详解Vue中Computed与watch的用法与区别

<script setup>
import { ref,computed } from 'vue'
const num = ref(1)
//computed返回一个只读响应式ref对象computedNum
//computedNum是只读属性
let computedNum = computed(() => num.value + 1)
</script>
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<template>
    <p> num:{{ num }} </p>
    <p>computedNum:{{ computedNum }}</p>
    <!-- 修改响应式变量num 触发与之绑定的computed的getter-->  
    <button @click="num++">num++</button>
    <!-- computedNum是只读属性-->  
    <button @click="computedNum++">computedNum++</button>
</template>

<script setup>
import { ref, computed } from 'vue'
const num = ref(1)
//getter（只读）
let computedNum = computed(() => num.value + 1)
//getter和setter （可读可写）
let computedNum2 = computed({
    get: () => num.value + 1,
    set: (val) => {
         console.log(val);
        //setter中修改ref响应式变量num，将触发关联的num的getter计算
        //computedNum和computedNum2的getter同时触发
         num.value++
          }
})
</script>
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<template>
    <p> num:{{ num }} </p>
    <p>computedNum:{{ computedNum }}</p>
    <p>computedNum2:{{ computedNum2 }}</p>
    <button @click="num++">num++</button>
    <!-- computedNum是只读属性，会有警告提醒 Write operation failed: computed value is readonly-->  
    <button @click="computedNum++">computedNum++</button>
     <!-- computedNum2是可读可写属性-->  
    <button @click="computedNum2++">computedNum2++</button>
</template>

<script setup>
import { ref, computed } from 'vue'
const num = ref(1)
let computedNum = computed(() => num.value + 1, {
    onTrack: (e) => {
        console.log('onTrack');
        console.log(e);
    },
    onTrigger: (e) => {
        console.log('onTrigger');
        console.log(e);
    }
})
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</script>
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<template>
    <p> num:{{ num }} </p>
    <p>computedNum:{{ computedNum }}</p>
    <!--每次 num++将触发onTrigger -->
    <button @click="num++">num++</button>
</template>

<script setup>
import { ref, watchEffect } from 'vue'
const num = ref(1)
//会立即执行一次
watchEffect(() => {
    console.log('watchEffect');
    num.value++
})
</script>
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<template>
    <p>num: {{ num }}</p>
    <button @click="num++">num++</button>
</template>

const stop = watchEffect(() => {
  /* ... */
})
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// 显式停止
stop()

import { ref, watchEffect } from 'vue'
const num = ref(1)
watchEffect((onInvalidate ) => {
    console.log('watchEffect-1');
    num.value++
    onInvalidate (()=>{
        console.log('onInvalidate-2');
    })
    console.log('watchEffect-3');
})
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//1、watchEffect   被重新触发时
// onInvalidate-2
// watchEffect-1
// watchEffect-3
//2、组件卸载时
// onInvalidate-2
//3、关联的响应式数据首次被修改（组件被挂载时）
//  watchEffect-1
//  watchEffect-3

//推迟触发watchEffect
watchEffect(
  () => {
    /* ... */
  },
  {
    flush: 'post'
  }
)
//Vue3.2语法糖watchPostEffect
watchPostEffect(()=>{
  /* ... */
})

//实验watchEffect第二参数 flush: 'post'属性
watchEffect(() => {
    console.log("实验watchEffect第二参数 {flush: 'post'}属性");
    console.log(obj.age);
},{
   flush:'post' 
})
watchEffect(() => {
    console.log("watchEffect正常时机触发");
    console.log(obj.age);
})
//生命周期onUpdated
onUpdated(()=>{
    console.log('onUpdated()');  
})
//生命周期onBeforeUpdate
onBeforeUpdate(()=>{
    console.log('onBeforeUpdate()');
})

watchEffect(
  () => {
    /* ... */
},
  {
    flush: 'sync'
  }
)
//Vue3.2语法糖watchSyncEffect
watchSyncEffect(()=>{
  /* ... */
})

//假设修改了对象的属性值-修改了obj.age
const obj = reactive({ name: '小明', age: 18 })
//watchEffect不能监听对象变化
watchEffect(() => {
    console.log('watchEffect监听对象变化');
    console.log(obj);
})
//watchEffect可以监听对象属性变化
watchEffect(() => {
    console.log('watchEffect监听对象属性变化');
    console.log(obj.age);
})
//watch监听对象变化
watch(obj, (obj) => {
    console.log('watch监听对象变化');
    console.log(obj);
})

// 侦听一个 getter
//被监听数据传入一个带返回值的回调函数
const state = reactive({ count: 0 })
watch(
  () => state.count,
  (count, prevCount) => {
    /* ... */
  }
)
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// 直接侦听一个 ref
const count = ref(0)
watch(count, (count, prevCount) => {
  /* ... */
})

watch([fooRef, barRef], ([foo, bar], [prevFoo, prevBar]) => {
  /* ... */
})

// 只读的
function computed<T>(
  getter: () => T,
  debuggerOptions?: DebuggerOptions
): Readonly<Ref<Readonly<T>>>
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// 可写的
function computed<T>(
  options: {
    get: () => T
    set: (value: T) => void
  },
  debuggerOptions?: DebuggerOptions
): Ref<T>
interface DebuggerOptions {
  onTrack?: (event: DebuggerEvent) => void
  onTrigger?: (event: DebuggerEvent) => void
}
interface DebuggerEvent {
  effect: ReactiveEffect
  target: any
  type: OperationTypes
  key: string | symbol | undefined
}

function watchEffect(
  effect: (onInvalidate: InvalidateCbRegistrator) => void,
  options?: WatchEffectOptions
): StopHandle
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interface WatchEffectOptions {
  flush?: 'pre' | 'post' | 'sync' // 默认：'pre'
  onTrack?: (event: DebuggerEvent) => void
  onTrigger?: (event: DebuggerEvent) => void
}
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interface DebuggerEvent {
  effect: ReactiveEffect
  target: any
  type: OperationTypes
  key: string | symbol | undefined
}
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type InvalidateCbRegistrator = (invalidate: () => void) => void
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type StopHandle = () => void

// 侦听单一源
function watch<T>(
  source: WatcherSource<T>,
  callback: (
    value: T,
    oldValue: T,
    onInvalidate: InvalidateCbRegistrator
  ) => void,
  options?: WatchOptions
): StopHandle
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// 侦听多个源
function watch<T extends WatcherSource<unknown>[]>(
  sources: T
  callback: (
    values: MapSources<T>,
    oldValues: MapSources<T>,
    onInvalidate: InvalidateCbRegistrator
  ) => void,
  options? : WatchOptions
): StopHandle
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type WatcherSource<T> = Ref<T> | (() => T)
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type MapSources<T> = {
  [K in keyof T]: T[K] extends WatcherSource<infer V> ? V : never
}
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// 参见 `watchEffect` 共享选项的类型声明
interface WatchOptions extends WatchEffectOptions {
  immediate?: boolean // 默认：false
  deep?: boolean
}