react源码分析：组件的创建和更新

这一章节就来讲讲ReactDOM.render()方法的内部实现与流程吧。
因为初始化的源码文件部分所涵盖的内容很多，包括创建渲染、更新渲染、Fiber树的创建与diff，element的创建与插入，还包括一些优化算法，所以我就整个的React执行流程画了一个简单的示意图。

React源码执行流程图

从图中我们很清晰的看到ReactDOM.render()之后我们的组件具体干了什么事情，那么我们进入源码文件一探究竟吧。

// packages/react-dom/src/client/ReactDOMLegacy.js
export function render(element: React$Element<any>, // 经过babel解析后的elementcontainer: Container, // 根组件节点： document.getElementById('root')..callback: ?Function,// 回调
) {// 做合法容器的验证(根组件)invariant(isValidContainer(container),'Target container is not a DOM element.',);// 开发模式下if (__DEV__) {const isModernRoot =isContainerMarkedAsRoot(container) &&container._reactRootContainer === undefined;if (isModernRoot) {console.error('You are calling ReactDOM.render() on a container that was previously ' +'passed to ReactDOM.createRoot(). This is not supported. ' +'Did you mean to call root.render(element)?',);}}// 返回 legacyRenderSubtreeIntoContainerreturn legacyRenderSubtreeIntoContainer(null,element,container,false,callback,);
}

所以当前render函数仅仅只是做了部分逻辑，阅读React源码，给你一个直观的感受就是他拆分的颗粒度非常的细，很多重复命名的函数，可能是见名知意的变量名只有那么几个常见的组合吧，这也是React作者的用心良苦吧。

追根究底我们还是得看一看legacyRenderSubtreeIntoContainer究竟干了些不为人知的事情呢

legacyRenderSubtreeIntoContainer

function legacyRenderSubtreeIntoContainer(parentComponent: ?React$Component<any, any>, // 父级组件children: ReactNodeList, // 当前元素container: Container, // 容器 eg：getElementById('root')forceHydrate: boolean,  callback: ?Function,
) {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: RootType = (container._reactRootContainer: any);let fiberRoot;// 如果有根组件，表示不是初始化渲染，则走下面的批量更新// 没有根组件，那么就要去创建根组件了if (!root) {// 初始化挂载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);};}// 不必要的批量更新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);};}// 批量更新updateContainer(children, fiberRoot, parentComponent, callback);}return getPublicRootInstance(fiberRoot);
}

• 有根节点的情况下，我们判定为非首次渲染状态，执行updateContainer
• 没有根节点的情况下，我们判定为首次渲染，接着去创建根节点，执行legacyCreateRootFromDOMContainer，拿到了root之后，我们会去触发执行updateContainer

legacyCreateRootFromDOMContainer

function legacyCreateRootFromDOMContainer(container: Container, // 容器forceHydrate: boolean, // value:false
): RootType {const shouldHydrate =forceHydrate || shouldHydrateDueToLegacyHeuristic(container);// First clear any existing content.if (!shouldHydrate) {let warned = false;let rootSibling;while ((rootSibling = container.lastChild)) {if (__DEV__) {if (!warned &&rootSibling.nodeType === ELEMENT_NODE &&(rootSibling: any).hasAttribute(ROOT_ATTRIBUTE_NAME)) {warned = true;console.error('render(): Target node has markup rendered by React, but there ' +'are unrelated nodes as well. This is most commonly caused by ' +'white-space inserted around server-rendered markup.',);}}container.removeChild(rootSibling);}}if (__DEV__) {if (shouldHydrate && !forceHydrate && !warnedAboutHydrateAPI) {warnedAboutHydrateAPI = true;console.warn('render(): Calling ReactDOM.render() to hydrate server-rendered markup ' +'will stop working in React v18. Replace the ReactDOM.render() call ' +'with ReactDOM.hydrate() if you want React to attach to the server HTML.',);}}// 关注createLegacyRootreturn createLegacyRoot(container,shouldHydrate? {hydrate: true,}: undefined,);
}

createLegacyRoot

export function createLegacyRoot(container: Container, // 容器options?: RootOptions,
): RootType {//关注ReactDOMBlockingRootreturn new ReactDOMBlockingRoot(container, LegacyRoot, options);
}

ReactDOMBlockingRoot

function ReactDOMBlockingRoot(container: Container, // 容器tag: RootTag, // LegacyRoot = 0;BlockingRoot = 1;ConcurrentRoot = 2;options: void | RootOptions,
) {this._internalRoot = createRootImpl(container, tag, options);
}

• 我们在这里看到this._internalRoot出来了，因为在先前这个值会给到fiberRoot，所以我们再去看一看这个_internalRoot是怎么创建出来的

createRootImpl

function createRootImpl(container: Container,  tag: RootTag,  options: void | RootOptions,
) {// Tag is either LegacyRoot or Concurrent Rootconst hydrate = options != null && options.hydrate === true;const hydrationCallbacks =(options != null && options.hydrationOptions) || null;const mutableSources =(options != null &&options.hydrationOptions != null &&options.hydrationOptions.mutableSources) ||null;// 关注createContainerconst root = createContainer(container, tag, hydrate, hydrationCallbacks);markContainerAsRoot(root.current, container);const containerNodeType = container.nodeType;if (enableEagerRootListeners) {const rootContainerElement =container.nodeType === COMMENT_NODE ? container.parentNode : container;listenToAllSupportedEvents(rootContainerElement);} else {if (hydrate && tag !== LegacyRoot) {const doc =containerNodeType === DOCUMENT_NODE? container: container.ownerDocument;// We need to cast this because Flow doesn't work// with the hoisted containerNodeType. If we inline// it, then Flow doesn't complain. We intentionally// hoist it to reduce code-size.eagerlyTrapReplayableEvents(container, ((doc: any): Document));} else if (containerNodeType !== DOCUMENT_FRAGMENT_NODE &&containerNodeType !== DOCUMENT_NODE) {ensureListeningTo(container, 'onMouseEnter', null);}}if (mutableSources) {for (let i = 0; i < mutableSources.length; i++) {const mutableSource = mutableSources[i];registerMutableSourceForHydration(root, mutableSource);}}// 关注rootreturn root;
}

相关参考视频讲解：进入学习

• 见名知意关注createContainer为创建容器，看其源码

createContainer

// packages/react-reconciler/src/ReactFiberReconciler.old.js
export function createContainer(containerInfo: Container, // 容器tag: RootTag, // LegacyRoot = 0;BlockingRoot = 1;ConcurrentRoot = 2;hydrate: boolean,  hydrationCallbacks: null | SuspenseHydrationCallbacks,
): OpaqueRoot {// 关注createFiberRootreturn createFiberRoot(containerInfo, tag, hydrate, hydrationCallbacks);
}

createFiberRoot

export function createFiberRoot(containerInfo: any,  tag: RootTag,  hydrate: boolean,  hydrationCallbacks: null | SuspenseHydrationCallbacks,
): FiberRoot {const root: FiberRoot = (new FiberRootNode(containerInfo, tag, hydrate): any);if (enableSuspenseCallback) {root.hydrationCallbacks = hydrationCallbacks;}// 关注createHostRootFiberconst uninitializedFiber = createHostRootFiber(tag);root.current = uninitializedFiber;uninitializedFiber.stateNode = root;// 初始化更新队列initializeUpdateQueue(uninitializedFiber);return root;
}

• 关注 root.current、uninitializedFiber.stateNode这两个玩意儿，后面有大作用，我们还是看看createHostRootFiber吧

createHostRootFiber

export function createHostRootFiber(tag: RootTag): Fiber {let mode;if (tag === ConcurrentRoot) {mode = ConcurrentMode | BlockingMode | StrictMode;} else if (tag === BlockingRoot) {mode = BlockingMode | StrictMode;} else {mode = NoMode;}if (enableProfilerTimer && isDevToolsPresent) {// Always collect profile timings when DevTools are present.// This enables DevTools to start capturing timing at any point–// Without some nodes in the tree having empty base times.mode |= ProfileMode;}return createFiber(HostRoot, null, null, mode);
}

• 一眼望去这里便是对tag的处理，到了后面便是去创建fiber节点

createFiber

const createFiber = function(tag: WorkTag,  pendingProps: mixed,  key: null | string,  mode: TypeOfMode,
): Fiber {// $FlowFixMe: the shapes are exact here but Flow doesn't like constructorsreturn new FiberNode(tag, pendingProps, key, mode);
};

• 那么主角出来了，就是我们的FiberNode，这里才走完初始化的创建流程，

所以大致的流程就是上面的图里画的那样子，创建流程我们就告一段落，那我们再去看看更新的流程是怎么玩的。

我们知道除了ReactDOM.render()会触发更新流程之外，我们还有setState、强制更新、hooks里面的setXxxx等等手段可以触发更新，所谓setState那么不正好是我们Component原型上挂的方法嘛。我们回顾一下Component，那些更新都是调用了updater触发器上的方法，那么我们去看一下这个东西。

const classComponentUpdater = {isMounted,// setStateenqueueSetState(inst, payload, callback) {const fiber = getInstance(inst);const eventTime = requestEventTime(); // 获取更新触发的时间const lane = requestUpdateLane(fiber); // 获取任务优先级//根据更新触发时间 + 更新优先级来创建更新任务对象const update = createUpdate(eventTime, lane); // 创建更新任务对象// const update: Update<*> = {//   eventTime, // 更新时间//   lane, // 优先级//   tag: UpdateState, // 更新类型：0更新，1替换。，2强制替换，3捕获型更新//   payload: null,// 需要更新的内容//   callback: null, // 更新完后的回调//   next: null, // 指向下一个更新// };// 把内容填上update.payload = payload;if (callback !== undefined && callback !== null) {if (__DEV__) {// 开发环境下腰给个警告warnOnInvalidCallback(callback, 'setState');}// 如果有回调，那么加上回调update.callback = callback;}// const update: Update<*> = {//   eventTime, // 更新时间 you//   lane, // 优先级 you //   tag: UpdateState, // 更新类型：0更新，1替换。，2强制替换，3捕获型更新//   payload: null,// 需要更新的内容 you//   callback: null, // 更新完后的回调 you//   next: null, // 指向下一个更新// };enqueueUpdate(fiber, update);// 推入更新队列scheduleUpdateOnFiber(fiber, lane, eventTime);// 调度if (__DEV__) {if (enableDebugTracing) {if (fiber.mode & DebugTracingMode) {const name = getComponentName(fiber.type) || 'Unknown';logStateUpdateScheduled(name, lane, payload);}}}if (enableSchedulingProfiler) {markStateUpdateScheduled(fiber, lane);}},// replaceStateenqueueReplaceState(inst, payload, callback) {const fiber = getInstance(inst);const eventTime = requestEventTime();const lane = requestUpdateLane(fiber);const update = createUpdate(eventTime, lane);update.tag = ReplaceState;update.payload = payload;if (callback !== undefined && callback !== null) {if (__DEV__) {warnOnInvalidCallback(callback, 'replaceState');}update.callback = callback;}enqueueUpdate(fiber, update);scheduleUpdateOnFiber(fiber, lane, eventTime);if (__DEV__) {if (enableDebugTracing) {if (fiber.mode & DebugTracingMode) {const name = getComponentName(fiber.type) || 'Unknown';logStateUpdateScheduled(name, lane, payload);}}}if (enableSchedulingProfiler) {markStateUpdateScheduled(fiber, lane);}},// forceUpdateenqueueForceUpdate(inst, callback) {const fiber = getInstance(inst);const eventTime = requestEventTime();const lane = requestUpdateLane(fiber);const update = createUpdate(eventTime, lane);update.tag = ForceUpdate;if (callback !== undefined && callback !== null) {if (__DEV__) {warnOnInvalidCallback(callback, 'forceUpdate');}update.callback = callback;}enqueueUpdate(fiber, update);scheduleUpdateOnFiber(fiber, lane, eventTime);if (__DEV__) {if (enableDebugTracing) {if (fiber.mode & DebugTracingMode) {const name = getComponentName(fiber.type) || 'Unknown';logForceUpdateScheduled(name, lane);}}}if (enableSchedulingProfiler) {markForceUpdateScheduled(fiber, lane);}},
};

updateContainer

export function updateContainer(element: ReactNodeList,  container: OpaqueRoot,  parentComponent: ?React$Component<any, any>,  callback: ?Function,
): Lane {if (__DEV__) {onScheduleRoot(container, element);}const current = container.current;const eventTime = requestEventTime();if (__DEV__) {// $FlowExpectedError - jest isn't a global, and isn't recognized outside of testsif ('undefined' !== typeof jest) {warnIfUnmockedScheduler(current);warnIfNotScopedWithMatchingAct(current);}}const lane = requestUpdateLane(current);if (enableSchedulingProfiler) {markRenderScheduled(lane);}const context = getContextForSubtree(parentComponent);if (container.context === null) {container.context = context;} else {container.pendingContext = context;}if (__DEV__) {if (ReactCurrentFiberIsRendering &&ReactCurrentFiberCurrent !== null &&!didWarnAboutNestedUpdates) {didWarnAboutNestedUpdates = true;console.error('Render methods should be a pure function of props and state; ' +'triggering nested component updates from render is not allowed. ' +'If necessary, trigger nested updates in componentDidUpdate.\n\n' +'Check the render method of %s.',getComponentName(ReactCurrentFiberCurrent.type) || 'Unknown',);}}const update = createUpdate(eventTime, lane);// 创建更新任务// Caution: React DevTools currently depends on this property// being called "element".update.payload = {element};callback = callback === undefined ? null : callback;if (callback !== null) {if (__DEV__) {if (typeof callback !== 'function') {console.error('render(...): Expected the last optional `callback` argument to be a ' +'function. Instead received: %s.',callback,);}}update.callback = callback;}enqueueUpdate(current, update); // 推入更新队列scheduleUpdateOnFiber(current, lane, eventTime); // 进行调度return lane;
}

• 我们看到了enqueueSetState、enqueueReplaceState、enqueueForceUpdate还是初始化时候走的updateContainer都是走了几乎一样的逻辑：requestEventTime => requestUpdateLane => createUpdate => enqueueUpdate => scheduleUpdateOnFiber

总结

本章从ReactDOM.render()开始讲解了，初始化的时候，根节点的创建与更新流程，以及在类组件原型上挂载的一些更新的方法，但是为什么这一章不直接把他更新流程讲完呢？因为下一章要讲一下fiberNode这个东西，简而言之他只是一个架构概念，并不是React独有的，但是现在很有必要一起来看一看这个，那么下一章我们来一起揭开FiberNode的神秘面纱吧