摘要：一的含義和作用每一個對應(yīng)一個對象記錄節(jié)點的各種狀態(tài)比如中的和的狀態(tài)就是記錄在對象上的。只有當(dāng)對象更新后，才會更新到上的和上上的和是根據(jù)對象的更新的。

一、Fiber的含義和作用
（1）每一個ReactElement對應(yīng)一個Fiber對象

（2）記錄節(jié)點的各種狀態(tài)
比如ClassComponent中的state和props的狀態(tài)就是記錄在Fiber對象上的。

只有當(dāng)Fiber對象更新后，才會更新到ClassComponent上的this.state和this.props上

this上的state和props是根據(jù)Fiber對象的state、props更新的。

這實際上也方便了ReactHooks，因為hooks是為FunctionalComponent服務(wù)的。雖然FunctionalComponent沒有this，但Fiber上有，是可以拿到state和props的

（3）串聯(lián)整個應(yīng)用形成樹結(jié)構(gòu)
每個ReactElement通過props.children與其他ReactElement連結(jié)起來

說明：
① ReactElement只會把子節(jié)點（props.children）的第一個子節(jié)點當(dāng)做child節(jié)點，其余的子節(jié)點（也就是第一個子節(jié)點的兄弟節(jié)點）都是從第一個子節(jié)點開始，依次單向連接至后一個兄弟節(jié)點

② 每個子節(jié)點都會指向父節(jié)點（紅箭頭），也就是Fiber對象的return屬性

export type Fiber = {|
  //指向該對象在Fiber節(jié)點樹中的`parent`，用來在處理完該節(jié)點后返回
  //即流程圖上的紅線
  return: Fiber | null,
}

串聯(lián)過程：
① 任一 葉子 節(jié)點A，如果有兄弟節(jié)點，則去單向向后遍歷兄弟節(jié)點，最后return到父節(jié)點
② 父節(jié)點的child節(jié)點不是剛剛的子節(jié)點A的話，則從child節(jié)點遍歷到A前的節(jié)點，并再次return到父節(jié)點
③ 該父節(jié)點執(zhí)行 ①、②

根據(jù)圖1舉例：
比如從左下角的input節(jié)點開始，它沒有兄弟節(jié)點，則return到父組件Input（因為父節(jié)點有且只有一個，所以必定return到父節(jié)點）

Input有兄弟節(jié)點List，List又有child節(jié)點，則從child節(jié)點往后單向遍歷兄弟節(jié)點，最后return到List

List又return到div，div的child節(jié)點已被遍歷，則return到App節(jié)點，App，App又return到所有Fiber對象的根對象RootFiber對象

這樣，就將整個應(yīng)用遍歷完了。

二、Fiber對象
源碼：

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

//Fiber對應(yīng)一個即將update或已經(jīng)update的組件，
// 一個組件可以有一個或多個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.

  //標(biāo)記不同的組件類型
  //有原生的DOM節(jié)點，有React自己的節(jié)點
  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，也就是我們調(diào)用createElement的第一個參數(shù)
  elementType: any,

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

  //異步組件resolve之后返回的內(nèi)容，一般是function或class
  //比如懶加載
  type: any,

  // The local state associated with this fiber.

  //當(dāng)前Fiber的狀態(tài)（比如瀏覽器環(huán)境就是DOM節(jié)點）

  //不同類型的實例都會記錄在stateNode上
  //比如DOM組件對應(yīng)DOM節(jié)點實例
  //ClassComponent對應(yīng)Class實例
  //FunctionComponent沒有實例，所以stateNode值為null

  //state更新了或props更新了均會更新到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.

  //指向該對象在Fiber節(jié)點樹中的`parent`，用來在處理完該節(jié)點后返回
  //即流程圖上的紅線
  return: Fiber | null,

  // Singly Linked List Tree Structure.
  //單鏈表樹結(jié)構(gòu)

  //指向自己的第一個子節(jié)點
  child: Fiber | null,

  //指向自己的兄弟結(jié)構(gòu)
  //兄弟節(jié)點的return指向同一個父節(jié)點
  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屬性
  ref: null | (((handle: mixed) => void) & {_stringRef: ?string}) | RefObject,

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

  //新的變動帶來的新的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.

  //該Fiber對應(yīng)的組件，所產(chǎn)生的update，都會放在該隊列中
  updateQueue: UpdateQueue | null,

  // The state used to create the output

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

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

  //一個列表，存在該Fiber依賴的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

  //用來描述當(dāng)前Fiber和其他子樹的Bitfield
  //共存的模式表示這個子樹是否默認(rèn)是 異步渲染的

  //Fiber剛被創(chuàng)建時，會繼承父Fiber
  //其他標(biāo)識也可以在創(chuàng)建的時候被設(shè)置，但是創(chuàng)建之后不該被修改，特別是它的子Fiber創(chuàng)建之前
  mode: TypeOfMode,

  //以下屬性是副作用
  //副作用是 標(biāo)記組件哪些需要更新的工具、標(biāo)記組件需要執(zhí)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.

  //代表任務(wù)在未來的哪個時間點 應(yīng)該被完成
  //不包括該Fiber的子樹產(chǎn)生的任務(wù)
  expirationTime: ExpirationTime,

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

  //快速確定子樹中是否有 update
  //如果子節(jié)點有update的話，就記錄應(yīng)該更新的時間
  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樹更新的過程中，每個Fiber都有與其對應(yīng)的Fiber
  //我們稱之為 current <==> workInProgress
  //在渲染完成后，會交換位置

  //doubleBuffer Fiber在更新后，不用再重新創(chuàng)建對象，
  // 而是復(fù)制自身，并且兩者相互復(fù)用，用來提高性能

  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 | null,
|};

解析：
熟悉Fiber的含義和屬性含義就可以了，之后講React更新的時候，還會提到它。

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

三、總結(jié)
（1）Fiber的三個作用
（2）單向遍歷
（3）props.children連接
（4）子指父
（5）doubleBuffer

（完）