简单理解Vue中的nextTick方法(2)

日期：2020-05-11 栏目：程序人生浏览：次

例如，当你设置vm.someData = 'new value'，该组件不会立即重新渲染。当刷新队列时，组件会在事件循环队列清空时的下一个“tick”更新。多数情况我们不需要关心这个过程，但是如果你想在 DOM 状态更新后做点什么，这就可能会有些棘手。虽然 Vue.js 通常鼓励开发人员沿着“数据驱动”的方式思考，避免直接接触 DOM，但是有时我们确实要这么做。为了在数据变化之后等待 Vue 完成更新 DOM ，可以在数据变化之后立即使用Vue.nextTick(callback) 。这样回调函数在 DOM 更新完成后就会调用。

三、nextTick源码浅析

作用

Vue.nextTick用于延迟执行一段代码，它接受2个参数（回调函数和执行回调函数的上下文环境），如果没有提供回调函数，那么将返回promise对象。

源码

/**
 * Defer a task to execute it asynchronously.
 */
export const nextTick = (function () {
 const callbacks = []
 let pending = false
 let timerFunc

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

 // 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 if */
 if (typeof Promise !== 'undefined' && isNative(Promise)) {
  var p = Promise.resolve()
  var logError = err => { console.error(err) }
  timerFunc = () => {
   p.then(nextTickHandler).catch(logError)
   // 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)
  }
 } 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
  var counter = 1
  var observer = new MutationObserver(nextTickHandler)
  var textNode = document.createTextNode(String(counter))
  observer.observe(textNode, {
   characterData: true
  })
  timerFunc = () => {
   counter = (counter + 1) % 2
   textNode.data = String(counter)
  }
 } else {
  // fallback to setTimeout
  /* istanbul ignore next */
  timerFunc = () => {
   setTimeout(nextTickHandler, 0)
  }
 }

 return function queueNextTick (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()
  }
  if (!cb && typeof Promise !== 'undefined') {
   return new Promise((resolve, reject) => {
    _resolve = resolve
   })
  }
 }
})()

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简单理解Vue中的nextTick方法(2)

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