Angular DI和继承:注入基础服务的扩展

问题描述

我有一个基本服务和两个继承服务:

I have a base service and two inhering services:

@Injectable({ providedIn: 'root' })
export class BaseService {
  foo(src?: string){
    return `speaking from ${src || 'BaseService'}`;
  }
}


@Injectable({ providedIn: 'root' })
export class SomeService extends BaseService {
  foo(){
    return super.foo('SomeService')
  }
}

@Injectable({ providedIn: 'root' })
export class AnotherService extends BaseService {
  foo(){
    return super.foo('AnotherService')
  }
}

我希望将它们注入某些组件中并检索三个单独类的实例:

I wish to inject them in some component and retrieve instances of the three separate classes:

@Component({
    selector: 'my-app',
    template: `
        <div>
            <p>Who's there?</p>
            <p>{{ base }}</p>
            <p>{{ some }}</p>
            <p>{{ another }}</p>

        </div>
    `,
})
export class App {
    base: string;
    some: string;
    another: string;

    constructor(base: BaseService, some: SomeService, another: AnotherService) {
        this.base = base.foo();
        this.some = some.foo();
        this.another = another.foo();

    }
}

相反，我得到了三个相同类的实例(HTML输出):

Instead, I get three instances of the same class (HTML output):

Who's there?

speaking from BaseService

speaking from BaseService

speaking from BaseService

• 为什么这行不通?
• 为什么SomeService，AnotherService和BaseService不是Angular DI的唯一标记?

似乎推杆

...
{ provide: SomeService , useClass: SomeService },
{ provide: AnotherService , useClass: AnotherService },
...

提供程序中的

将使其起作用.

in the providers will make it work.

• 为什么这是明确需要的?

一个plnkr: https://next.plnkr.co/edit/BvmppLHRbFbz9CFZ

推荐答案

SomeService和AnotherService继承了BaseService的装饰器元数据，因此angular将BaseService的实例插入其位置.

SomeService and AnotherService inherit the decorator metadata from BaseService, so angular injects an instance of BaseService in their place.

这很危险，因为从SomeService或AnotherService调用任何实例成员(不是从BaseService继承的)都会触发运行时错误.

This is dangerous, as calling any instance member from either SomeService or AnotherService which isnt inherited from BaseService will trigger a run-time error.

存档所需行为的最简单方法是从没有装饰器的通用抽象基类继承:

The simplest way to archive the behavior you are looking for, would be to inherit from a common abstract base class, with no decorator:

export abstract class AbstractBaseService {
  foo(src?: string) {
    return `speaking from ${src || 'AbstractBaseService'}`;
  }
}

@Injectable({ providedIn: 'root' })
export class BaseService extends AbstractBaseService {
  foo() {
    return super.foo('BaseService');
  }
}

@Injectable({ providedIn: 'root'})
export class SomeService extends AbstractBaseService {
  foo() {
    return super.foo('SomeService');
  }
}

@Injectable({ providedIn: 'root' })
export class AnotherService extends AbstractBaseService {
  foo() {
    return super.foo('AnotherService');
  }
}

我修改了plnkr 来测试这种方法.

这篇关于Angular DI和继承:注入基础服务的扩展的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！