vue中nextTick的使用及實現原理

一、介紹

vue中的nextTick（組件實例是$nextTick）方法是比較常見的，一般想延遲執行某個邏輯時可以使用它。之前我們一般是用setTimeout(0)來模擬類似的操作。

其實，在瀏覽器中nextTick是用es6中的Promise實現的。它創建了一個resolve狀態的Promise示例，以在下一輪微任務中執行它。在不支持Promise的環境中，優先用setImmediate，如果setImmediate也不支持則用setTimeout。

二、示例

來個簡單的示例：

<template>
  <div>
    <a-button @click="changeState">點我</a-button>
  </div>
</template>
<script lang="ts">
import { Component, Prop, Vue, Watch } from "vue-property-decorator";
import { getWeekdayConfig } from "./http";
@Component({})
export default class Nexttick extends Vue {
  public state = 0;
  public changeState() {
    window.setTimeout(() => {
      console.log("settimeout 1");
    }, 0);
    console.log("同步 1");
    Promise.resolve().then(() => {
      console.log("promise 1");
    });
    this.$nextTick(() => {
      console.log("nexttick");
    });
    Promise.resolve().then(() => {
      console.log("promise 2");
    });
    console.log("同步 2");
    window.setTimeout(() => {
      console.log("settimeout 2");
    }, 0);
  }
}
</script>

結果：

來看vue的源碼：

export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  if (!pending) {
    pending = true
    timerFunc()
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}
if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  timerFunc = () => {
    p.then(flushCallbacks)
    // In problematic UIWebViews, Promise.then doesn't completely break, but
    // it can get stuck in a weird state where callbacks are pushed into the
    // microtask queue but the queue isn't being flushed, until the browser
    // needs to do some other work, e.g. handle a timer. Therefore we can
    // "force" the microtask queue to be flushed by adding an empty timer.
    if (isIOS) setTimeout(noop)
  }
  isUsingMicroTask = true
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
  isNative(MutationObserver) ||
  // PhantomJS and iOS 7.x
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
  // Use MutationObserver where native Promise is not available,
  // e.g. PhantomJS, iOS7, Android 4.4
  // (#6466 MutationObserver is unreliable in IE11)
  let counter = 1
  const observer = new MutationObserver(flushCallbacks)
  const textNode = document.createTextNode(String(counter))
  observer.observe(textNode, {
    characterData: true
  })
  timerFunc = () => {
    counter = (counter + 1) % 2
    textNode.data = String(counter)
  }
  isUsingMicroTask = true
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  // Fallback to setImmediate.
  // Techinically it leverages the (macro) task queue,
  // but it is still a better choice than setTimeout.
  timerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else {
  // Fallback to setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

三、拓展-手動實現一個nextTick方法

現在，我們自己寫一個瀏覽器下的nextTick方法

<template>
  <div>
    <a-button @click="changeState">點我</a-button>
  </div>
</template>
<script lang="ts">
import { Component, Prop, Vue, Watch } from "vue-property-decorator";
import { getWeekdayConfig } from "./http";
@Component({})
export default class Nexttick extends Vue {
  public state = 0;
  public changeState() {
    window.setTimeout(() => {
      console.log("settimeout 1");
    }, 0);
    console.log("同步 1");
    this.myNextTick(() => {
      console.log("mypromise 1");
    });
    this.$nextTick(() => {
      console.log("nexttick");
    });
    this.myNextTick(() => {
      console.log("mypromise 2");
    });
    console.log("同步 2");
    window.setTimeout(() => {
      console.log("settimeout 2");
    }, 0);
  }
  public myNextTick(callback: Function) {
    return Promise.resolve().then(() => {
      callback();
    });
  }
}
</script>