Vue-Property-Decorator源码分析

概述

vue-property-decorator是基于vue组织里vue-class-component所做的拓展,先来了解一下vue-class-component

Vue-Class-Component

vue-class-component是一个Class Decorator,也就是类的装饰器,但目前装饰器在vue中只属于草案阶段.

原理简述

vue2.x只有Object一种声明组件的方式, 比如这样:

const App = Vue.extend({
    // data
    data() {
        return {
            hello: 'world',
        };
    },
    computed: {
        world() {
            return this.hello + 'world';
        },
    },
    // hooks
    mounted() {
        this.sayHello();
    },
    // methods
    methods: {
        sayHello() {
            console.log(this.hello);
        },
    },
});

用了vue-class-component就成了以下写法:

import Component from 'vue-class-component';

@Component({
    name: 'App'
})
class App extends Vue {
    hello = 'world';

    get world() {
        return this.hello + 'world';
    }

    mounted() {
        this.sayHello();
    }

    sayHello() {
        console.log(this.hello);
    }
}

在这个例子中,很容易发现几个疑点:

@Component()是什么?
hello = 'world'这是什么语法?
App类没有constructor构造函数;
导出的类没有被new就直接使用了;

疑点1:

对装饰器的有一定了解. 装饰器种类有好几种, vue-class-component中主要使用了类装饰器.
更多关于装饰器信息请参阅阮老师的文章: ECMAScript6入门

看完阮老师所写的文章已经可以解决了疑点1

简述: @Component就是一个修饰器, 用来修改类的行为

疑点2:

在JS语法中, class中都是需要在constructor中给属性赋值, 在chrome上像vue-class-component中定义class是会报错的,
但vue-class-component中却又这么做了.

然后我们看看class通过webpack + babel-loader解析后会变成什么样子

// 转换前
class App {
    hello = 'world';

    sayHello() {
        console.log(this.hello);
    }
}

// 转换后
function App () {
    this.hello = 'world'
}

App.prototype.sayHello = function () {
    console.log(this.hello);
}

接下来看看入口文件index.ts所做的东西:

// Component实际上是既作为工厂函数,又作为装饰器函数
function Component (options: ComponentOptions<Vue> | VueClass<Vue>): any {
  if (typeof options === 'function') {
    // 区别一下。这里的命名虽然是工厂，其实它才是真正封装装饰器逻辑的函数
    return componentFactory(options)
  }
  return function (Component: VueClass<Vue>) {
    return componentFactory(Component, options)
  }
}

再看看componentFactory所做的东西:

import Vue, { ComponentOptions } from 'vue'
import { copyReflectionMetadata, reflectionIsSupported } from './reflect'
import { VueClass, DecoratedClass } from './declarations'
import { collectDataFromConstructor } from './data'
import { hasProto, isPrimitive, warn } from './util'

export const $internalHooks = [
  'data',
  'beforeCreate',
  'created',
  'beforeMount',
  'mounted',
  'beforeDestroy',
  'destroyed',
  'beforeUpdate',
  'updated',
  'activated',
  'deactivated',
  'render',
  'errorCaptured', // 2.5
  'serverPrefetch' // 2.6
]

export function componentFactory (
  Component: VueClass<Vue>,
  options: ComponentOptions<Vue> = {}
): VueClass<Vue> {
  // 为component的name赋值
  options.name = options.name || (Component as any)._componentTag || (Component as any).name
  // prototype props.
  // 获取原型
  const proto = Component.prototype
  // 遍历原型
  Object.getOwnPropertyNames(proto).forEach(function (key) {
    // 如果是constructor, 则不处理
    if (key === 'constructor') {
      return
    }

    // hooks
    // 如果原型属性(方法)名是vue生命周期钩子名，则直接作为钩子函数挂载在options最外层
    if ($internalHooks.indexOf(key) > -1) {
      options[key] = proto[key]
      return
    }
    // getOwnPropertyDescriptor 返回描述对象
    const descriptor = Object.getOwnPropertyDescriptor(proto, key)!
    // void 0 === undefined
    if (descriptor.value !== void 0) {
      // methods
      // 如果是方法名就挂载到methods上
      if (typeof descriptor.value === 'function') {
        (options.methods || (options.methods = {}))[key] = descriptor.value
      } else {
        // typescript decorated data
        // 把成员变量作为mixin放到options上，一个变量一个mixin，而不是直接统计好放到data或者同一个mixin中
        // 因为data我们已经作为了保留字段，可以在类中声明成员方法data()和options中声明data同样的方法声明变量
        (options.mixins || (options.mixins = [])).push({
          data (this: Vue) {
            return { [key]: descriptor.value }
          }
        })
      }
    } else if (descriptor.get || descriptor.set) {
      // computed properties
      // 转换成计算属性的getter和setter
      (options.computed || (options.computed = {}))[key] = {
        get: descriptor.get,
        set: descriptor.set
      }
    }
  })

  // add data hook to collect class properties as Vue instance's data
  // 这里再次添加了一个mixin，会把这个类实例化，然后把对象中的值放到mixin中
  // 只有在这里我们声明的class的constructor被调用了
  ;(options.mixins || (options.mixins = [])).push({
    data (this: Vue) {
      return collectDataFromConstructor(this, Component)
    }
  })

  // decorate options
  // 如果这个类还有其他的装饰器，也逐个调用. vue-class-component只提供了类装饰器
  // props、components、watch等特殊参数只能写在Component(options)的options参数里
  // 因此我们使用vue-property-decorator库的属性装饰器
  // 通过下面这个循环应用属性装饰器就可以合并options(ps: 不明白可以看看createDecorator这个函数)
  const decorators = (Component as DecoratedClass).__decorators__
  if (decorators) {
    decorators.forEach(fn => fn(options))
    delete (Component as DecoratedClass).__decorators__
  }

  // find super
  // 找到这个类的父类，如果父类已经是继承于Vue的，就直接调用它的extend方法，否则调用Vue.extend
  const superProto = Object.getPrototypeOf(Component.prototype)
  const Super = superProto instanceof Vue
    ? superProto.constructor as VueClass<Vue>
    : Vue
  // 最后生成我们要的Vue组件
  const Extended = Super.extend(options)

  // 处理静态成员
  forwardStaticMembers(Extended, Component, Super)

  // 如果我们支持反射，那么也把对应的反射收集的内容绑定到Extended上
  if (reflectionIsSupported) {
    copyReflectionMetadata(Extended, Component)
  }

  return Extended
}

const reservedPropertyNames = [
  // Unique id
  'cid',

  // Super Vue constructor
  'super',

  // Component options that will be used by the component
  'options',
  'superOptions',
  'extendOptions',
  'sealedOptions',

  // Private assets
  'component',
  'directive',
  'filter'
]

const shouldIgnore = {
  prototype: true,
  arguments: true,
  callee: true,
  caller: true
}

function forwardStaticMembers (
  Extended: typeof Vue,
  Original: typeof Vue,
  Super: typeof Vue
): void {
  // We have to use getOwnPropertyNames since Babel registers methods as non-enumerable
  Object.getOwnPropertyNames(Original).forEach(key => {
    // Skip the properties that should not be overwritten
    if (shouldIgnore[key]) {
      return
    }

    // Some browsers does not allow reconfigure built-in properties
    const extendedDescriptor = Object.getOwnPropertyDescriptor(Extended, key)
    if (extendedDescriptor && !extendedDescriptor.configurable) {
      return
    }

    const descriptor = Object.getOwnPropertyDescriptor(Original, key)!

    // If the user agent does not support `__proto__` or its family (IE <= 10),
    // the sub class properties may be inherited properties from the super class in TypeScript.
    // We need to exclude such properties to prevent to overwrite
    // the component options object which stored on the extended constructor (See #192).
    // If the value is a referenced value (object or function),
    // we can check equality of them and exclude it if they have the same reference.
    // If it is a primitive value, it will be forwarded for safety.
    if (!hasProto) {
      // Only `cid` is explicitly exluded from property forwarding
      // because we cannot detect whether it is a inherited property or not
      // on the no `__proto__` environment even though the property is reserved.
      if (key === 'cid') {
        return
      }

      const superDescriptor = Object.getOwnPropertyDescriptor(Super, key)

      if (
        !isPrimitive(descriptor.value) &&
        superDescriptor &&
        superDescriptor.value === descriptor.value
      ) {
        return
      }
    }

    // Warn if the users manually declare reserved properties
    if (
      process.env.NODE_ENV !== 'production' &&
      reservedPropertyNames.indexOf(key) >= 0
    ) {
      warn(
        `Static property name '${key}' declared on class '${Original.name}' ` +
        'conflicts with reserved property name of Vue internal. ' +
        'It may cause unexpected behavior of the component. Consider renaming the property.'
      )
    }

    Object.defineProperty(Extended, key, descriptor)
  })
}

下面简单总结一下vue-class-component做了什么:

收集class中的属性, 如果是方法就放到Methods里, 如果是普通变量就放到mixin中的data里
实例化class, 把这个class的属性也作为mixin中的data, 我们所写class的构造函数只会被这里所调用
利用Options执行生成组件
处理静态属性
反射相关处理

https://zhuanlan.zhihu.com/p/48371638

http://www.lutoyvan.cn/2019/03/05/vue-class-component-source-code-analysis.html