React入口详解

在使用React进行构建应用时，我们总会有一个步骤将组建或者虚拟DOM元素渲染到真实的DOM上，将任务交给浏览器，进而进行layout和paint等步骤，这个函数就是React.render()。首先看下该函数的接口定义：

ReactComponent render( ReactElement element, DOMElement container, [function callback] )

接收2-3个参数，并返回ReactComponent类型的对象，当组件被添加到DOM中后，执行回调。在这里涉及到了两个React类型--ReactComponent和ReactElement，着重分析。

ReactElement类型解读

ReactElement类型通过函数React.createElement()创建，接口定义如下：

ReactElement createElement( string/ReactClass type, [object props], [children ...] )

第一个参数可以接受字符串（如“p”，“div”等HTML的tag）或ReactClass，第二个参数为传递的参数，第三个为子元素，可以为字符串和ReactElement。

下面着重分析createElement的具体实现：

ReactElement.createElement = function(type, config, children) {
        var propName; // Reserved names are extractedvar
        props = {};
        var key = null;
        var ref = null;
        if (config != null) {
                ref = config.ref === undefined ? null : config.ref;
                key = config.key === undefined ? null : '' + config.key;
                // Remaining properties are added to a new props object
                for (propName in config) {
                        if (config.hasOwnProperty(propName) && !RESERVED_PROPS.hasOwnProperty(propName)) {
                                props[propName] = config[propName];
                        }
                }
        }
        // Children can be more than one argument, and those are transferred onto
        // // the newly allocated props object.
        var childrenLength = arguments.length - 2;
        if (childrenLength === 1) {
                props.children = children;
        } else if (childrenLength > 1) {
                var childArray = Array(childrenLength);
                for (var i = 0; i < childrenLength; i++) {
                        childArray[i] = arguments[i + 2];
                }
                props.children = childArray;
        }
        // Resolve default props
        if (type && type.defaultProps) {
                var defaultProps = type.defaultProps;
                for (propName in defaultProps) {
                        if (typeof props[propName] === 'undefined') {
                                props[propName] = defaultProps[propName];
                        }
                }
        }
        return new ReactElement(type, key, ref, ReactCurrentOwner.current, ReactContext.current, props);
};
var ReactElement = function(type, key, ref, owner, context, props) {
        // Built-in properties that belong on the element
        this.type = type;
        this.key = key;
        this.ref = ref;
        // Record the component responsible for creating this element.
        this._owner = owner;
        // TODO: Deprecate withContext, and then the context becomes accessible
        // through the owner.
        this._context = context;
        if ("production" !== process.env.NODE_ENV) {
                // The validation flag and props are currently mutative. We put them on
                // an external backing store so that we can freeze the whole object.
                // This can be replaced with a WeakMap once they are implemented in
                // commonly used development environments.
                this._store = {
                        props: props,
                        originalProps: assign({}, props)
                };
                // To make comparing ReactElements easier for testing purposes, we make
                // the validation flag non-enumerable (where possible, which should
                // include every environment we run tests in), so the test framework
                // ignores it.
                try {
                        Object.defineProperty(this._store, 'validated', {
                                configurable: false,
                                enumerable: false,
                                writable: true
                        });
                } catch (x) {}
                this._store.validated = false;
                // We're not allowed to set props directly on the object so we early
                // return and rely on the prototype membrane to forward to the backing store.
                if (useMutationMembrane) {
                        Object.freeze(this);
                        return;
                }
        }
        this.props = props;
};

在ReactElement.js中实现了该方法，首先保存传入的参数，其中ref和key这两个参数比较特别，ref用于父组件引用子组件的真实DOM，key用于调和算法，判断该组件是否update或remove；保存children到props中，并根据type是否有defaultProps属性对props进行mixin；最后创建ReactElement实例。其中reactElement有个实例属性_owner，用于保存所属的组件。

ReactElement的原型对象只有一个简单的方法用于判断是否是ReactElement对象，没有额外的方法。

综上，我们可以看出ReactElement有4个属性：type，ref，key，props，并且轻量，没有状态，是一个虚拟化的DOM元素。

ReactClass类型解读

React的核心是ReactElement类型，但是精髓确实ReactComponent，即组件。但是组件的创建却并不简单，我们通过React.createClass创建ReactClass类，它是ReactComponent的构造函数，不同于正常的对象创建，组件的创建由React接管，即我们无须对其实例化（new MyComponent（））。相对于ReactElement的无状态，ReactComponent是有状态的，先看接口定义：

ReactClass createClass(object specification)

传入的spec参数必须包含render方法，用于渲染虚拟DOM，render返回ReactElement类型；另外还有一些getInitialState和生命周期方法，可以根据需要定义。

下面根据createClass的实现来深入分析：

createClass: function(spec) {
                var Constructor = function(props, context) {
                        // Wire up auto-binding
                        if (this.__reactAutoBindMap) {
                                bindAutoBindMethods(this);
                        }
                        this.props = props;
                        this.context = context;
                        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;
                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;
                        }
                }
                // Legacy hookConstructor.
                type = Constructor;
                return Constructor;
        } // Constructor的原型
var ReactClassComponent = function() {};
// assign类似于mixinassign(ReactClassComponent.prototype,ReactComponent.prototype,ReactClassMixin);
// mixin到Constructor的原型上
function mixSpecIntoComponent(Constructor, spec) {
        if (!spec) {
                return;
        }
        var proto = Constructor.prototype;
        // By handling mixins before any other properties, we ensure the same
        // chaining order is applied to methods with DEFINE_MANY policy, whether
        // mixins are listed before or after these methods in the spec.
        if (spec.hasOwnProperty(MIXINS_KEY)) {
                RESERVED_SPEC_KEYS.mixins(Constructor, spec.mixins);
        }
        for (var name in spec) {
                if (!spec.hasOwnProperty(name)) {
                        continue;
                }
                if (name === MIXINS_KEY) {
                        // We have already handled mixins in a special case abovecontinue;}
                        var property = spec[name];
                        validateMethodOverride(proto, name);
                        if (RESERVED_SPEC_KEYS.hasOwnProperty(name)) {
                                RESERVED_SPEC_KEYS[name](Constructor, property);
                        } else {
                                // Setup methods on prototype:
                                // The following member methods should not be automatically bound:
                                // 1. Expected ReactClass methods (in the "interface").
                                // 2. Overridden methods (that were mixed in).
                                var isReactClassMethod = ReactClassInterface.hasOwnProperty(name);
                                var isAlreadyDefined = proto.hasOwnProperty(name);
                                var markedDontBind = property && property.__reactDontBind;
                                var isFunction = typeof property === 'function';
                                var shouldAutoBind = isFunction && !isReactClassMethod && !isAlreadyDefined && !markedDontBind;
                                if (shouldAutoBind) {
                                        if (!proto.__reactAutoBindMap) {
                                                proto.__reactAutoBindMap = {};
                                        }
                                        proto.__reactAutoBindMap[name] = property;
                                        proto[name] = property;
                                } else {
                                        if (isAlreadyDefined) {
                                                var specPolicy = ReactClassInterface[name];
                                                // For methods which are defined more than once, call the existing
                                                // methods before calling the new property, merging if appropriate.
                                                if (specPolicy === SpecPolicy.DEFINE_MANY_MERGED) {
                                                        proto[name] = createMergedResultFunction(proto[name], property);
                                                } else if (specPolicy === SpecPolicy.DEFINE_MANY) {
                                                        proto[name] = createChainedFunction(proto[name], property);
                                                }
                                        } else {
                                                proto[name] = property;
                                                if ("production" !== process.env.NODE_ENV) {
                                                        // Add verbose displayName to the function, which helps when looking
                                                        // at profiling tools.
                                                        if (typeof property === 'function' && spec.displayName) {
                                                                proto[name].displayName = spec.displayName + '_' + name;
                                                        }
                                                }
                                        }
                                }
                        }
                }
        }

createClass返回一个Constructor构造函数或者事件处理函数）保存在原型的__reactAutoBindMap的属性上。最后再设置Constructor的defaultProps和type（Constructor.type = Constructor）。

在上节中提到了createElement的第一个参数可以是ReactClass，因此在Constructor实现上赋予了type和defaultProps属性。

React的入口—React.render()

React.render的实现是在ReactMount中，我们通过源码进行进一步的分析。

render: function(nextElement, container, callback) {
    var prevComponent = instancesByReactRootID[getReactRootID(container)];
    if (prevComponent) {
        var prevElement = prevComponent._currentElement;
        if (shouldUpdateReactComponent(prevElement, nextElement)) {
            return ReactMount._updateRootComponent(prevComponent, nextElement, container, callback).getPublicInstance();
        } else {
            ReactMount.unmountComponentAtNode(container);
        }
    }
    var reactRootElement = getReactRootElementInContainer(container);
    var containerHasReactMarkup = reactRootElement && ReactMount.isRenderedByReact(reactRootElement);
    var shouldReuseMarkup = containerHasReactMarkup && !prevComponent;
    var component = ReactMount._renderNewRootComponent(nextElement, container, shouldReuseMarkup).getPublicInstance();
    if (callback) {
        callback.call(component);
    }
    return component;
}

如果是第一次挂载该ReactElement，直接添加即可；如果之前已挂载过，则通过instancesByReactRootID获取渲染之前container的旧组件，即prevComponent，具体通过获取container的firstChild，并根据缓存获取该对象对应的id，并根据id得到prevComponent。每个component对象都有对应的虚拟DOM，即ReactElement，通过shouldUpdateReactComponent(prevElement, nextElement）进行判断对组件进行update还是delete。

具体shouldUpdateReactComponent的比较算法是：如果prevElement类型为string或者number，那么nextElement类型为string或number时为true；如果prevElement和nextElement为object，并且key和type属性相同，则prevElement._owner == nextElement._owner相等时为true，否则为false。

如果需要更新，则调用ReactMount.._updateRootComponent函数进行Reconciliation，并返回该组件；否则删除该组件，具体操作则是删除container的所有子元素。然后判断shouldReuseMarkup，对于初次挂载的ReactElement而言，该标记为false。最后通过调用_renderNewRootComponent方法将ReactElement渲染到DOM上，并获取对应的ReactComponent对象，最后执行回调并返回组件对象。

对于_renderNewRootComponent方法，通过调用instantiateReactComponent(nextElement, null)来实例化组件，并在ReactMount的缓存中注册组件，批量执行更新ReactUpdates.batchedUpdates，最终通过_mountImageIntoNode方法将虚拟节点插入到DOM中。