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。
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）
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