computed

function initComputed(vm: Component, computed: Object) {
  const watchers = (vm._computedWatchers = Object.create(null));
  const isSSR = isServerRendering();

  for (const key in computed) {
    const userDef = computed[key];
    const getter = typeof userDef === "function" ? userDef : userDef.get;
    if (process.env.NODE_ENV !== "production" && getter == null) {
      warn(`Getter is missing for computed property "${key}".`, vm);
    }

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

    if (!(key in vm)) {
      defineComputed(vm, key, userDef);
    } else if (process.env.NODE_ENV !== "production") {
      if (key in vm.$data) {
        warn(`The computed property "${key}" is already defined in data.`, vm);
      } else if (vm.$options.props && key in vm.$options.props) {
        warn(
          `The computed property "${key}" is already defined as a prop.`,
          vm
        );
      } else if (vm.$options.methods && key in vm.$options.methods) {
        warn(
          `The computed property "${key}" is already defined as a method.`,
          vm
        );
      }
    }
  }
}

• 直接遍历定义的computed，获取到用户定义的函数或者getter函数。
• 创建空对象watchers/this._computedWatchers,然后初始化watcher对象，传入lazy:true的参数。
• 判断computed中的key，是否存在data,props,methods,如果存在则报错,否则调用defineComputed。
• 如果是子组件，不会在这里调用defineComputed,而是在Vue.extend中调用。

export function defineComputed(
  target: any,
  key: string,
  userDef: Object | Function
) {
  const 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;
  }
  if (
    process.env.NODE_ENV !== "production" &&
    sharedPropertyDefinition.set === noop
  ) {
    sharedPropertyDefinition.set = function () {
      warn(
        `Computed property "${key}" was assigned to but it has no setter.`,
        this
      );
    };
  }
  Object.defineProperty(target, key, sharedPropertyDefinition);
}

根据不同类型的computed,对sharedPropertyDefinition的get和set进行赋值。

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

createComputedGetter返回一个函数,这个函数在数据获取的时候会执行，因为lazy=true，所以watcher.dirty= true，执行watcher.evaluate()

class Watcher {
  evaluate() {
    this.value = this.get();
    this.dirty = false;
  }
}

最后就会执行一个getter获取数据。