Vue手写实现异步更新详解

class Watcher {
  // some code ...
  update () {
    // 直接执行更新操作
    this.get()
  }
}

class Watcher {
  // some code
  update () {
    queueWatcher(this);
  }

  run () {
    this.get();
  }
}

export default Watcher;

let queue = [];
let has = {}; // 使用对象来保存id，进行去重操作
let pending = false; // 如果异步队列正在执行，将不会再次执行

function flushSchedulerQueue () {
  queue.forEach(watcher => {
    watcher.run();
    if (watcher.options.render) { // 在更新之后执行对应的回调: 这里是updated钩子函数
      watcher.cb();
    }
  });
  // 执行完成后清空队列
  queue = [];
  has = {};
  pending = false;
}

function queueWatcher (watcher) {
  const id = watcher.id;
  if (!has[id]) {
    queue.push(watcher);
    has[id] = true;
    if (!pending) {
      pending = true;
      // 异步执行watcher的更新方法
      setTimeout(flushSchedulerQueue)
    }
  }
}

let callbacks = [];
let pending = false;

function flushCallbacks () {
  callbacks.forEach(cb => cb());
  callbacks = [];
  pending = false;
}

export function nextTick (cb) {
  callbacks.push(cb);
  if (!pending) {
    pending = true;
    timerFunc();
  }
}

let timerFunc;
if (Promise) {
  timerFunc = function () {
    return Promise.resolve().then(flushCallbacks);
  };
} else if (MutationObserver) {
  timerFunc = function () {
    const textNode = document.createTextNode('1');
    const observer = new MutationObserver(() => {
      flushCallbacks();
      observer.disconnect();
    });
    const observe = observer.observe(textNode, { characterData: true });
    textNode.textContent = '2';
  };
} else if (setImmediate) {
  timerFunc = function () {
    setImmediate(flushCallbacks);
  };
} else {
  timerFunc = function () {
    setTimeout(flushCallbacks);
  };
}

function queueWatcher (watcher) {
  const id = watcher.id;
  if (!has[id]) {
    queue.push(watcher);
    has[id] = true;
    if (!pending) {
      pending = true;
      nextTick(flushSchedulerQueue);
    }
  }
}

Vue.prototype.$nextTick = function (cb) {
  nextTick(cb);
};

<div id="app">{{name}}</div>
<script>
  const vm = new Vue({
    el: '#app',
    data () {
      return {
        name: 'zs'
      };
    }
  });
  vm.name = 'ls';
  console.log('$el', vm.$el);
  vm.$nextTick(() => {
    console.log('next tick $el', vm.$el);
  });
</script>