How to extend a component with dependency injection?

Question

I have the following class ModuleWithHttp:

@Injectable()
export default class {
  constructor(private fetchApi: FetchApi) {}
}

and I want to use it as follows:

@Component({
  selector: 'main',
  providers: [FetchApi]
})
export default class extends ModuleWithHttp {
  onInit() {
    this.fetchApi.call();
  }
}

So by extending a super class that already injects a dependency I want to have an access to it in its children.

I tried many different ways, even having super-class as a component:

@Component({
  providers: [FetchApi]
})
export default class {
  constructor(private fetchApi: FetchApi) {}
}

But still, this.fetchApi is null, even in super-class.

Answer 1

If you want to avoid this "boiler plate" code injecting services in child classes just for injection in parent classes' constructor and then effectively using that services in child classes through inheritance, you could do this:

edit: from Angular 5.0.0 ReflectiveInjector has been deprecated in favour of StaticInjector, below is updated code to reflect this change

Have a services map with deps,

export const services: {[key: string]: {provide: any, deps: any[], useClass?: any}} = {   'FetchApi': {     provide: FetchApi,     deps: []   } } 

Have an Injector holder,

import {Injector} from "@angular/core";  export class ServiceLocator {   static injector: Injector; } 

set it up in AppModule,

@NgModule(...) export class AppModule {   constructor() {     ServiceLocator.injector = Injector.create(       Object.keys(services).map(key => ({         provide: services[key].provide,         useClass: services[key].provide,         deps: services[key].deps       }))     );   } } 

use the injector in parent class,

export class ParentClass {    protected fetchApi: FetchApi;    constructor() {     this.fetchApi = ServiceLocator.injector.get(FetchApi);     ....   } } 

and extend parent class so you don't need to inject the FetchApi service.

export class ChildClass extends ParentClass {   constructor() {     super();     ...   }    onInit() {     this.fetchApi.call();   } } 

Answer 2

You have to inject fetchAPI in the super class and pass it down to the child class

export default class extends ModuleWithHttp {

  constructor(fetchApi: FetchApi) {
     super(fetchApi);
  }   

  onInit() {
    this.fetchApi.call();
  }
}

This is a feature of how DI works in general. The super class will instantiate the child via inheritance, but you have to supply the required parameters to the child.