ReactMount = {
// ReactDOM.render直接引用此方法
render: function (nextElement, container, callback) {
return ReactMount._renderSubtreeIntoContainer(null, nextElement, container, callback);
},
// 實際執(zhí)行render的方法
_renderSubtreeIntoContainer: function (parentComponent, nextElement, container, callback) {
ReactUpdateQueue.validateCallback(callback, 'ReactDOM.render');

// 將傳入的element用TopLevelWrapper包裝索昂，
// 包裝后的元素载庭，標記有rootID挣惰，并且擁有render方法，
// 具體可看TopLevelWrapper的源碼
var nextWrappedElement = React.createElement(TopLevelWrapper, {
  child: nextElement
});

// ReactDOM.render方法調(diào)用時链烈，parentComponent為null
var nextContext;
if (parentComponent) {
  var parentInst = ReactInstanceMap.get(parentComponent);
  nextContext = parentInst._processChildContext(parentInst._context);
} else {
  nextContext = emptyObject;
}

// 第一次執(zhí)行時，prevComponent為null挚躯，具體可看此方法源碼
var prevComponent = getTopLevelWrapperInContainer(container);

if (prevComponent) {
  var prevWrappedElement = prevComponent._currentElement;
  var prevElement = prevWrappedElement.props.child;

  // 判斷上一次的prevElement和nextElement是否是同一個組件强衡，或者僅僅是數(shù)字、字符串码荔，如果是漩勤，則直接update，
  // 否則目胡，重新渲染整個Element
  if (shouldUpdateReactComponent(prevElement, nextElement)) {
    var publicInst = prevComponent._renderedComponent.getPublicInstance();
    var updatedCallback = callback && function () {
      callback.call(publicInst);
    };
    // 更新vdom
    ReactMount._updateRootComponent(prevComponent, nextWrappedElement, nextContext, container, updatedCallback);
    return publicInst;
  } else {
    ReactMount.unmountComponentAtNode(container);
  }
}

var reactRootElement = getReactRootElementInContainer(container);
var containerHasReactMarkup = reactRootElement && !!internalGetID(reactRootElement);
var containerHasNonRootReactChild = hasNonRootReactChild(container);


var shouldReuseMarkup = containerHasReactMarkup && !prevComponent && !containerHasNonRootReactChild;
// 本次為首次渲染锯七，因此調(diào)用ReactMount._renderNewRootComponent
var component = ReactMount._renderNewRootComponent(nextWrappedElement, container, shouldReuseMarkup, nextContext)._renderedComponent.getPublicInstance();
if (callback) {
  callback.call(component);
}
return component;

},
/**

• Render a new component into the DOM. Hooked by hooks!
• @param {ReactElement} nextElement element to render
• @param {DOMElement} container container to render into
• @param {boolean} shouldReuseMarkup if we should skip the markup insertion
• @return {ReactComponent} nextComponent
  */
  _renderNewRootComponent: function (nextElement, container, shouldReuseMarkup, context) {

ReactBrowserEventEmitter.ensureScrollValueMonitoring();
// 初始化組件實例，并增加組件掛載(mount)誉己、更新(update)眉尸、卸載(unmount)等方法
var componentInstance = instantiateReactComponent(nextElement, false);

// The initial render is synchronous but any updates that happen during
// rendering, in componentWillMount or componentDidMount, will be batched
// according to the current batching strategy.

ReactUpdates.batchedUpdates(batchedMountComponentIntoNode, componentInstance, container, shouldReuseMarkup, context);

var wrapperID = componentInstance._instance.rootID;
instancesByReactRootID[wrapperID] = componentInstance;

return componentInstance;

},
}