next-tick

在下次 DOM 更新循环结束之后执行延迟回调。在修改数据之后立即使用这个方法，获取更新后的 DOM

// 修改数据
vm.msg = 'Hello'
// DOM 还没有更新
Vue.nextTick(function () {
  // DOM 更新了
})

// 作为一个 Promise 使用 (2.1.0 起新增，详见接下来的提示)
Vue.nextTick()
  .then(function () {
    // DOM 更新了
  })

next-tick 源码

/* @flow */
/* globals MutationObserver */

import { noop } from 'shared/util'
import { handleError } from './error'
import { isIE, isIOS, isNative } from './env'

export let isUsingMicroTask = false

const callbacks = []
let pending = false

function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}

// Here we have async deferring wrappers using microtasks.
// In 2.5 we used (macro) tasks (in combination with microtasks).
// However, it has subtle problems when state is changed right before repaint
// (e.g. #6813, out-in transitions).
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// So we now use microtasks everywhere, again.
// A major drawback of this tradeoff is that there are some scenarios
// where microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690, which have workarounds)
// or even between bubbling of the same event (#6566).
let timerFunc

// The nextTick behavior leverages the microtask queue, which can be accessed
// via either native Promise.then or MutationObserver.
// MutationObserver has wider support, however it is seriously bugged in
// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
// completely stops working after triggering a few times... so, if native
// Promise is available, we will use it:
/* istanbul ignore next, $flow-disable-line */
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.
  // Technically 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)
  }
}

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
    })
  }
}

next-tick 源码分析

1.定义变量，设置一个存放函数数组，循环遍历数组里面的函数，并且执行。 2.判断什么条件环境使用什么样的异步延迟函数（宏任务，微任务） (这是核心关键的代码) 3.封装并导出nextTick的函数，设置this指向（这样组件才能正常使用这个函数） 4.当nextTick 没有传入函数参数的时候，返回一个 Promise 化的调用

微任务

1.Promise

2.MutationObserver(HTML5新特性)

3.MessageChannel

console.log('1')

// 这是一个宏任务
setTimeout(function () {
  console.log('2')
});

new Promise(function (resolve) {
  // 这里是同步任务
  console.log('3');
  resolve();
  // then是一个微任务
}).then(function () {
  console.log('4')
  setTimeout(function () {
    console.log('5')
  });
});

//结果 1 3 4 2 5

宏任务

2.setTimeout

3.setInterval

4.setImmediate

console.log('1')

// 这是一个宏任务
setTimeout(function () {
  console.log('2')
});

new Promise(function (resolve) {
  // 这里是同步任务
  console.log('3');
  resolve();
  // then是一个微任务
}).then(function () {
  console.log('4')
  setTimeout(function () {
    console.log('5')
  });
});

//结果 1 3 4 2 5

总结

把回调函数放入callbacks等待执行
将执行函数放到微任务或者宏任务中
事件循环到了微任务或者宏任务，执行函数依次执行callbacks中的回调再回到我们开头说的setTimeout，可以看出来nextTick是对setTimeout进行了多种兼容性的处理，宽泛的也可以理解为将回调函数放入setTimeout中执行；不过nextTick优先放入微任务执行，而setTimeout是宏任务，因此nextTick一般情况下总是先于setTimeout执行，

setTimeout(() => {
  console.log(1)
}, 0)
this.$nextTick(() => {
  console.log(2)
})
this.$nextTick(() => {
  console.log(3)
})
//运行结果 2 3 1

Vue源码解析之Next-Tick

next-tick

next-tick 源码

next-tick 源码分析

微任务

宏任务

总结