Vue3源码解析watch函数实例

// reactivity/src/effect.ts
export interface DebuggerOptions {
onTrack?: (event: DebuggerEvent) => void
onTrigger?: (event: DebuggerEvent) => void
}
​
// runtime-core/apiWatch.ts
export interface WatchOptionsBase extends DebuggerOptions {
flush?: 'pre' | 'post' | 'sync'
}
​
export interface WatchOptions<Immediate = boolean> extends WatchOptionsBase {
immediate?: Immediate
deep?: boolean
}

// 副作用
export type WatchEffect = (onCleanup: OnCleanup) => void
​
export type WatchCallback<V = any, OV = any> = (
value: V,
oldValue: OV,
onCleanup: OnCleanup
) => any
​
type OnCleanup = (cleanupFn: () => void) => void

// 单数据源类型：可以是 Ref 或 ComputedRef 或 函数
export type WatchSource<T = any> = Ref<T> | ComputedRef<T> | (() => T)
​
// 多数据源类型
type MultiWatchSources = (WatchSource<unknown> | object)[]
​

// watch的重载与实现
export function watch<
T extends MultiWatchSources,
Immediate extends Readonly<boolean> = false
>(
sources: [...T],
cb: WatchCallback<MapSources<T, false>, MapSources<T, Immediate>>,
options?: WatchOptions<Immediate>
): WatchStopHandle
​
// overload: multiple sources w/ `as const`
// watch([foo, bar] as const, () => {})
// somehow [...T] breaks when the type is readonly
export function watch<
T extends Readonly<MultiWatchSources>,
Immediate extends Readonly<boolean> = false
>(
source: T,
cb: WatchCallback<MapSources<T, false>, MapSources<T, Immediate>>,
options?: WatchOptions<Immediate>
): WatchStopHandle
​
// overload: single source + cb
export function watch<T, Immediate extends Readonly<boolean> = false>(
source: WatchSource<T>,
cb: WatchCallback<T, Immediate extends true ? T | undefined : T>,
options?: WatchOptions<Immediate>
): WatchStopHandle
​
// overload: watching reactive object w/ cb
export function watch<
T extends object,
Immediate extends Readonly<boolean> = false
>(
source: T,
cb: WatchCallback<T, Immediate extends true ? T | undefined : T>,
options?: WatchOptions<Immediate>
): WatchStopHandle
​
// implementation
export function watch<T = any, Immediate extends Readonly<boolean> = false>(
source: T | WatchSource<T>,
cb: any,
options?: WatchOptions<Immediate>
): WatchStopHandle {
if (__DEV__ && !isFunction(cb)) {
warn(
``watch(fn, options?)` signature has been moved to a separate API. ` +
`Use `watchEffect(fn, options?)` instead. `watch` now only ` +
`supports `watch(source, cb, options?) signature.`
)
}
return doWatch(source as any, cb, options)
}

function doWatch(
source: WatchSource | WatchSource[] | WatchEffect | object,
cb: WatchCallback | null,
{ immediate, deep, flush, onTrack, onTrigger }: WatchOptions = EMPTY_OBJ
): WatchStopHandle {
// ...
}

if (__DEV__ && !cb) {
if (immediate !== undefined) {
warn(
`watch() "immediate" option is only respected when using the ` +
`watch(source, callback, options?) signature.`
)
}
if (deep !== undefined) {
warn(
`watch() "deep" option is only respected when using the ` +
`watch(source, callback, options?) signature.`
)
}
}
​
// 声明一个source参数不合法的警告函数
const warnInvalidSource = (s: unknown) => {
warn(
`Invalid watch source: `,
s,
`A watch source can only be a getter/effect function, a ref, ` +
`a reactive object, or an array of these types.`
)
}
// ...

// 拿到当前实例，声明了空的getter，初始化关闭强制触发，且默认为单数据源的侦听
const instance = currentInstance
let getter: () => any
let forceTrigger = false
let isMultiSource = false
​
// 根据侦听数据源的类型做相应的处理
if (isRef(source)) {
getter = () => source.value
forceTrigger = isShallow(source)
} else if (isReactive(source)) {
getter = () => source
deep = true
} else if (isArray(source)) {
isMultiSource = true
forceTrigger = source.some(s => isReactive(s) || isShallow(s))
getter = () =>
// 可见，数组成员只能是Ref、Reactive或者函数，其它类型无法通过校验，将引发告警
source.map(s => {
if (isRef(s)) {
return s.value
} else if (isReactive(s)) {
return traverse(s)
} else if (isFunction(s)) {
return callWithErrorHandling(s, instance, ErrorCodes.WATCH_GETTER)
} else {
__DEV__ && warnInvalidSource(s)
}
})
} else if (isFunction(source)) {
if (cb) {
// getter with cb
getter = () =>
callWithErrorHandling(source, instance, ErrorCodes.WATCH_GETTER)
} else {
// no cb -> simple effect
getter = () => {
if (instance && instance.isUnmounted) {
return
}
if (cleanup) {
cleanup()
}
return callWithAsyncErrorHandling(
source,
instance,
ErrorCodes.WATCH_CALLBACK,
[onCleanup]
)
}
}
} else {
getter = NOOP
__DEV__ && warnInvalidSource(source)
}

// 2.x array mutation watch compat
if (__COMPAT__ && cb && !deep) {
const baseGetter = getter
getter = () => {
const val = baseGetter()
if (
isArray(val) &&
checkCompatEnabled(DeprecationTypes.WATCH_ARRAY, instance)
) {
traverse(val)
}
return val
}
}

export function traverse(value: unknown, seen?: Set<unknown>) {
if (!isObject(value) || (value as any)[ReactiveFlags.SKIP]) {
return value
}
seen = seen || new Set()
if (seen.has(value)) {
return value
}
seen.add(value)
// Ref
if (isRef(value)) {
traverse(value.value, seen)
} else if (isArray(value)) {
// 数组
for (let i = 0; i < value.length; i++) {
traverse(value[i], seen)
}
} else if (isSet(value) || isMap(value)) {
// 集合与映射
value.forEach((v: any) => {
traverse(v, seen)
})
} else if (isPlainObject(value)) {
// 普通对象
for (const key in value) {
traverse((value as any)[key], seen)
}
}
return value
}

let cleanup: () => void
let onCleanup: OnCleanup = (fn: () => void) => {
cleanup = effect.onStop = () => {
callWithErrorHandling(fn, instance, ErrorCodes.WATCH_CLEANUP)
}
}
// in SSR there is no need to setup an actual effect, and it should be noop
// unless it's eager
if (__SSR__ && isInSSRComponentSetup) {
// we will also not call the invalidate callback (+ runner is not set up)
onCleanup = NOOP
if (!cb) {
getter()
} else if (immediate) {
callWithAsyncErrorHandling(cb, instance, ErrorCodes.WATCH_CALLBACK, [
getter(),
isMultiSource ? [] : undefined,
onCleanup
])
}
return NOOP
}

let oldValue = isMultiSource ? [] : INITIAL_WATCHER_VALUE
// job为当前watch要做的工作，后续通过调度器来处理
const job: SchedulerJob = () => {
// 当前effect不在active状态，说明没有触发该effect的响应式变化，直接返回
if (!effect.active) {
return
}
// cb存在，说明是watch，而不是watchEffect
if (cb) {
// watch(source, cb)
// 调用 effect.run 得到新的值 newValue
const newValue = effect.run()
if (
deep ||
forceTrigger ||
// 取到的新值和旧值是否相同，如果有变化则进入分支
(isMultiSource
? (newValue as any[]).some((v, i) =>
hasChanged(v, (oldValue as any[])[i])
)
: hasChanged(newValue, oldValue)) ||
// 兼容2.x
(__COMPAT__ &&
isArray(newValue) &&
isCompatEnabled(DeprecationTypes.WATCH_ARRAY, instance))
) {
// cleanup before running cb again
if (cleanup) {
cleanup()
}
// 用异步异常处理程序包裹了一层来调用cb
callWithAsyncErrorHandling(cb, instance, ErrorCodes.WATCH_CALLBACK, [
newValue,
// pass undefined as the old value when it's changed for the first time
oldValue === INITIAL_WATCHER_VALUE ? undefined : oldValue,
onCleanup
])
// cb执行完成，当前的新值就变成了旧值
oldValue = newValue
}
} else {
// cb不存在，则是watchEffect
// watchEffect
effect.run()
}
}
// 设置allowRecurse，让调度器知道它可以自己触发
job.allowRecurse = !!cb

let scheduler: EffectScheduler
// 根据flush的值来创建不同的调度器
if (flush === 'sync') {
scheduler = job as any // the scheduler function gets called directly
} else if (flush === 'post') {
scheduler = () => queuePostRenderEffect(job, instance && instance.suspense)
} else {
// default: 'pre'
scheduler = () => queuePreFlushCb(job)
}
// 为 watch 创建 effect ，watchEffect就不必了，因为自带的有
const effect = new ReactiveEffect(getter, scheduler)
// 主要是调试用的onTrack和onTrigger，当收集依赖和触发更新时做一些操作
if (__DEV__) {
effect.onTrack = onTrack
effect.onTrigger = onTrigger
}

// initial run
// 有cb，是 watch
if (cb) {
if (immediate) {
job()
} else {
// 获取一下当前的值作为旧值
oldValue = effect.run()
}
} else if (flush === 'post') {
// 没有cb，是watchEffect，副作用的时机在组件更新之后，用queuePostRenderEffect包裹一层来调整时机
queuePostRenderEffect(
effect.run.bind(effect),
instance && instance.suspense
)
} else {
// watchEffect，副作用的时机在组件更新之前，直接执行一次effect.run
effect.run()
}
// 返回一个WatchStopHandle，内部执行 effect.stop来达到停止侦听的作用
return () => {
effect.stop()
// 移除当前实例作用域下的当前effect
if (instance && instance.scope) {
remove(instance.scope.effects!, effect)
}
}