nextIndex,此时新集合中A._mountIndex = 1,nextIndex++ 进入下一个节点的判断。
- 从新集合中取得 D,判断老集合中存在相同节点 D,通过对比节点位置判断是否进行移动操作,D 在老集合中的位置 D._mountIndex= 3,此时 lastIndex = 1,不满足 child._mountIndex < lastIndex的条件,因此不对 D 进行移动操作;更新 lastIndex = Math.max(prevChild._mountIndex,lastIndex),则lastIndex = 3,并将 D 的位置更新为新集合中的位置 prevChild._mountIndex = nextIndex,此时新集合中D._mountIndex = 2,nextIndex++ 进入下一个节点的判断。
- 从新集合中取得 C,判断老集合中存在相同节点 C,通过对比节点位置判断是否进行移动操作,C 在老集合中的位置 C._mountIndex= 2,此时 lastIndex = 3,满足 child._mountIndex < lastIndex 的条件,因此对 C 进行移动操作 enqueueMove(this,toIndex);更新 lastIndex = Math.max(prevChild._mountIndex,lastIndex),则 lastIndex = 3,并将 C的位置更新为新集合中的位置 prevChild._mountIndex = nextIndex,此时新集合中 C._mountIndex= 3,nextIndex++ 进入下一个节点的判断,由于 C 已经是最后一个节点,因此 diff 到此完成。
- 当有新的 Component 插入时,逻辑一致,不做具体分析了
- 当完成集合中所有节点 diff,还需要遍历老集合,如果存在新集合中没有但老集合中有的节点,则删除
代码(ReactMultiChild.js),针对 element diff(tree diff 和 component diff 在之前的代码中已经提到过):
_updateChildren: function (nextNestedChildrenElements,context) {
var prevChildren = this._renderedChildren;
var removedNodes = {};
var mountImages = [];
var nextChildren = this._reconcilerUpdateChildren(prevChildren,nextNestedChildrenElements,mountImages,removedNodes,context);
if (!nextChildren && !prevChildren) {
return;
}
var updates = null;
var name;
// `nextIndex` will increment for each child in `nextChildren`,but
// `lastIndex` will be the last index visited in `prevChildren`.
var nextIndex = 0;
var lastIndex = 0;
// `nextMountIndex` will increment for each newly mounted child.
var nextMountIndex = 0;
var lastPlacedNode = null;
for (name in nextChildren) {
if (!nextChildren.hasOwnProperty(name)) {
continue;
}
var prevChild = prevChildren && prevChildren[name];
var nextChild = nextChildren[name];
if (prevChild === nextChild) {
updates = enqueue(updates,this.moveChild(prevChild,lastPlacedNode,nextIndex,lastIndex));
lastIndex = Math.max(prevChild._mountIndex,lastIndex);
prevChild._mountIndex = nextIndex;
} else {
if (prevChild) {
// Update `lastIndex` before `_mountIndex` gets unset by unmounting.
lastIndex = Math.max(prevChild._mountIndex,lastIndex);
// The `removedNodes` loop below will actually remove the child.
}
// The child must be instantiated before it's mounted.
updates = enqueue(updates,this._mountChildAtIndex(nextChild,mountImages[nextMountIndex],context));
nextMountIndex++;
}
nextIndex++;
lastPlacedNode = ReactReconciler.getHostNode(nextChild);
}
// Remove children that are no longer present.
for (name in removedNodes) {
if (removedNodes.hasOwnProperty(name)) {
updates = enqueue(updates,this._unmountChild(prevChildren[name],removedNodes[name]));
}
}
if (updates) {
processQueue(this,updates);
}
this._renderedChildren = nextChildren;
},
综上,在开发中,保持稳定的结构有助于性能提升,当有一组节点时,除了要设置 key,也要避免将靠后的节点移动到靠前的位置
一些其他的点
interface(ReactClass.js)
var ReactClassInterface = {
mixins: 'DEFINE_MANY',statics: 'DEFINE_MANY',propTypes: 'DEFINE_MANY',contextTypes: 'DEFINE_MANY',childContextTypes: 'DEFINE_MANY',// ==== Definition methods ====
getDefaultProps: 'DEFINE_MANY_MERGED',getInitialState: 'DEFINE_MANY_MERGED',getChildContext: 'DEFINE_MANY_MERGED',render: 'DEFINE_ONCE',// ==== Delegate methods ====
componentWillMount: 'DEFINE_MANY',componentDidMount: 'DEFINE_MANY',componentWillReceiveProps: 'DEFINE_MANY',shouldComponentUpdate: 'DEFINE_ONCE',componentWillUpdate: 'DEFINE_MANY',componentDidUpdate: 'DEFINE_MANY',componentWillUnmount: 'DEFINE_MANY',// ==== Advanced methods ====
updateComponent: 'OVERRIDE_BASE'
};
function validateMethodOverride(isAlreadyDefined,name) {
var specPolicy = ReactClassInterface.hasOwnProperty(name) ? ReactClassInterface[name] : null;
// Disallow overriding of base class methods unless explicitly allowed.
if (ReactClassMixin.hasOwnProperty(name)) {
!(specPolicy === 'OVERRIDE_BASE') ? process.env.NODE_ENV !== 'production' ? invariant(false,'ReactClassInterface: You are attempting to override `%s` from your class specification. Ensure that your method names do not overlap with React methods.',name) : _prodInvariant('73',name) : void 0;
}
// Disallow defining methods more than once unless explicitly allowed.
if (isAlreadyDefined) {
!(specPolicy === 'DEFINE_MANY' || specPolicy === 'DEFINE_MANY_MERGED') ? process.env.NODE_ENV !== 'production' ? invariant(false,'ReactClassInterface: You are attempting to define `%s` on your component more than once. This conflict may be due to a mixin.',name) : _prodInvariant('74',name) : void 0;
}
}
可以看到,和后端中interface
(或是抽象类)还是有区别的,但是可以起到规范和检查的作用,实际项目中可以借鉴