解析vue的provide和inject使用方法及其原理

1.父组件提供向子组件要传递的参数
provide() {
    return {
      listType: this.listType,
    }
  }
2.子组件使用：
inject: ['listType'],

inject:{
  listType:{
  from:"par"//provide定义的名字
  default:1
  }
}

export function provide<T>(key: InjectionKey<T> | string | number, value: T) {
  if (!currentInstance) {
    if (__DEV__) {
      warn(`provide() can only be used inside setup().`)
    
    }
  } else {
    //获取当前组件的provides,默认实例继承父类的provides对象
    let provides = currentInstance.provides
    //使用父provide对象作为原型来创建自己的provide对象
    const parentProvides =
      currentInstance.parent && currentInstance.parent.provides
    if (parentProvides === provides) {
      provides = currentInstance.provides = Object.create(parentProvides)
    }
    provides[key as string] = value
  }
}
​

export function inject(
  key: InjectionKey<any> | string,
  defaultValue?: unknown,
  treatDefaultAsFactory = false
) {
  //获取当前组件实例
  const instance = currentInstance || currentRenderingInstance
  if (instance) {
  //获取provides
    const provides =
      instance.parent == null
        ? instance.vnode.appContext && instance.vnode.appContext.provides
        : instance.parent.provides
​
    if (provides && (key as string | symbol) in provides) {
      //如果key存在就直接返回
      return provides[key as string]
    } else if (arguments.length > 1) {
      //如果key不存在，设置了默认值就直接返回默认值
      return treatDefaultAsFactory && isFunction(defaultValue)
        ? defaultValue.call(instance.proxy)
        : defaultValue
    } else if (__DEV__) {
      //如果都没有就提示
      warn(`injection "${String(key)}" not found.`)
    }
  } else if (__DEV__) {
    warn(`inject() can only be used inside setup() or functional components.`)
  }
}
​