原文链接地址：https://github.com/Nealyang%EF%BC%9A%E7%8A%B6%E6%80%81%E3%80%81%E5%B1%9E%E6%80%A7%E6%9B%B4%E6%96%B0%20-%3E%20setState.md) 转载请注明出处
状态更新
此次分析setState基于0.3版本，实现比较简单，后续会再分析目前使用的版本以及事务机制。
流程图大概如下
setState的源码比较简单，而在执行更新的过程比较复杂。我们直接跟着源码一点一点屡清楚。
- ReactCompositeComponent.js
/**
* Sets a subset of the state. Always use this or `replaceState` 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.
*
* @param {object} partialState Next partial state to be merged with state.
* @final
* @protected
*/
setState: function(partialState) {
// Merge with `_pendingState` if it exists, otherwise with existing state.
this.replaceState(merge(this._pendingState || this.state, partialState));
},注释部分说的很明确，setState后我们不能够立即拿到我们设置的值。
而这段代码也非常简单，就是将我们传入的state和this._pendingState做一次merge，merge的代码在util.js下
var merge = function(one, two) {
var result = {};
mergeInto(result, one);
mergeInto(result, two);
return result;
};
function mergeInto(one, two) {
checkMergeObjectArg(one);
if (two != null) {
checkMergeObjectArg(two);
for (var key in two) {
if (!two.hasOwnProperty(key)) {
continue;
}
one[key] = two[key];
}
}
}
checkMergeObjectArgs: function(one, two) {
mergeHelpers.checkMergeObjectArg(one);
mergeHelpers.checkMergeObjectArg(two);
},
/**
* @param {*} arg
*/
checkMergeObjectArg: function(arg) {
throwIf(isTerminal(arg) || Array.isArray(arg), ERRORS.MERGE_CORE_FAILURE);
},
var isTerminal = function(o) {
return typeof o !== 'object' || o === null;
};
var throwIf = function(condition, err) {
if (condition) {
throw new Error(err);
}
};诊断代码的逻辑非常简单，其实功能就是Object.assign() ,但是从上面代码我们可以看出react源码中的function大多都具有小而巧的特点。
最终，将merge后的结果传递给replaceState
replaceState: function(completeState) {
var compositeLifeCycleState = this._compositeLifeCycleState;
invariant(
this._lifeCycleState === ReactComponent.LifeCycle.MOUNTED ||
compositeLifeCycleState === CompositeLifeCycle.MOUNTING,
'replaceState(...): Can only update a mounted (or mounting) component.'
);
invariant(
compositeLifeCycleState !== CompositeLifeCycle.RECEIVING_STATE &&
compositeLifeCycleState !== CompositeLifeCycle.UNMOUNTING,
'replaceState(...): Cannot update while unmounting component or during ' +
'an existing state transition (such as within `render`).'
);
this._pendingState = completeState;
// Do not trigger a state transition if we are in the middle of mounting or
// receiving props because both of those will already be doing this.
if (compositeLifeCycleState !== CompositeLifeCycle.MOUNTING &&
compositeLifeCycleState !== CompositeLifeCycle.RECEIVING_PROPS) {
this._compositeLifeCycleState = CompositeLifeCycle.RECEIVING_STATE;
var nextState = this._pendingState;
this._pendingState = null;
var transaction = ReactComponent.ReactReconcileTransaction.getPooled();
transaction.perform(
this._receivePropsAndState,
this,
this.props,
nextState,
transaction
);
ReactComponent.ReactReconcileTransaction.release(transaction);
this._compositeLifeCycleState = null;
}
},撇开50% 判断warning代码不说，从上面代码我们可以看出，只有在componsiteLifeState不等于mounting和receiving_props 时，才会调用 _receivePropsAndState函数来更新组件。
我们可以演示下：
var ExampleApplication = React.createClass({
getInitialState() {
return {}
},
componentWillMount() {
this.setState({
a: 1,
})
console.log('componentWillMount', this.state.a)
this.setState({
a: 2,
})
console.log('componentWillMount', this.state.a)
this.setState({
a: 3,
})
console.log('componentWillMount', this.state.a)
setTimeout(() => console.log('a5'), 0)
setTimeout(() => console.log(this.state.a,'componentWillMount'))
Promise.resolve('a4').then(console.log)
},
componentDidMount() {
this.setState({
a: 4,
})
console.log('componentDidMount', this.state.a)
this.setState({
a: 5,
})
console.log('componentDidMount', this.state.a)
this.setState({
a: 6,
})
console.log('componentDidMount', this.state.a)
},
render: function () {
var elapsed = Math.round(this.props.elapsed / 100);
var seconds = elapsed / 10 + (elapsed % 10 ? '' : '.0');
var message =
'React has been successfully running for ' + seconds + ' seconds.';
return React.DOM.p(null, message);
}
});
所以以上结果我们可以看出，在componentWillMount生命周期内setState后this.state不会改变，在componentDidMount是正常的。因为在上一篇文章中我们也有说到，在mountComponent过程中，会把compositeLifeCycleState设置为MOUNTING状态，在这个过程中，是不会执行receivePropsAndState的，所以this.state也就不会更新，同理，在receivePropsAndState的过程中，会把compositeLifeCycleState置成RECEIVING_PROPS状态，也不会执行state更新以及render执行，在updateComponent过程中又执行了mountComponent函数，mountComponent函数调用了render函数。
而在现在我们使用16或者15版本中，我们发现：
componentDidMount() {
this.setState({val: this.state.val + 1});
console.log(this.state.val); // 第 1 次 log
this.setState({val: this.state.val + 1});
console.log(this.state.val); // 第 2 次 log
setTimeout(() => {
this.setState({val: this.state.val + 1});
console.log(this.state.val); // 第 3 次 log
this.setState({val: this.state.val + 1});
console.log(this.state.val); // 第 4 次 log
}, 0);
}最后打印的结果为：0，0，2，3
为什么有这样呢？其实源于源码中的这段代码：
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);
}因为这里涉及到事务的概念、批量更新以及benchUpdate等，在我们目前分析的版本中还未迭代上去，后面我们会跟着版本升级慢慢说道。
属性更新
首先我们知道，属性的更新必然是由于state的更新，所以其实组件属性的更新流程就是setState执行更新的延续，换句话说，也就是setState才能出发组件属性的更新，源码里就是我在处理state更新的时候，顺带检测了属性的更新。所以这段源码的开始，还是从setState中看
_receivePropsAndState: function(nextProps, nextState, transaction) {
if (!this.shouldComponentUpdate ||
this.shouldComponentUpdate(nextProps, nextState)) {
this._performComponentUpdate(nextProps, nextState, transaction);
} else {
this.props = nextProps;
this.state = nextState;
}
},代码非常的简单，一句话解释：当shouldComponentUpdate为true时，则执行更新操作。
_performComponentUpdate: function(nextProps, nextState, transaction) {
var prevProps = this.props;
var prevState = this.state;
if (this.componentWillUpdate) {
this.componentWillUpdate(nextProps, nextState, transaction);
}
this.props = nextProps;
this.state = nextState;
this.updateComponent(transaction);
if (this.componentDidUpdate) {
transaction.getReactOnDOMReady().enqueue(
this,
this.componentDidUpdate.bind(this, prevProps, prevState)
);
}
},这段代码的核心就是调用this.updateComponent，然后对老的属性和状态存一下，新的更新一下而已。如果存在componentWillUpdate就执行一下，然后走更新流程。最后是把执行componentDidUpdate推入getReactOnDOMReady的队列中，等待组件的更新。
_renderValidatedComponent: function() {
ReactCurrentOwner.current = this;
var renderedComponent = this.render();
ReactCurrentOwner.current = null;
return renderedComponent;
},
...
...
updateComponent: function(transaction) {
var currentComponent = this._renderedComponent;
var nextComponent = this._renderValidatedComponent();
if (currentComponent.constructor === nextComponent.constructor) {
if (!nextComponent.props.isStatic) {
currentComponent.receiveProps(nextComponent.props, transaction);
}
} else {
var thisID = this._rootNodeID;
var currentComponentID = currentComponent._rootNodeID;
currentComponent.unmountComponent();
var nextMarkup = nextComponent.mountComponent(thisID, transaction);
ReactComponent.DOMIDOperations.dangerouslyReplaceNodeWithMarkupByID(
currentComponentID,
nextMarkup
);
this._renderedComponent = nextComponent;
}
},这里我们直接看updateComponent更新流程，首先获取当前render函数的组件，然后获取下一次render函数的组件，_renderValidatedComponent就是获取下一次的render组件。 通过Constructor来判断组件是否相同，如果相同且组件为非静态，则更新组件的属性，否则卸载当前组件，然后重新mount下一个render组件并且直接暴力更新。
接着会调用render组件的receiveProps方法，其实一开始这个地方我也是非常困惑的，this指向傻傻分不清楚，后来经过各种查阅资料知道，它其实是一个多态方法，如果是复合组件，则执行ReactCompositeComponent.receiveProps，如果是原生组件，则执行ReactNativeComponent.receiveProps。源码分别如下：
receiveProps: function(nextProps, transaction) {
if (this.constructor.propDeclarations) {
this._assertValidProps(nextProps);
}
ReactComponent.Mixin.receiveProps.call(this, nextProps, transaction);
this._compositeLifeCycleState = CompositeLifeCycle.RECEIVING_PROPS;
if (this.componentWillReceiveProps) {
this.componentWillReceiveProps(nextProps, transaction);
}
this._compositeLifeCycleState = CompositeLifeCycle.RECEIVING_STATE;
var nextState = this._pendingState || this.state;
this._pendingState = null;
this._receivePropsAndState(nextProps, nextState, transaction);
this._compositeLifeCycleState = null;
},有人可能注意到这里的this._receivePropsAndState函数，这不是刚才调用过么？怎么又调用一遍？没错，调用这个的this已经是currentComponent了，并不是上一个this。currentComponent是当前组件的render组件，也就是当前组件的子组件。子组件同样也可能是复合组件或者原生组件。正式通过这种多态的方式，递归的解析每级嵌套组件。最终完成从当前组件到下面的所有叶子节点的树更新。
其实话说回来，compositeComponent最终还是会遍历递归到解析原生组件，通过我们整体浏览下ReactNativeComponent.js代码可以看出。
我们先从 receiveProps方法开始看
receiveProps: function(nextProps, transaction) {
assertValidProps(nextProps);
ReactComponent.Mixin.receiveProps.call(this, nextProps, transaction);
this._updateDOMProperties(nextProps);
this._updateDOMChildren(nextProps, transaction);
this.props = nextProps;
},
function assertValidProps(props) {
if (!props) {
return;
}
var hasChildren = props.children != null ? 1 : 0;
var hasContent = props.content != null ? 1 : 0;
var hasInnerHTML = props.dangerouslySetInnerHTML != null ? 1 : 0;
}删除安全警告和注释其实代码非常简答，首先assertValidProps就是校验props是否合法的，更新属性的方法就是_updateDOMProperties
_updateDOMProperties: function(nextProps) {
var lastProps = this.props;
for (var propKey in nextProps) {
var nextProp = nextProps[propKey];
var lastProp = lastProps[propKey];
//判断新老属性中的值是否相等
if (!nextProps.hasOwnProperty(propKey) || nextProp === lastProp) {
continue;
}
//如果是style样式，遍历新style，如果去旧style不相同，则把变化的存入styleUpdates对象中。最后调用 updateStylesByID 统一修改dom的style属性。
if (propKey === STYLE) {
if (nextProp) {
nextProp = nextProps.style = merge(nextProp);
}
var styleUpdates;
for (var styleName in nextProp) {
if (!nextProp.hasOwnProperty(styleName)) {
continue;
}
if (!lastProp || lastProp[styleName] !== nextProp[styleName]) {
if (!styleUpdates) {
styleUpdates = {};
}
styleUpdates[styleName] = nextProp[styleName];
}
}
if (styleUpdates) {
ReactComponent.DOMIDOperations.updateStylesByID(
this._rootNodeID,
styleUpdates
);
}
} else if (propKey === DANGEROUSLY_SET_INNER_HTML) {
var lastHtml = lastProp && lastProp.__html;
var nextHtml = nextProp && nextProp.__html;
if (lastHtml !== nextHtml) {
ReactComponent.DOMIDOperations.updateInnerHTMLByID(//注意这里是innerHtml，所以dangerouslyInnerHTML会展示正常的HTML
this._rootNodeID,
nextProp
);
}
} else if (propKey === CONTENT) {
ReactComponent.DOMIDOperations.updateTextContentByID(//这里是innerText，所以content与children原封不动的把HTML代码打印到页面上
this._rootNodeID,
'' + nextProp
);
} else if (registrationNames[propKey]) {
putListener(this._rootNodeID, propKey, nextProp);
} else {
ReactComponent.DOMIDOperations.updatePropertyByID(
this._rootNodeID,
propKey,
nextProp
);
}
}
},这里面方法没有太多的hack技巧，非常的简单直白，不单独拧出来说，我直接写到注释里面了。
最后直接更新组件的属性
setValueForProperty: function(node, name, value) {
if (DOMProperty.isStandardName[name]) {
var mutationMethod = DOMProperty.getMutationMethod[name];
if (mutationMethod) {
mutationMethod(node, value);
} else if (DOMProperty.mustUseAttribute[name]) {
if (DOMProperty.hasBooleanValue[name] && !value) {
node.removeAttribute(DOMProperty.getAttributeName[name]);
} else {
node.setAttribute(DOMProperty.getAttributeName[name], value);
}
} else {
var propName = DOMProperty.getPropertyName[name];
if (!DOMProperty.hasSideEffects[name] || node[propName] !== value) {
node[propName] = value;
}
}
} else if (DOMProperty.isCustomAttribute(name)) {
node.setAttribute(name, value);
}
}整体属性更新的流程图大概如下：
结束语
通篇读完，是不是有种
react源码中包含很多的点的知识，比如我们之前说的VDOM、包括后面要去学习dom-diff、事务、缓存等等，都是一个点，而但从一个点来切入难免有的会有些枯燥没卵用，别急别急~
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