React源码解析之RootFiber

一、Fiber的含义和作用
（1）每一个ReactElement对应一个Fiber对象

（2）记录节点的各种状态
比如ClassComponent中的state和props的状态就是记录在Fiber对象上的。

只有当Fiber对象更新后，才会更新到ClassComponent上的this.state和this.props上

this上的state和props是根据Fiber对象的state、props更新的。

这实际上也方便了ReactHooks，因为hooks是为FunctionalComponent服务的。虽然FunctionalComponent没有this，但Fiber上有，是可以拿到state和props的

（3）串联整个应用形成树结构
每个ReactElement通过props.children与其他ReactElement连结起来

说明：
① ReactElement只会把子节点（props.children）的第一个子节点当做child节点，其余的子节点（也就是第一个子节点的兄弟节点）都是从第一个子节点开始，依次单向连接至后一个兄弟节点

② 每个子节点都会指向父节点（红箭头），也就是Fiber对象的return属性

export type Fiber = {|
  //指向该对象在Fiber节点树中的`parent`，用来在处理完该节点后返回
  //即流程图上的红线
  return: Fiber | null,
}

串联过程：
① 任一 叶子 节点A，如果有兄弟节点，则去单向向后遍历兄弟节点，最后return到父节点
② 父节点的child节点不是刚刚的子节点A的话，则从child节点遍历到A前的节点，并再次return到父节点
③ 该父节点执行 ①、②

根据图1举例：
比如从左下角的input节点开始，它没有兄弟节点，则return到父组件input（因为父节点有且只有一个，所以必定return到父节点）

input有兄弟节点List，List又有child节点，则从child节点往后单向遍历兄弟节点，最后return到List

List又return到div，div的child节点已被遍历，则return到App节点，App，App又return到所有Fiber对象的根对象RootFiber对象

这样，就将整个应用遍历完了。

二、Fiber对象
源码：

// A Fiber is work on a Component that needs to be done or was done. There can
// be more than one per component.

//Fiber对应一个即将update或已经update的组件，
// 一个组件可以有一个或多个Fiber
export type Fiber = {|
  // These first fields are conceptually members of an Instance. This used to
  // be split into a separate type and intersected with the other Fiber fields,
  // but until Flow fixes its intersection bugs, we've merged them into a
  // single type.

  // An Instance is shared between all versions of a component. We can easily
  // break this out into a separate object to avoid copying so much to the
  // alternate versions of the tree. We put this on a single object for now to
  // minimize the number of objects created during the initial render.

  // Tag identifying the type of fiber.

  //标记不同的组件类型
  //有原生的DOM节点，有React自己的节点
  tag: WorkTag,

  // Unique identifier of this child.

  //ReactElement里面的key
  key: null | string,

  // The value of element.type which is used to preserve the identity during
  // reconciliation of this child.

  //ReactElement.type，也就是我们调用createElement的第一个参数
  elementType: any,

  // The resolved function/class/ associated with this fiber.

  //异步组件resolve之后返回的内容，一般是function或class
  //比如懒加载
  type: any,

  // The local state associated with this fiber.

  //当前Fiber的状态（比如浏览器环境就是DOM节点）

  //不同类型的实例都会记录在stateNode上
  //比如DOM组件对应DOM节点实例
  //ClassComponent对应Class实例
  //FunctionComponent没有实例，所以stateNode值为null

  //state更新了或props更新了均会更新到stateNode上
  stateNode: any,

  // Conceptual aliases
  // parent : Instance -> return The parent happens to be the same as the
  // return fiber since we've merged the fiber and instance.

  // Remaining fields belong to Fiber

  // The Fiber to return to after finishing processing this one.
  // This is effectively the parent, but there can be multiple parents (two)
  // so this is only the parent of the thing we're currently processing.
  // It is conceptually the same as the return address of a stack frame.

  //指向该对象在Fiber节点树中的`parent`，用来在处理完该节点后返回
  //即流程图上的红线
  return: Fiber | null,

  // Singly Linked List Tree Structure.
  //单链表树结构

  //指向自己的第一个子节点
  child: Fiber | null,

  //指向自己的兄弟结构
  //兄弟节点的return指向同一个父节点
  sibling: Fiber | null,
  index: number,

  // The ref last used to attach this node.
  // I'll avoid adding an owner field for prod and model that as functions.

  //ref属性
  ref: null | (((handle: mixed) => void) & {_stringRef: ?string}) | RefObject,

  // Input is the data coming into process this fiber. Arguments. Props.

  //新的变动带来的新的props，即nextProps
  pendingProps: any, // This type will be more specific once we overload the tag.

  //上一次渲染完成后的props,即 props
  memoizedProps: any, // The props used to create the output.

  // A queue of state updates and callbacks.

  //该Fiber对应的组件，所产生的update，都会放在该队列中
  updateQueue: UpdateQueue<any> | null,

  // The state used to create the output

  //上次渲染的state，即 state
  //新的state由updateQueue计算得出，并覆盖memoizedState
  memoizedState: any,

  // Dependencies (contexts, events) for this fiber, if it has any

  //一个列表，存在该Fiber依赖的contexts，events
  dependencies: Dependencies | null,

  // Bitfield that describes properties about the fiber and its subtree. E.g.
  // the ConcurrentMode flag indicates whether the subtree should be async-by-
  // default. When a fiber is created, it inherits the mode of its
  // parent. Additional flags can be set at creation time, but after that the
  // value should remain unchanged throughout the fiber's lifetime, particularly
  // before its child fibers are created.

  //mode有conCurrentMode和strictMode

  //用来描述当前Fiber和其他子树的Bitfield
  //共存的模式表示这个子树是否默认是 异步渲染的

  //Fiber刚被创建时，会继承父Fiber
  //其他标识也可以在创建的时候被设置，但是创建之后不该被修改，特别是它的子Fiber创建之前
  mode: TypeOfMode,

  //以下属性是副作用
  //副作用是 标记组件哪些需要更新的工具、标记组件需要执行哪些生命周期的工具

  // Effect
  effectTag: SideEffectTag,

  // Singly linked list fast path to the next fiber with side-effects.
  nextEffect: Fiber | null,

  // The first and last fiber with side-effect within this subtree. This allows
  // us to reuse a slice of the linked list when we reuse the work done within
  // this fiber.
  firstEffect: Fiber | null,
  lastEffect: Fiber | null,

  // Represents a time in the future by which this work should be completed.
  // Does not include work found in its subtree.

  //代表任务在未来的哪个时间点 应该被完成
  //不包括该Fiber的子树产生的任务
  expirationTime: ExpirationTime,

  // This is used to quickly determine if a subtree has no pending changes.

  //快速确定子树中是否有 update
  //如果子节点有update的话，就记录应该更新的时间
  childExpirationTime: ExpirationTime,

  // This is a pooled version of a Fiber. Every fiber that gets updated will
  // eventually have a pair. There are cases when we can clean up pairs to save
  // memory if we need to.

  // 在FIber树更新的过程中，每个Fiber都有与其对应的Fiber
  //我们称之为 current <==> workInProgress
  //在渲染完成后，会交换位置

  //doubleBuffer Fiber在更新后，不用再重新创建对象，
  // 而是复制自身，并且两者相互复用，用来提高性能

  alternate: Fiber | null,

  // Time spent rendering this Fiber and its descendants for the current update.
  // This tells us how well the tree makes use of sCU for memoization.
  // It is reset to 0 each time we render and only updated when we don't bailout.
  // This field is only set when the enableProfilerTimer flag is enabled.
  actualDuration?: number,

  // If the Fiber is currently active in the "render" phase,
  // This marks the time at which the work began.
  // This field is only set when the enableProfilerTimer flag is enabled.
  actualStartTime?: number,

  // Duration of the most recent render time for this Fiber.
  // This value is not updated when we bailout for memoization purposes.
  // This field is only set when the enableProfilerTimer flag is enabled.
  selfBaseDuration?: number,

  // Sum of base times for all descedents of this Fiber.
  // This value bubbles up during the "complete" phase.
  // This field is only set when the enableProfilerTimer flag is enabled.
  treeBaseDuration?: number,

  // Conceptual aliases
  // workInProgress : Fiber ->  alternate The alternate used for reuse happens
  // to be the same as work in progress.
  // __DEV__ only
  _debugID?: number,
  _debugSource?: Source | null,
  _debugOwner?: Fiber | null,
  _debugIsCurrentlyTiming?: boolean,
  _debugNeedsRemount?: boolean,

  // Used to verify that the order of hooks does not change between renders.
  _debugHookTypes?: Array<HookType> | null,
|};

解析：
熟悉Fiber的含义和属性含义就可以了，之后讲React更新的时候，还会提到它。

GitHub：
https://github.com/AttackXiaoJinJin/reactExplain/blob/master/react16.8.6/packages/react-reconciler/src/ReactFiber.js

三、总结
（1）Fiber的三个作用
（2）单向遍历
（3）props.children连接
（4）子指父
（5）doubleBuffer

（完）