render()

ReactDOM.render(element, container[, callback])

在提供的 container 里渲染一個 React 元素磁携，并返回對該組件的引用（或者針對無狀態(tài)組件返回 null）。

如果 React 元素之前已經(jīng)在 container 里渲染過膝舅，這將會對其執(zhí)行更新操作，并僅會在必要時改變 DOM 以映射最新的 React 元素窑多。

如果提供了可選的回調(diào)函數(shù)铸史，該回調(diào)將在組件被渲染或更新之后被執(zhí)行。

注意：

ReactDOM.render() 會控制你傳入容器節(jié)點(diǎn)里的內(nèi)容怯伊。當(dāng)首次調(diào)用時琳轿，容器節(jié)點(diǎn)里的所有 DOM 元素都會被替換，后續(xù)的調(diào)用則會使用 React 的 DOM 差分算法（DOM diffing algorithm）進(jìn)行高效的更新耿芹。

ReactDOM.render() 不會修改容器節(jié)點(diǎn)（只會修改容器的子節(jié)點(diǎn)）崭篡。可以在不覆蓋現(xiàn)有子節(jié)點(diǎn)的情況下吧秕，將組件插入已有的 DOM 節(jié)點(diǎn)中琉闪。

ReactDOM.render() 目前會返回對根組件 ReactComponent 實(shí)例的引用。 但是砸彬，目前應(yīng)該避免使用返回的引用颠毙，因?yàn)樗菤v史遺留下來的內(nèi)容，而且在未來版本的 React 中砂碉，組件渲染在某些情況下可能會是異步的蛀蜜。 如果你真的需要獲得對根組件 ReactComponent 實(shí)例的引用，那么推薦為根元素添加 callback ref增蹭。

使用 ReactDOM.render() 對服務(wù)端渲染容器進(jìn)行 hydrate 操作的方式已經(jīng)被廢棄滴某，并且會在 React 17 被移除。作為替代滋迈，請使用 hydrate()霎奢。

packages/react-dom/ReactDOM.js 源碼解析

時序圖.png

render(
    element,
    container,
    callback,
  ) {
    invariant(
      isValidContainer(container),
      'Target container is not a DOM element.',
    );
    if (__DEV__) {
      warningWithoutStack(
        !container._reactHasBeenPassedToCreateRootDEV,
        'You are calling ReactDOM.render() on a container that was previously ' +
          'passed to ReactDOM.%s(). This is not supported. ' +
          'Did you mean to call root.render(element)?',
        enableStableConcurrentModeAPIs ? 'createRoot' : 'unstable_createRoot',
      );
    }
    return legacyRenderSubtreeIntoContainer(
      null,
      element,
      container,
      false,
      callback,
    );
  }

在ReactDOM.render方法中調(diào)用了legacyRenderSubtreeIntoContainer方法：

function legacyRenderSubtreeIntoContainer(
  parentComponent,
  children,
  container,
  forceHydrate,
  callback,
) {
  if (__DEV__) {
    topLevelUpdateWarnings(container);
    warnOnInvalidCallback(callback === undefined ? null : callback, 'render');
  }

  // TODO: Without `any` type, Flow says "Property cannot be accessed on any
  // member of intersection type." Whyyyyyy.
  let root = container._reactRootContainer;
  let fiberRoot;
  if (!root) {
    // Initial mount
    root = container._reactRootContainer = legacyCreateRootFromDOMContainer(
      container,
      forceHydrate,
    );
    fiberRoot = root._internalRoot;
    if (typeof callback === 'function') {
      const originalCallback = callback;
      callback = function() {
        const instance = getPublicRootInstance(fiberRoot);
        originalCallback.call(instance);
      };
    }
    // Initial mount should not be batched.
    unbatchedUpdates(() => {
      updateContainer(children, fiberRoot, parentComponent, callback);
    });
  } else {
    fiberRoot = root._internalRoot;
    if (typeof callback === 'function') {
      const originalCallback = callback;
      callback = function() {
        const instance = getPublicRootInstance(fiberRoot);
        originalCallback.call(instance);
      };
    }
    // Update
    updateContainer(children, fiberRoot, parentComponent, callback);
  }
  return getPublicRootInstance(fiberRoot);
}

• 如果沒有container._reactRootContainer值，則說明是第一次掛載到該container下饼灿，這時候幕侠，react在執(zhí)行legacyCreateRootFromDOMContainer的會清空該dom下的子元素，并返回一個ReactSyncRoot類型的對象碍彭。并且第一次掛載的時候晤硕，shouldHydrate是為false的。
• 經(jīng)過legacyCreateRootFromDOMContainer 之后硕旗，container的數(shù)據(jù)是這樣的：

container._reactRootContainer = ReactRoot (from legacyCreateRootFromDOMContainer);
container._reactRootContainer._internalRoot = FiberRoot
container._reactRootContainer._internalRoot.current = FiberNode (HostRoot)
container._reactRootContainer._internalRoot.current.stateNode = container._reactRootContainer._internalRoot = FiberRoot

FiberRoot和Fiber的數(shù)據(jù)結(jié)構(gòu)：

function FiberRootNode(containerInfo, tag, hydrate) {
  this.tag = tag;
  this.current = null;
  this.containerInfo = containerInfo;
  this.pendingChildren = null;
  this.pingCache = null;
  this.finishedExpirationTime = NoWork;
  this.finishedWork = null;
  this.timeoutHandle = noTimeout;
  this.context = null;
  this.pendingContext = null;
  this.hydrate = hydrate;
  this.firstBatch = null;
  this.callbackNode = null;
  this.callbackExpirationTime = NoWork;
  this.firstPendingTime = NoWork;
  this.lastPendingTime = NoWork;
  this.pingTime = NoWork;

  if (enableSchedulerTracing) {
    this.interactionThreadID = unstable_getThreadID();
    this.memoizedInteractions = new Set();
    this.pendingInteractionMap = new Map();
  }
}

function FiberNode(
  tag,
  pendingProps,
  key,
  mode,
) {
  // Instance
  this.tag = tag;
  this.key = key;
  this.elementType = null;
  this.type = null;
  this.stateNode = null;

  // Fiber
  this.return = null;
  this.child = null;
  this.sibling = null;
  this.index = 0;

  this.ref = null;

  this.pendingProps = pendingProps;
  this.memoizedProps = null;
  this.updateQueue = null;
  this.memoizedState = null;
  this.dependencies = null;

  this.mode = mode;

  // Effects
  this.effectTag = NoEffect;
  this.nextEffect = null;

  this.firstEffect = null;
  this.lastEffect = null;

  this.expirationTime = NoWork;
  this.childExpirationTime = NoWork;

  this.alternate = null;
}

接下去就是關(guān)鍵的updateContainer函數(shù)了窗骑。這個函數(shù)牽涉的面比較多，我們下一片再說漆枚。