vue compute 初始化

步骤：

• 遍历所有computed属性，获取对应的getter
• 添加内部 watcher，跟踪val的变化
• 将computed属性上的所有值挂载到当前vm实例上，defineComputed
• 重写computed的getter, 添加响应式依赖，追踪数据变化

{

  function initState (vm) {
    vm._watchers = [];
    var opts = vm.$options;
    
    if (opts.computed) { initComputed(vm, opts.computed); }
  }


  var computedWatcherOptions = { lazy: true };

  function initComputed (vm, computed) {
    // $flow-disable-line
    var watchers = vm._computedWatchers = Object.create(null);
    // computed properties are just getters during SSR
    var isSSR = isServerRendering();

    for (var key in computed) {
      var userDef = computed[key];
      var getter = typeof userDef === 'function' ? userDef : userDef.get;

      if (!isSSR) {
        // create internal watcher for the computed property.
        watchers[key] = new Watcher(
          vm,
          getter || noop,
          noop,
          computedWatcherOptions
        );
      }

      // component-defined computed properties are already defined on the
      // component prototype. We only need to define computed properties defined
      // at instantiation here.
      if (!(key in vm)) {
        defineComputed(vm, key, userDef);
      }
    }
  }

  function defineComputed (
    target,
    key,
    userDef
  ) {
    var shouldCache = !isServerRendering();
    if (typeof userDef === 'function') {
      sharedPropertyDefinition.get = shouldCache
        ? createComputedGetter(key)
        : createGetterInvoker(userDef);
      sharedPropertyDefinition.set = noop;
    } else {
      sharedPropertyDefinition.get = userDef.get
        ? shouldCache && userDef.cache !== false
          ? createComputedGetter(key)
          : createGetterInvoker(userDef.get)
        : noop;
      sharedPropertyDefinition.set = userDef.set || noop;
    }
    Object.defineProperty(target, key, sharedPropertyDefinition);
  }

  function createComputedGetter (key) {
    return function computedGetter () {
      var watcher = this._computedWatchers && this._computedWatchers[key];
      if (watcher) {
        if (watcher.dirty) {
          watcher.evaluate();
        }
        if (Dep.target) {
          watcher.depend();
        }
        return watcher.value
      }
    }
  }

  function createGetterInvoker(fn) {
    return function computedGetter () {
      return fn.call(this, this)
    }
  }
}