前面的章节，我们在render方法里，简单涉及到了fiber，那么：

fiber究竟是什么？
fiber的出现解决了什么问题？
为什么fiber之前的版本，没办法解决

下面，我们带着这些问题，来仔细聊聊Fiber

一. Fiber是什么

在fiber出现之前，react的架构体系只有协调器reconciler和渲染器render。

当前有新的update时，react会递归所有的vdom节点，如果dom节点过多，会导致其他事件影响滞后，造成卡顿。即之前的react版本无法中断工作过程，一旦递归开始无法停留下来。

为了解决这一系列问题，react历时多年重构了底层架构，引入了fiber。 fiber的出现使得react能够异步可中断工作任务，并且可以在浏览器空闲时，从中断处继续往下工作。当出现多个高优先任务并行时，react引入lane模型取代之前的expireTime机制。

这里提及下vue工作原理，为什么有新的update任务时，vue不需要做全量递归，而react需要？（留个悬念，大家可以先思考下）

fiber本质上是一种数据结构，在react17后，没有vdom概念，一切皆是Fiber，但Fiber != vdom。

二. FiberRoot

FiberRoot是react启动阶段，要构建的fiber对象。与之容易混淆是rootFiber，下面会具体介绍。

fiberRoot生成

  function createFiberRoot(containerInfo, tag, hydrate, hydrationCallbacks) {
    var root = new FiberRootNode(containerInfo, tag, hydrate);
    // stateNode is any.

    
    var uninitializedFiber = createHostRootFiber(tag);
    root.current = uninitializedFiber;
    uninitializedFiber.stateNode = root;
    initializeUpdateQueue(uninitializedFiber);
    return root;
  }

fiberRoot类

function FiberRootNode(containerInfo, tag, hydrate) {
    this.tag = tag;
    this.containerInfo = containerInfo;
    this.pendingChildren = null;
    this.current = null;
    this.pingCache = null;
    this.finishedWork = null;
    this.timeoutHandle = noTimeout;
    this.context = null;
    this.pendingContext = null;
    this.hydrate = hydrate;
    this.callbackNode = null;
    this.callbackPriority = NoLanePriority;
    this.eventTimes = createLaneMap(NoLanes);
    this.expirationTimes = createLaneMap(NoTimestamp);
    this.pendingLanes = NoLanes;
    this.suspendedLanes = NoLanes;
    this.pingedLanes = NoLanes;
    this.expiredLanes = NoLanes;
    this.mutableReadLanes = NoLanes;
    this.finishedLanes = NoLanes;
    this.entangledLanes = NoLanes;
    this.entanglements = createLaneMap(NoLanes);

    {
      this.mutableSourceEagerHydrationData = null;
    }

    {
      this.interactionThreadID = unstable_getThreadID();
      this.memoizedInteractions = new Set();
      this.pendingInteractionMap = new Map();
    }

    {
      switch (tag) {
        case BlockingRoot:
          this._debugRootType = 'createBlockingRoot()';
          break;

        case ConcurrentRoot:
          this._debugRootType = 'createRoot()';
          break;

        case LegacyRoot:
          this._debugRootType = 'createLegacyRoot()';
          break;
      }
    }
  }

fiberRoot本质上fiber的顶层对象，其中tag记录了几种启动模式：

0普通模式
1 小部分并发模式
2 并发模式

启动模式的不同，在后协调阶段有具体差异。

该类引用的实例，即current对象是rootFiber。finishedWork是fiber完成协调器work之后的结果，下面有许多字段都带有lane，这里可以先不关注，后面章节我们单独聊聊Lane模型

三. RootFiber

rootFiber生成

  function createFiberRoot(containerInfo, tag, hydrate, hydrationCallbacks) {
    var root = new FiberRootNode(containerInfo, tag, hydrate);
    // stateNode is any.

    
    var uninitializedFiber = createHostRootFiber(tag);
    root.current = uninitializedFiber;
    uninitializedFiber.stateNode = root;
    initializeUpdateQueue(uninitializedFiber);
    return root;
  }

createHostRootFiber

  function createHostRootFiber(tag) {
    var mode;

    if (tag === ConcurrentRoot) {
      mode = ConcurrentMode | BlockingMode | StrictMode;
    } else if (tag === BlockingRoot) {
      mode = BlockingMode | StrictMode;
    } else {
      mode = NoMode;
    }

    if ( 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);
  }

FiberNode

function FiberNode(tag, pendingProps, key, mode) {
    this.tag = tag;
    this.key = key;
    this.elementType = null;
    this.type = null;
    this.stateNode = null;

    this.return = null;
    this.child = null;
    this.sibling = null;
    
    this.index = 0;
    this.ref = null;

    this.pendingProps = pendingProps; 
    this.memoizedProps = null; 
    this.updateQueue = null; 

    this.memoizedState = null;  
    this.dependencies = null;
    this.mode = mode; // Effects

    this.flags = NoFlags;     
    this.nextEffect = null;  
    this.firstEffect = null; 
    this.lastEffect = null; 
    this.lanes = NoLanes;
    this.childLanes = NoLanes;
    this.alternate = null;

    {
      this.actualDuration = Number.NaN;
      this.actualStartTime = Number.NaN;
      this.selfBaseDuration = Number.NaN;
      this.treeBaseDuration = Number.NaN; // It's okay to replace the initial doubles with smis after initialization.

      this.actualDuration = 0;
      this.actualStartTime = -1;
      this.selfBaseDuration = 0;
      this.treeBaseDuration = 0;
    }

    {
      // This isn't directly used but is handy for debugging internals:
      this._debugID = debugCounter++;
      this._debugSource = null;
      this._debugOwner = null;
      this._debugNeedsRemount = false;
      this._debugHookTypes = null;

      if (!hasBadMapPolyfill && typeof Object.preventExtensions === 'function') {
        Object.preventExtensions(this);
      }
    }
  }

到这里，fiberNode是一个类，往下的所有dom都将实例化这个类。

这里的tag依旧是启动模式，return是父节点fiber，child是子节点第一个fiber，sibling是兄弟节点的fiber。

另外，flags很重要，在后续work阶段会大量使用，另外flags和lane都是二进制数据对象，后面大量运用位运算。

effect对象，会在work loop阶段生成，也就是副作用，比如我们写的useEffect，都在work lopp阶段被挂载。

每个fiber的stateNode指向具体实例节点。

flags

export type Flags = number;

export const NoFlags = /*                      */ 0b000000000000000000;
export const PerformedWork = /*                */ 0b000000000000000001;

export const Placement = /*                    */ 0b000000000000000010;
export const Update = /*                       */ 0b000000000000000100;
export const PlacementAndUpdate = /*           */ 0b000000000000000110;
export const Deletion = /*                     */ 0b000000000000001000;
export const ContentReset = /*                 */ 0b000000000000010000;
export const Callback = /*                     */ 0b000000000000100000;
export const DidCapture = /*                   */ 0b000000000001000000;
export const Ref = /*                          */ 0b000000000010000000;
export const Snapshot = /*                     */ 0b000000000100000000;
export const Passive = /*                      */ 0b000000001000000000;
// TODO (effects) Remove this bit once the new reconciler is synced to the old.
export const PassiveUnmountPendingDev = /*     */ 0b000010000000000000;
export const Hydrating = /*                    */ 0b000000010000000000;
export const HydratingAndUpdate = /*           */ 0b000000010000000100;

// Passive & Update & Callback & Ref & Snapshot
export const LifecycleEffectMask = /*          */ 0b000000001110100100;

// Union of all host effects
export const HostEffectMask = /*               */ 0b000000011111111111;

// These are not really side effects, but we still reuse this field.
export const Incomplete = /*                   */ 0b000000100000000000;
export const ShouldCapture = /*                */ 0b000001000000000000;
export const ForceUpdateForLegacySuspense = /* */ 0b000100000000000000;


export const PassiveStatic = /*                */ 0b001000000000000000;


export const BeforeMutationMask = /*           */ 0b000000001100001010;
export const MutationMask = /*                 */ 0b000000010010011110;
export const LayoutMask = /*                   */ 0b000000000010100100;
export const PassiveMask = /*                  */ 0b000000001000001000;


export const StaticMask = /*                   */ 0b001000000000000000;


export const MountLayoutDev = /*               */ 0b010000000000000000;
export const MountPassiveDev = /*              */ 0b100000000000000000;

lane

export const NoLanes: Lanes = /*                        */ 0b0000000000000000000000000000000;
export const NoLane: Lane = /*                          */ 0b0000000000000000000000000000000;

export const SyncLane: Lane = /*                        */ 0b0000000000000000000000000000001;
export const SyncBatchedLane: Lane = /*                 */ 0b0000000000000000000000000000010;

export const InputDiscreteHydrationLane: Lane = /*      */ 0b0000000000000000000000000000100;
const InputDiscreteLanes: Lanes = /*                    */ 0b0000000000000000000000000011000;

const InputContinuousHydrationLane: Lane = /*           */ 0b0000000000000000000000000100000;
const InputContinuousLanes: Lanes = /*                  */ 0b0000000000000000000000011000000;

export const DefaultHydrationLane: Lane = /*            */ 0b0000000000000000000000100000000;
export const DefaultLanes: Lanes = /*                   */ 0b0000000000000000000111000000000;

const TransitionHydrationLane: Lane = /*                */ 0b0000000000000000001000000000000;
const TransitionLanes: Lanes = /*                       */ 0b0000000001111111110000000000000;

const RetryLanes: Lanes = /*                            */ 0b0000011110000000000000000000000;

export const SomeRetryLane: Lanes = /*                  */ 0b0000010000000000000000000000000;

export const SelectiveHydrationLane: Lane = /*          */ 0b0000100000000000000000000000000;

const NonIdleLanes = /*                                 */ 0b0000111111111111111111111111111;

export const IdleHydrationLane: Lane = /*               */ 0b0001000000000000000000000000000;
const IdleLanes: Lanes = /*                             */ 0b0110000000000000000000000000000;

export const OffscreenLane: Lane = /*                   */ 0b1000000000000000000000000000000;

关于flags和lane，我们先有个感性认知，后面章节单独分析

initializeUpdateQueue

  function initializeUpdateQueue(fiber) {
    var queue = {
      baseState: fiber.memoizedState,
      firstBaseUpdate: null,
      lastBaseUpdate: null,
      shared: {
        pending: null
      },
      effects: null
    };
    fiber.updateQueue = queue;
  }

rootFiber上调用initializeUpdateQueue，初始化queue对象，这里仅仅是初始化对象而已，并不是许多文章说fiber加入更新队列。fiber的更新队列和这里没有任何关系，fiber的更新队列是后续schedule调度的task queue。

四. Root

root对象是legacyCreateRootFromDOMContainer方法的返回对象，这个对象是全局唯一，并贯穿了react后续的各阶段计算。

至此，我们对应fiber有个感性的认知。另外需要说明的是，每个dom节点都是fiber，fiber通过return, child, sibling关联其他fiber，本质上fiber是个链表数据结构，这一点和后续的effect数据结构还是有区别的。

在root生成后，首次初始化应用，将进入核心updateContainer方法

updateContainer(children, fiberRoot, parentComponent, callback);

updateContainer

  function updateContainer(element, container, parentComponent, callback) {
      // ...省略eventTime和lane相关，后续单独介绍

    var update = createUpdate(eventTime, lane); // Caution: React DevTools currently depends on this property
    // being called "element".

    update.payload = {
      element: element
    };
    callback = callback === undefined ? null : callback;

    if (callback !== null) {
      {
        if (typeof callback !== 'function') {
          error('render(...): Expected the last optional `callback` argument to be a ' + 'function. Instead received: %s.', callback);
        }
      }

      update.callback = callback;
    }

    enqueueUpdate(current$1, update);
    scheduleUpdateOnFiber(current$1, lane, eventTime);
    return lane;
  }

createUpdate

  function createUpdate(eventTime, lane) {
    var update = {
      eventTime: eventTime,
      lane: lane,
      tag: UpdateState,
      payload: null,
      callback: null,
      next: null
    };
    return update;
  }

  function enqueueUpdate(fiber, update) {
    var updateQueue = fiber.updateQueue;

    if (updateQueue === null) {
      // Only occurs if the fiber has been unmounted.
      return;
    }

    var sharedQueue = updateQueue.shared;
    var pending = sharedQueue.pending;

    if (pending === null) {
      // This is the first update. Create a circular list.
      update.next = update;
    } else {
      update.next = pending.next;
      pending.next = update;
    }

    sharedQueue.pending = update;
  }

在rootFiber上，创建更新对象，并挂载至enqueueUpdate上。

update对象上payload很重要，后面在协调阶段-fiber树构建阶段是重要的输入。

update对象也是链表结构，通过next关联下一个update对象。

至此，fiber的初始化对象到这里就结束了，目前内存中只存在fiberRoot和rootFiber对象，下面调的scheduleUpdateOnFiber方法，将正式进入协调阶段。

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React源码解析-Fiber介绍