概述

  相对于typeScript的class写法,是对当前类的封装（个人理解）

首先看2.0 js的写法

 const App=Vue.extent({
     data（）{return{'hello':'hello'}},
     computed: {
     world() {
        return this.hello + 'world';
       },
   },

   mounted(){
    this.sayHello();
   }
 methods:{
        sayHello() {
                    console.log(this.hello);
                },
             
        }
  })

3.0 ts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);
        }
    }

疑问点

 1：@Component()  是？（@Component就是一个修饰器, 用来修改类的行为，更多关于装饰器信息请参阅阮老师Es6） 

 2：hello = 'world';  这是啥写法
 js语法中使用的是constructor去给属性赋值，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';
    sathello(){
     console.log(this.hello);
   } 
  }
 // 转换后  挂在原型上
    function App () {
        this.hello = 'world'
    }
    
    App.prototype.sayHello = function () {
        console.log(this.hello);
    }


 3：App类没有constructor 构造函数
 4：导出的类没有new 直接使用？

重点分析（index.js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)
  }
}

componentFactory此方法的封装所要做的东西

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)
 })
 }