ReactJS分析之入口函数render

前言

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 extracted
  var 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 hook
    Constructor.type = Constructor;

    return Constructor;
  }
// Constructor的原型
var ReactClassComponent = function() {};
// assign类似于mixin
assign(
  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 above
      continue;
    }

    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，它的原型是new ReactClassComponent()对象，该对象有mixin的组件的方法(在spec对象中的mixins属性的对象的方法)和ReactComponent的方法（setState和forceUpdate），并且在mixSpecIntoComponent(Constructor, spec)方法中将spec中实现的方法绑定到Constructor的原型上，在这里对于非React提供的方法（即个人实现的一些功能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。

至此，React中比较重要的方法讲解完毕。下一步计划是分析组件的实例化过程，敬请期待。