new Vue({
  el: '.app',
  data: {
    msg: 'Hello Vue.',
    msg1: '',
    msg2: '',
    msg3: '',
  },
  methods: {
    changeMsg() {
      this.msg = 'Hello world.';
      this.msg1 = this.$refs.msgDiv.innerHTML;
      this.$nextTick(() => {
        this.msg2 = this.$refs.msgDiv.innerHTML;
      });
      this.msg3 = this.$refs.msgDiv.innerHTML;
    },
  },
});

/**
 * 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;
      });
    }
  };
})();

var domTarget = 你想要监听的dom节点;
var mo = new MutationObserver(callback);
mo.observe(domTarget, {
  characterData: true, //说明监听文本内容的修改。
});