如何使用带角度5的@Injectable providedIn根获取

Question

When I use the

@Injectable({
    providedIn: 'root'
})

The lines keep getting underlined in red with the message Expects 0 arguments but got 1. Any ideas on what to do.

I've been following the documentation here https://angular.io/tutorial/toh-pt4#injectable-services Here is the code:

import { Injectable } from '@angular/core';
import { Observable, of } from 'rxjs';
import { Hero } from './hero';
import { HEROES } from './mock-heroes';
import { MessageService } from './message.service';

@Injectable({
  providedIn: 'root',
})
export class HeroService {

//Modify the constructor with a parameter that declares a private 
messageService property. 
 // Angular will inject the singleton MessageService into that property when 
it creates 
  // the HeroService.
 constructor(private messageService: MessageService) { }

getHeroes(): Observable<Hero[]> {
// TODO: send the message _after_ fetching the heroes
this.messageService.add('HeroService: fetched heroes');
return of(HEROES);
}
}

Answer 1

I think this feature was introduced in Angular 6, so you'll have to remove that and instead provide the service in your component's module:

@NgModule
....
providers: [MyService]