JS运行机制

JS 执行是单线程的，它是基于事件循环的。事件循环大致分为以下几个步骤：

  1. 所有同步任务都在主线程上执行，形成一个执行栈
  2. 主线程之外还存在一个“任务队列”。只要异步任务有了运行结果就在“任务队列”之中放置一个事件
  3. 一旦执行栈中所有同步任务执行完毕，系统就会读取“任务队列”，看看里面有哪些事件。那些对应的异步任务于是结束等待状态进入执行栈开始执行
  4. 主线程不断重复上面第三步

image.png
主线程·的执行过程就是一个tick，而所有的异步结果都是通过“任务队列”来调度
消息队列中存放的是一个个的任务（task）,规范中规定task分为两大类

  • macro task
  • micro task

每个macro task结束后都要清空所有的micro task
在浏览器环境中，常见的 macro task 有 setTimeout、MessageChannel、postMessage、setImmediate；常见的 micro task 有 MutationObsever 和 Promise.then

nextTick在Vue中的实现

定义在src/core/util/next-tick.js中

  1. import { noop } from 'shared/util'
  2. import { handleError } from './error'
  3. import { isIE, isIOS, isNative } from './env'
  5. export let isUsingMicroTask = false
  7. const callbacks = []
  8. let pending = false
  10. // 对callbacks遍历，然后执行相应的回调函数
  11. function flushCallbacks () {
  12.   pending = false
  13.   const copies = callbacks.slice(0)
  14.   callbacks.length = 0
  15.   for (let i = 0; i < copies.length; i++) {
  16.     copies[i]()
  17.   }
  18. }
  20. // Here we have async deferring wrappers using microtasks.
  21. // In 2.5 we used (macro) tasks (in combination with microtasks).
  22. // However, it has subtle problems when state is changed right before repaint
  23. // (e.g. #6813, out-in transitions).
  24. // Also, using (macro) tasks in event handler would cause some weird behaviors
  25. // that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
  26. // So we now use microtasks everywhere, again.
  27. // A major drawback of this tradeoff is that there are some scenarios
  28. // where microtasks have too high a priority and fire in between supposedly
  29. // sequential events (e.g. #4521, #6690, which have workarounds)
  30. // or even between bubbling of the same event (#6566).
  31. let timerFunc
  33. // The nextTick behavior leverages the microtask queue, which can be accessed
  34. // via either native Promise.then or MutationObserver.
  35. // MutationObserver has wider support, however it is seriously bugged in
  36. // UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
  37. // completely stops working after triggering a few times... so, if native
  38. // Promise is available, we will use it:
  39. /* istanbul ignore next, $flow-disable-line */
  40. // 根据判断结果给timerFunc不同的函数内容
  41. if (typeof Promise !== 'undefined' && isNative(Promise)) {
  42.   const p = Promise.resolve()
  43.   timerFunc = () => {
  44.     p.then(flushCallbacks)
  45.     // In problematic UIWebViews, Promise.then doesn't completely break, but
  46.     // it can get stuck in a weird state where callbacks are pushed into the
  47.     // microtask queue but the queue isn't being flushed, until the browser
  48.     // needs to do some other work, e.g. handle a timer. Therefore we can
  49.     // "force" the microtask queue to be flushed by adding an empty timer.
  50.     if (isIOS) setTimeout(noop)
  51.   }
  52.   isUsingMicroTask = true
  53. } else if (!isIE && typeof MutationObserver !== 'undefined' && (
  54.   isNative(MutationObserver) ||
  55.   // PhantomJS and iOS 7.x
  56.   MutationObserver.toString() === '[object MutationObserverConstructor]'
  57. )) {
  58.   // Use MutationObserver where native Promise is not available,
  59.   // e.g. PhantomJS, iOS7, Android 4.4
  60.   // (#6466 MutationObserver is unreliable in IE11)
  61.   let counter = 1
  62.   const observer = new MutationObserver(flushCallbacks)
  63.   const textNode = document.createTextNode(String(counter))
  64.   observer.observe(textNode, {
  65.     characterData: true
  66.   })
  67.   timerFunc = () => {
  68.     counter = (counter + 1) % 2
  69.     textNode.data = String(counter)
  70.   }
  71.   isUsingMicroTask = true
  72. } else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  73.   // Fallback to setImmediate.
  74.   // Technically it leverages the (macro) task queue,
  75.   // but it is still a better choice than setTimeout.
  76.   timerFunc = () => {
  77.     setImmediate(flushCallbacks)
  78.   }
  79. } else {
  80.   // Fallback to setTimeout.
  81.   timerFunc = () => {
  82.     setTimeout(flushCallbacks, 0)
  83.   }
  84. }
  86. export function nextTick (cb?: Function, ctx?: Object) {
  87.   let _resolve
  88.   // 把传入的回调函数cb压入callbacks数组
  89.   callbacks.push(() => {
  90.     if (cb) {
  91.       try {
  92.         cb.call(ctx)
  93.       } catch (e) {
  94.         handleError(e, ctx, 'nextTick')
  95.       }
  96.     } else if (_resolve) {
  97.       _resolve(ctx)
  98.     }
  99.   })
  100.   if (!pending) {
  101.     pending = true
  102.     timerFunc()
  103.   }
  104.   // $flow-disable-line
  105.   // 当nextTick不传cb参数时提供一个Promise化的调用 nextTick().then(() => {})
  106.   if (!cb && typeof Promise !== 'undefined') {
  107.     return new Promise(resolve => {
  108.       // 当_resolve函数执行就会跳到then的逻辑中
  109.       _resolve = resolve
  110.     })
  111.   }
  112. }

Vue.js 提供了 2 种调用 nextTick 的方式，一种是全局 API Vue.nextTick，一种是实例上的方法 vm.$nextTick，无论我们使用哪一种，最后都是调用 next-tick.js 中实现的 nextTick 方法