React16源码: React中的IndeterminateComponent的源码实现

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IndeterminateComponent

1 ）概述

这是一个比较特殊的component的类型，就是还没有被指定类型的component
在一个fibrer被创建的时候，它的tag可能会是 IndeterminateComponent
在 packages/react-reconciler/src/ReactFiber.js 中，有一个方法

createFiberFromTypeAndProps 中，一开始就声明了

let fiberTag = IndeterminateComponent;
let resolvedType = type;
// 一开始
if (typeof type === 'function') {// 在 function 下只是判断了 constructor是否存在// 在不存在的时候，是否认为它是一个 FunctionComponent 呢，实际上并没有// 实际上我们写的任何一个 function component 一开始就是 IndeterminateComponent 类型if (shouldConstruct(type)) {// 存在，则赋值为 ClassComponentfiberTag = ClassComponent;}
} else if (typeof type === 'string') {fiberTag = HostComponent;
} else {// 省略
}

在最终调用 createFiber 创建 Fiber 对象

2 ）源码

定位到 packages/react-reconciler/src/ReactFiber.js
进入 mountIndeterminateComponent 方法

// 
function mountIndeterminateComponent(_current,workInProgress,Component,renderExpirationTime,
) {// 首先判断 _current 是否存在，存在则进行初始化操作// 因为只有在第一次渲染的时候，才有 indeterminate component 这种情况// 经过第一次渲染之后，我们就会发现 indeterminate component 的具体的类型// 这种情况，可能是中途抛出一个 error 或 promise 等情况，比如 Suspense 组件// 这时候初始化是为了 去除 _current 和 workInProgress 相关联系，因为需要重新进行初次渲染的流程if (_current !== null) {// An indeterminate component only mounts if it suspended inside a non-// concurrent tree, in an inconsistent state. We want to treat it like// a new mount, even though an empty version of it already committed.// Disconnect the alternate pointers._current.alternate = null;workInProgress.alternate = null;// Since this is conceptually a new fiber, schedule a Placement effectworkInProgress.effectTag |= Placement;}const props = workInProgress.pendingProps;const unmaskedContext = getUnmaskedContext(workInProgress, Component, false);const context = getMaskedContext(workInProgress, unmaskedContext);prepareToReadContext(workInProgress, renderExpirationTime);prepareToUseHooks(null, workInProgress, renderExpirationTime);let value;if (__DEV__) {if (Component.prototype &&typeof Component.prototype.render === 'function') {const componentName = getComponentName(Component) || 'Unknown';if (!didWarnAboutBadClass[componentName]) {warningWithoutStack(false,"The <%s /> component appears to have a render method, but doesn't extend React.Component. " +'This is likely to cause errors. Change %s to extend React.Component instead.',componentName,componentName,);didWarnAboutBadClass[componentName] = true;}}if (workInProgress.mode & StrictMode) {ReactStrictModeWarnings.recordLegacyContextWarning(workInProgress, null);}ReactCurrentOwner.current = workInProgress;value = Component(props, context); } else {value = Component(props, context); // 调用 Component 方法}// React DevTools reads this flag.workInProgress.effectTag |= PerformedWork;// 符合这个条件，认为是 ClassComponent, 具有 render 方法if (typeof value === 'object' &&value !== null &&typeof value.render === 'function' &&value.$$typeof === undefined) {// Proceed under the assumption that this is a class instanceworkInProgress.tag = ClassComponent; // 这里认为它是一个 ClassComponent// Throw out any hooks that were used.resetHooks();// Push context providers early to prevent context stack mismatches.// During mounting we don't know the child context yet as the instance doesn't exist.// We will invalidate the child context in finishClassComponent() right after rendering.let hasContext = false;if (isLegacyContextProvider(Component)) {hasContext = true;pushLegacyContextProvider(workInProgress);} else {hasContext = false;}workInProgress.memoizedState =value.state !== null && value.state !== undefined ? value.state : null;const getDerivedStateFromProps = Component.getDerivedStateFromProps;if (typeof getDerivedStateFromProps === 'function') {applyDerivedStateFromProps(workInProgress,Component,getDerivedStateFromProps,props,);}adoptClassInstance(workInProgress, value);mountClassInstance(workInProgress, Component, props, renderExpirationTime);return finishClassComponent(null,workInProgress,Component,true,hasContext,renderExpirationTime,);} else {// 否则按照 FunctionComponent 来渲染// Proceed under the assumption that this is a function componentworkInProgress.tag = FunctionComponent; value = finishHooks(Component, props, value, context);if (__DEV__) {if (Component) {warningWithoutStack(!Component.childContextTypes,'%s(...): childContextTypes cannot be defined on a function component.',Component.displayName || Component.name || 'Component',);}if (workInProgress.ref !== null) {let info = '';const ownerName = ReactCurrentFiber.getCurrentFiberOwnerNameInDevOrNull();if (ownerName) {info += '\n\nCheck the render method of `' + ownerName + '`.';}let warningKey = ownerName || workInProgress._debugID || '';const debugSource = workInProgress._debugSource;if (debugSource) {warningKey = debugSource.fileName + ':' + debugSource.lineNumber;}if (!didWarnAboutFunctionRefs[warningKey]) {didWarnAboutFunctionRefs[warningKey] = true;warning(false,'Function components cannot be given refs. ' +'Attempts to access this ref will fail.%s',info,);}}if (typeof Component.getDerivedStateFromProps === 'function') {const componentName = getComponentName(Component) || 'Unknown';if (!didWarnAboutGetDerivedStateOnFunctionComponent[componentName]) {warningWithoutStack(false,'%s: Function components do not support getDerivedStateFromProps.',componentName,);didWarnAboutGetDerivedStateOnFunctionComponent[componentName] = true;}}if (typeof Component.contextType === 'object' &&Component.contextType !== null) {const componentName = getComponentName(Component) || 'Unknown';if (!didWarnAboutContextTypeOnFunctionComponent[componentName]) {warningWithoutStack(false,'%s: Function components do not support contextType.',componentName,);didWarnAboutContextTypeOnFunctionComponent[componentName] = true;}}}reconcileChildren(null, workInProgress, value, renderExpirationTime);return workInProgress.child;}
}

基于上述判断条件能认定是一个 ClassComponent，后续渲染一定会按照 ClassComponent 进行
- if ( typeof value === 'object' && value !== null && typeof value.render === 'function' && value.$$typeof === undefined ){}
现在来测试一下，看下 function component 是否可以执行
```
import React from 'react'export default function TestIndeterminationComponent() {return {componentDidMount() {console.log('invoker')},render() {return <span>aaa</span>}}
}
```
- 上述都能正常显示，以及打印 console 输出
- 也就是说，对于一个 function component, 如果里面 return 的对象，具有 render 方法
- 就认为它是一个 class component 一样的类型，去使用它
- 并且在里面声明的生命周期方法都会被它调用
- 这是 IndeterminateComponent 的特性
在最初我们渲染的时候，所有的 function component 都是 IndeterminateComponent 的类型
在第一次渲染之后，我们根据渲染类型的返回，最终得到具体类型
可以通过 function component 返回一个对象的方式去渲染一个类似 classComponent 这样的类型
注意，一般不推荐这么写