Vue中之nextTick函数源码分析详解

export const nextTick = (function () {
 const callbacks = []
 let pending = false
 let timerFunc

 function nextTickHandler () {
 pending = false;
 /*
  之所以要slice复制一份出来是因为有的cb执行过程中又会往callbacks中加入内容，比如$nextTick的回调函数里又有$nextTick，
  那么这些应该放入到下一个轮次的nextTick去执行，所以拷贝一份，遍历完成即可，防止一直循环下去。
  */
 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 */
 /*
 nextTick行为利用了microtask队列, 先使用 Promise.resolve().then(nextTickHandler)来将异步回调
 放入到microtask中，Promise 和 MutationObserver都可以使用，但是 MutationObserver 在IOS9.3以上的
 WebView中有bug，因此如果满足第一项的话就可以执行，如果没有原生Promise就用 MutationObserver。
 */
 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 (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 IE11, iOS7, Android 4.4
 /*
  创建一个MutationObserver，observe监听到DOM改动之后执行的回调 nextTickHandler 
  */
 var counter = 1
 var observer = new MutationObserver(nextTickHandler)
 var textNode = document.createTextNode(String(counter));
 // 使用MutationObserver的接口，监听文本节点的字符内容
 observer.observe(textNode, {
  characterData: true
 });
 /*
  每次执行timerFunc函数都会让文本节点的内容在0/1之间切换，切换之后将新赋值到那个我们MutationObserver监听的文本节点上去。
  */
 timerFunc = () => {
  counter = (counter + 1) % 2
  textNode.data = String(counter)
 }
 } else {
 // fallback to setTimeout
 /*
  如果上面的两种都不支持的话，我们就使用setTimeout来执行
  */
 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)
  }
 });
 /* 如果pending为true，表明本轮事件循环中已经执行过 timerFunc(nextTickHandler, 0) */
 if (!pending) {
  pending = true
  timerFunc()
 }
 if (!cb && typeof Promise !== 'undefined') {
  return new Promise((resolve, reject) => {
  _resolve = resolve
  })
 }
 }
})()

var nextTick2 = (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]()
 }
 }
 if (typeof Promise !== 'undefined') {
 var p = Promise.resolve()
 var logError = err => { console.error(err) }
 timerFunc = () => {
  p.then(nextTickHandler).catch(logError)
 }
 } else if (typeof MutationObserver !== 'undefined' ||
 // PhantomJS and iOS 7.x
 MutationObserver.toString() === '[object MutationObserverConstructor]'
 ) {
 // use MutationObserver where native Promise is not available,
 // e.g. PhantomJS IE11, 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, ctx) {
 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
  })
 }
 }
})();
nextTick2(function(){
 console.log(2222);
});

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

if (typeof Promise !== 'undefined' && isNative(Promise)) {
 var p = Promise.resolve();
 var logError = err => { console.error(err) }
 timerFunc = () => {
 p.then(nextTickHandler).catch(logError);
} else if (typeof MutationObserver !== 'undefined' && (
 isNative(MutationObserver) ||
 // PhantomJS and iOS 7.x
 MutationObserver.toString() === '[object MutationObserverConstructor]'
)){
 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 {
 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
 })
 }
}