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reconcileChildren

1 ）概述

• 在更新了一个节点之后，拿到它的props.children
• 要根据这个children里面的 ReactElement 来去创建子树的所有的 fiber 对象
• 要根据 props.children 来生成 fiber 子树，然后判断 fiber 对象它是否是可以复用的
  • 因为我们在第一次渲染的时候，就已经渲染了整个 fiber 子树
  • 再有一个更新进来之后，state 变化可能会导致一些子节点产生一个新的变化
  • 可能就不能复用之前的 fiber 节点了，它里面的很多东西都变得不一样
  • 大部分情况下所有 fiber 节点都是可以可以重复利用的
  • 这个时候我们根据什么进行判断，是这里面的一个非常重要的一个点
• 在这里就会拿出react当中的非常重要的key，列表根据key来优化

2 ）源码

在 packages/react-reconciler/src/ReactFiberBeginWork.js#L137 中

export function reconcileChildren(current: Fiber | null,workInProgress: Fiber,nextChildren: any,renderExpirationTime: ExpirationTime,
) {if (current === null) {// If this is a fresh new component that hasn't been rendered yet, we// won't update its child set by applying minimal side-effects. Instead,// we will add them all to the child before it gets rendered. That means// we can optimize this reconciliation pass by not tracking side-effects.workInProgress.child = mountChildFibers(workInProgress,null,nextChildren,renderExpirationTime,);} else {// If the current child is the same as the work in progress, it means that// we haven't yet started any work on these children. Therefore, we use// the clone algorithm to create a copy of all the current children.// If we had any progressed work already, that is invalid at this point so// let's throw it out.workInProgress.child = reconcileChildFibers(workInProgress,current.child,nextChildren,renderExpirationTime,);}
}

• 可见，根据 current 是否为 null 调用两个方法: mountChildFibers, reconcileChildFibers
  • 两个方法的区别，在第二个参数
  • 在前者方法中传递的是 null, 因为 第一次渲染，不存在 current.child
  • 因为第一次渲染的时候，都是父亲节点先渲染，子节点后渲染
  • 只有在后续的渲染过程当中，已经过第一次渲染了我们的父节点
  • 这个时候才有一个子节点存在，所以这是一个区别
  • 在 react-reconciler/src/ReactFiberBeginWork.js 中 的 reconcileChildren 调用这个 reconcileChildFibers
• 看一下这两个方法，它们来自于 ReactChildFiber.js

  export const reconcileChildFibers = ChildReconciler(true);
  export const mountChildFibers = ChildReconciler(false);
  

• 可见，上述两个方法的区别是 参数的 true 和 false
  • 这个参数的意思是: 表示是否要跟踪副作用
• 进入 ChildReconciler，这个方法 1k 多行，这里不去全部摘抄
• 用到哪里，逐个分析

整体框架

// 参数 shouldTrackSideEffects 表示是否要跟踪副作用
function ChildReconciler(shouldTrackSideEffects) {// ... 跳过很多代码// This API will tag the children with the side-effect of the reconciliation// itself. They will be added to the side-effect list as we pass through the// children and the parent.function reconcileChildFibers(returnFiber: Fiber,currentFirstChild: Fiber | null,newChild: any,expirationTime: ExpirationTime,): Fiber | null {// 省略很多代码// Remaining cases are all treated as empty.return deleteRemainingChildren(returnFiber, currentFirstChild);}return reconcileChildFibers;
}

• 在 ChildReconciler 方法的最后, return reconcileChildFibers;，先看下这个方法

  // This API will tag the children with the side-effect of the reconciliation
  // itself. They will be added to the side-effect list as we pass through the
  // children and the parent.
  function reconcileChildFibers(returnFiber: Fiber,currentFirstChild: Fiber | null,newChild: any,expirationTime: ExpirationTime,
  ): Fiber | null {// This function is not recursive.// If the top level item is an array, we treat it as a set of children,// not as a fragment. Nested arrays on the other hand will be treated as// fragment nodes. Recursion happens at the normal flow.// Handle top level unkeyed fragments as if they were arrays.// This leads to an ambiguity between <>{[...]}</> and <>...</>.// We treat the ambiguous cases above the same.// newChild 是计算出来的 新children// 这个节点是一个 top level 的节点，因为 <></> 这种类型会匹配 REACT_FRAGMENT_TYPE// 符合以下就是没有key的 topLevel节点const isUnkeyedTopLevelFragment =typeof newChild === 'object' &&newChild !== null &&newChild.type === REACT_FRAGMENT_TYPE &&newChild.key === null;// 如果匹配了，那么 Fragment 是没有任何的操作更新的, 本身并没有什么实际的意义// 如果匹配了，则将newChild赋值if (isUnkeyedTopLevelFragment) {newChild = newChild.props.children;}// Handle object typesconst isObject = typeof newChild === 'object' && newChild !== null;// 如果是对象，说明有下面两种情况// 下面两种方法调用相似，都是基于 placeSingleChild 传递不同参数// 一个是 reconcileSingleElement// 另一个是 reconcileSinglePortalif (isObject) {switch (newChild.$$typeof) {// 匹配 REACT_ELEMENT，是通过 React.createElement 产生的case REACT_ELEMENT_TYPE:return placeSingleChild(reconcileSingleElement(returnFiber,currentFirstChild,newChild,expirationTime,),);// 匹配 REACT_PORTAL, 是通过 ReactDOM.createPortalcase REACT_PORTAL_TYPE:return placeSingleChild(reconcileSinglePortal(returnFiber,currentFirstChild,newChild,expirationTime,),);}}// 匹配到 string 和 number 类就是 text类型的Nodeif (typeof newChild === 'string' || typeof newChild === 'number') {return placeSingleChild(reconcileSingleTextNode(returnFiber,currentFirstChild,'' + newChild,expirationTime,),);}// 匹配到数组if (isArray(newChild)) {return reconcileChildrenArray(returnFiber,currentFirstChild,newChild,expirationTime,);}// 匹配到可迭代的对象if (getIteratorFn(newChild)) {return reconcileChildrenIterator(returnFiber,currentFirstChild,newChild,expirationTime,);}// 以上都没有匹配，但仍然是对象，则 throw errorif (isObject) {throwOnInvalidObjectType(returnFiber, newChild);}// 忽略 DEVif (__DEV__) {if (typeof newChild === 'function') {warnOnFunctionType();}}// 匹配到 undefined 的对象if (typeof newChild === 'undefined' && !isUnkeyedTopLevelFragment) {// If the new child is undefined, and the return fiber is a composite// component, throw an error. If Fiber return types are disabled,// we already threw above.switch (returnFiber.tag) {// 匹配到 ClassComponent 忽略case ClassComponent: {if (__DEV__) {const instance = returnFiber.stateNode;if (instance.render._isMockFunction) {// We allow auto-mocks to proceed as if they're returning null.break;}}}// Intentionally fall through to the next case, which handles both// functions and classes// eslint-disable-next-lined no-fallthrough// 匹配到 FunctionComponent 提醒case FunctionComponent: {const Component = returnFiber.type;invariant(false,'%s(...): Nothing was returned from render. This usually means a ' +'return statement is missing. Or, to render nothing, ' +'return null.',Component.displayName || Component.name || 'Component',);}}}// Remaining cases are all treated as empty.// 以上都不符合，则 是 null, 删除现有所有 children// 新的 props.children 都是 null, 老的 props 节点都应该被删除return deleteRemainingChildren(returnFiber, currentFirstChild);
  }
  

• 接下来看不同节点的更新 reconcileSingleElement

  // 从当前已有的所有子节点中，找到一个可以复用新的子节点的Fiber对象
  // 复用它之后，把剩下兄弟节点全部删掉
  function reconcileSingleElement(returnFiber: Fiber,currentFirstChild: Fiber | null, // 是当前已有的fiber节点，初次渲染没有，后续渲染后很可能存在element: ReactElement,expirationTime: ExpirationTime,
  ): Fiber {// 获取 key 和 childconst key = element.key;let child = currentFirstChild;// child 存在while (child !== null) {// TODO: If key === null and child.key === null, then this only applies to// the first item in the list.if (child.key === key) {// 注意这个判断if (child.tag === Fragment? element.type === REACT_FRAGMENT_TYPE // 是否是 frament: child.elementType === element.type // 新老 element type) {// 删除 已存在当前节点的兄弟节点// 为何要删除兄弟节点，因为我们这次渲染只有一个节点，老的节点有兄弟节点，新的节点只有一个，删除之deleteRemainingChildren(returnFiber, child.sibling);const existing = useFiber(child,element.type === REACT_FRAGMENT_TYPE? element.props.children: element.props,expirationTime,);// 挂载属性existing.ref = coerceRef(returnFiber, child, element);existing.return = returnFiber;if (__DEV__) {existing._debugSource = element._source;existing._debugOwner = element._owner;}// 找到可复用节点，直接returnreturn existing;} else {// 条件不符合，删除节点并退出 deleteRemainingChildren(returnFiber, child);break;}} else {// key 不同，则删除deleteChild(returnFiber, child);}// 当前兄弟节点 等于 child 再次进入while循环child = child.sibling;}// 创建新的节点, 基于各种不同的类型，调用不同的创建 Fiber的方式if (element.type === REACT_FRAGMENT_TYPE) {const created = createFiberFromFragment(element.props.children, // 注意这个参数returnFiber.mode,expirationTime,element.key,);created.return = returnFiber;return created;} else {const created = createFiberFromElement(element,returnFiber.mode,expirationTime,);created.ref = coerceRef(returnFiber, currentFirstChild, element);created.return = returnFiber;return created;}
  }
  

  • 进入 createFiberFromFragment

    export function createFiberFromFragment(elements: ReactFragment,mode: TypeOfMode,expirationTime: ExpirationTime,key: null | string,
    ): Fiber {const fiber = createFiber(Fragment, elements, key, mode);fiber.expirationTime = expirationTime;return fiber;
    }// This is a constructor function, rather than a POJO constructor, still
    // please ensure we do the following:
    // 1) Nobody should add any instance methods on this. Instance methods can be
    //    more difficult to predict when they get optimized and they are almost
    //    never inlined properly in static compilers.
    // 2) Nobody should rely on `instanceof Fiber` for type testing. We should
    //    always know when it is a fiber.
    // 3) We might want to experiment with using numeric keys since they are easier
    //    to optimize in a non-JIT environment.
    // 4) We can easily go from a constructor to a createFiber object literal if that
    //    is faster.
    // 5) It should be easy to port this to a C struct and keep a C implementation
    //    compatible.
    const createFiber = function(tag: WorkTag,pendingProps: mixed, // 对于 fragment来说，props只有children, 直接把children作为propskey: null | string,mode: TypeOfMode,
    ): Fiber {// $FlowFixMe: the shapes are exact here but Flow doesn't like constructorsreturn new FiberNode(tag, pendingProps, key, mode);
    };
    

  • 以上的 fragment 可以在 ReactFiberBeginWork.js 中的 updateFragment 方法中找到验证

    function updateFragment(current: Fiber | null,workInProgress: Fiber,renderExpirationTime: ExpirationTime,
    ) {// 看这里，直接通过 .pendingProps 而非 props.children 获取，参考上述 createFiber 写明原因const nextChildren = workInProgress.pendingProps;reconcileChildren(current,workInProgress,nextChildren,renderExpirationTime,);return workInProgress.child;
    }
    

  • 进入 createFiberFromElement

    export function createFiberFromElement(element: ReactElement,mode: TypeOfMode,expirationTime: ExpirationTime,
    ): Fiber {let owner = null;if (__DEV__) {owner = element._owner;}// 获取各类属性const type = element.type;const key = element.key;const pendingProps = element.props;// 创建 Fiberconst fiber = createFiberFromTypeAndProps(type,key,pendingProps,owner,mode,expirationTime,);if (__DEV__) {fiber._debugSource = element._source;fiber._debugOwner = element._owner;}return fiber;
    }
    

  • 进入 createFiberFromTypeAndProps

    // 这个方法比较复杂，要判断不同的 type 给 fiber 对象增加 不同的 tag
    // 主要是去匹配 fiberTag
    export function createFiberFromTypeAndProps(type: any, // React$ElementTypekey: null | string,pendingProps: any,owner: null | Fiber,mode: TypeOfMode,expirationTime: ExpirationTime,
    ): Fiber {let fiber;// 组件tag未知时的初始化配置 未指定状态let fiberTag = IndeterminateComponent;// The resolved type is set if we know what the final type will be. I.e. it's not lazy.let resolvedType = type;// 根据不同类型来处理 fiberTagif (typeof type === 'function') {// 判断是否有 constructor 方法/*function shouldConstruct(Component: Function) {const prototype = Component.prototype;// 注意，这里 isReactComponent 是一个 {}return !!(prototype && prototype.isReactComponent);}*/if (shouldConstruct(type)) {fiberTag = ClassComponent;}} else if (typeof type === 'string') {fiberTag = HostComponent;} else {getTag: switch (type) {case REACT_FRAGMENT_TYPE:return createFiberFromFragment(pendingProps.children,mode,expirationTime,key,);case REACT_CONCURRENT_MODE_TYPE:return createFiberFromMode(pendingProps,mode | ConcurrentMode | StrictMode,expirationTime,key,);case REACT_STRICT_MODE_TYPE:return createFiberFromMode(pendingProps,mode | StrictMode,expirationTime,key,);case REACT_PROFILER_TYPE:return createFiberFromProfiler(pendingProps, mode, expirationTime, key);case REACT_SUSPENSE_TYPE:return createFiberFromSuspense(pendingProps, mode, expirationTime, key);default: {if (typeof type === 'object' && type !== null) {switch (type.$$typeof) {case REACT_PROVIDER_TYPE:fiberTag = ContextProvider;break getTag;case REACT_CONTEXT_TYPE:// This is a consumerfiberTag = ContextConsumer;break getTag;case REACT_FORWARD_REF_TYPE:fiberTag = ForwardRef;break getTag;case REACT_MEMO_TYPE:fiberTag = MemoComponent;break getTag;case REACT_LAZY_TYPE:fiberTag = LazyComponent;resolvedType = null;break getTag;}}let info = '';if (__DEV__) {if (type === undefined ||(typeof type === 'object' &&type !== null &&Object.keys(type).length === 0)) {info +=' You likely forgot to export your component from the file ' +"it's defined in, or you might have mixed up default and " +'named imports.';}const ownerName = owner ? getComponentName(owner.type) : null;if (ownerName) {info += '\n\nCheck the render method of `' + ownerName + '`.';}}invariant(false,'Element type is invalid: expected a string (for built-in ' +'components) or a class/function (for composite components) ' +'but got: %s.%s',type == null ? type : typeof type,info,);}}}// 最终创建 Fiberfiber = createFiber(fiberTag, pendingProps, key, mode);fiber.elementType = type;fiber.type = resolvedType;fiber.expirationTime = expirationTime;return fiber;
    }
    

  • 以上，reconcileSingleElement 就完了

• 进入 reconcileSingleTextNode

  function reconcileSingleTextNode(returnFiber: Fiber,currentFirstChild: Fiber | null,textContent: string,expirationTime: ExpirationTime,
  ): Fiber {// There's no need to check for keys on text nodes since we don't have a// way to define them.// 处理 原生 标签if (currentFirstChild !== null && currentFirstChild.tag === HostText) {// We already have an existing node so let's just update it and delete// the rest.// 删除兄弟节点，新节点只是一个纯的文本节点deleteRemainingChildren(returnFiber, currentFirstChild.sibling);// 基于 useFiber 复用当前节点const existing = useFiber(currentFirstChild, textContent, expirationTime);existing.return = returnFiber;return existing;}// The existing first child is not a text node so we need to create one// and delete the existing ones.deleteRemainingChildren(returnFiber, currentFirstChild);const created = createFiberFromText(textContent,returnFiber.mode,expirationTime,);created.return = returnFiber;return created;
  }
  

  • 这个流程就比较简单了

• 现在我们主要看下 deleteRemainingChildren 这个api

  function deleteRemainingChildren(returnFiber: Fiber, // 当前正在更新的节点currentFirstChild: Fiber | null, // 子节点
  ): null {// 这是第一次渲染的时候，没有子节点// 直接 returnif (!shouldTrackSideEffects) {// Noop.return null;}// TODO: For the shouldClone case, this could be micro-optimized a bit by// assuming that after the first child we've already added everything.// 一个个的删除let childToDelete = currentFirstChild;while (childToDelete !== null) {deleteChild(returnFiber, childToDelete);childToDelete = childToDelete.sibling;}return null;
  }
  

  • 进入 deleteChild

    // 可以看到这里没有实施删除的操作，只是改变了节点上的 effectTag
    // 这里我们只更新 Fiber Tree, 如果真的要 delete 就会影响到 dom节点
    // 所以删除的流程不在这里做，只有在下个阶段，也就是 commit 阶段来做
    // 就是把Fiber Tree 需要更新的流程都过一遍之后，把整个需要更新的属性
    // 通过 firstEffect，lastEffect 这种链条，最终链到根节点上面
    // 最终要执行更新 只需要在 commit 阶段拿到 根节点上面的 这种链条把每个节点去执行即可
    // 在处理更新的时候任务是可以中断的，但是在 commit 阶段任务是不可中断的
    function deleteChild(returnFiber: Fiber, childToDelete: Fiber): void {if (!shouldTrackSideEffects) {// Noop.return;}// Deletions are added in reversed order so we add it to the front.// At this point, the return fiber's effect list is empty except for// deletions, so we can just append the deletion to the list. The remaining// effects aren't added until the complete phase. Once we implement// resuming, this may not be true.const last = returnFiber.lastEffect;// 存在 lastif (last !== null) {last.nextEffect = childToDelete;returnFiber.lastEffect = childToDelete;} else {// 不存在 lastreturnFiber.firstEffect = returnFiber.lastEffect = childToDelete;}childToDelete.nextEffect = null; // 已经是要删除的节点，上面的其他副作用都是没有任何意义的// effectTag 就是告诉后续的 commit 阶段，对于这个节点需要执行什么操作，可以看到，这里是删除childToDelete.effectTag = Deletion; // 只需要设置这个即可
    }
    

• reconcileChildrenArray 和 reconcileChildrenIterator 这两个api比较复杂，先跳过

• 其他的 API 都是比较简单了