Test code:
export abstract class GreetingService {
abstract greet(name: string): string;
}
Implementation class annotated with @Injectable:
import { Injectable } from '@angular/core';
import { GreetingService } from './greeting.service';
@Injectable({ providedIn: 'root'})
export class EnglishGreetingService extends GreetingService {
greet(name: string): string {
return 'Hello ' + name;
}
constructor(){
super();
console.log('English class created!');
}
}
Attempt to inject through constructor parameters:
wrong information:
core.js:6241 ERROR NullInjectorError: R3InjectorError(AppModule)[GreetingService -> GreetingService -> GreetingService]:
NullInjectorError: No provider for GreetingService!
at NullInjector.get (http://localhost:4201/vendor.js:8310:27)
at R3Injector.get (http://localhost:4201/vendor.js:22317:33)
at R3Injector.get (http://localhost:4201/vendor.js:22317:33)
at R3Injector.get (http://localhost:4201/vendor.js:22317:33)
at NgModuleRef$1.get (http://localhost:4201/vendor.js:39618:33)
at Object.get (http://localhost:4201/vendor.js:37352:35)
at getOrCreateInjectable (http://localhost:4201/vendor.js:12112:39)
at Module.ɵɵdirectiveInject (http://localhost:4201/vendor.js:26132:12)
at NodeInjectorFactory.AppComponent_Factory [as factory] (http://localhost:4201/main.js:122:289)
at getNodeInjectable (http://localhost:4201/vendor.js:12257:44)
Solution
In the providers area of the module, maintain specific implementation classes for GreetingService:
providers: [{ provide: JerrySandBoxService },
{ provide: GreetingService, useClass: EnglishGreetingService}]
The problem can be solved:
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