var nextTick = (function () {
    var callbacks = []; // 存儲需要觸發(fā)的回調(diào)函數(shù)
    var pending = false; // 是否正在等待的標識(false:允許觸發(fā)在下次事件循環(huán)觸發(fā)callbacks中的回調(diào), true: 已經(jīng)觸發(fā)過,需要等到下次事件循環(huán))
    var timerFunc; // 設置在下次事件循環(huán)觸發(fā)callbacks的 觸發(fā)函數(shù)

    //處理callbacks的函數(shù)
    function nextTickHandler () {
      pending = false;// 可以觸發(fā)timeFunc
      var copies = callbacks.slice(0);//復制callback
      callbacks.length = 0;//清空callback
      for (var i = 0; i < copies.length; i++) {
        copies[i]();//觸發(fā)callback回調(diào)函數(shù)
      }
    }

    //如果支持Promise,使用Promise實現(xiàn)
    if (typeof Promise !== 'undefined' && isNative(Promise)) {
      var p = Promise.resolve();
      var logError = function (err) { console.error(err); };
      timerFunc = function () {
        p.then(nextTickHandler).catch(logError);
        // ios的webview下,需要強制刷新隊列,執(zhí)行上面的回調(diào)函數(shù)
        if (isIOS) { setTimeout(noop); }
      };

      //如果Promise不支持,但是支持MutationObserver(h5新特性,異步,當dom變動是觸發(fā),注意是所有的dom都改變結束后觸發(fā))
    } 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
      var counter = 1;
      var observer = new MutationObserver(nextTickHandler);
      //創(chuàng)建一個textnode dom節(jié)點,并讓MutationObserver 監(jiān)視這個節(jié)點;而 timeFunc正是改變這個dom節(jié)點的觸發(fā)函數(shù)
      var textNode = document.createTextNode(String(counter));
      observer.observe(textNode, {
        characterData: true
      });
      timerFunc = function () {
        counter = (counter + 1) % 2;
        textNode.data = String(counter);
      };
    } else {// 上面兩種不支持的話,就使用setTimeout

      timerFunc = function () {
        setTimeout(nextTickHandler, 0);
      };
    }
    //nextTick接受的函數(shù), 參數(shù)1:回調(diào)函數(shù) 參數(shù)2:回調(diào)函數(shù)的執(zhí)行上下文
    return function queueNextTick (cb, ctx) {
      var _resolve;//用于接受觸發(fā) promise.then中回調(diào)的函數(shù)
      //向回調(diào)數(shù)據(jù)中pushcallback
      callbacks.push(function () {
        //如果有回調(diào)函數(shù),執(zhí)行回調(diào)函數(shù)
        if (cb) { cb.call(ctx); }
        if (_resolve) { _resolve(ctx); }//觸發(fā)promise的then回調(diào)
      });
      if (!pending) {//是否執(zhí)行刷新callback隊列
        pending = true;
        timerFunc();
      }
      //如果沒有傳遞回調(diào)函數(shù),并且當前瀏覽器支持promise,使用promise實現(xiàn)
      if (!cb && typeof Promise !== 'undefined') {
        return new Promise(function (resolve) {
          _resolve = resolve;
        })
      }
    }
  })();

function queueWatcher (watcher) {
    var id = watcher.id;
    if (has[id] == null) {
      has[id] = true;
      if (!flushing) {
        queue.push(watcher);
      } else {
        // if already flushing, splice the watcher based on its id
        // if already past its id, it will be run next immediately.
        var i = queue.length - 1;
        while (i >= 0 && queue[i].id > watcher.id) {
          i--;
        }
        queue.splice(Math.max(i, index) + 1, 0, watcher);
      }
      // queue the flush
      if (!waiting) {
        waiting = true;
        nextTick(flushSchedulerQueue);
      }
    }
  }

Vue.prototype.$nextTick = function (fn) {
  return nextTick(fn, this)// 設置nextTick回調(diào)函數(shù)的上下文環(huán)境是當前Vue實例
};