定义服务属性并在组件中访问它

Question

不确定如何从 AuthService 访问 admin 属性。组件中完全相同的代码将 this.admin 设置为数据库中的值，但我不想重写每个组件中的代码。

@Injectable()
export class AuthService {

public admin: any;

constructor() {
    this.currentUser.take(1).subscribe(user => {
          if (user) {
            this.db.object(`/users/${user.uid}`).subscribe(usr => this.setAdmin(usr));
          }
        });
    }
}


setAdmin(usr) {
    console.log(usr.admin); // returns value from database
    this.admin = usr.admin;
    console.log(this.admin); // returns usr.admin value
}

分量

export class AppComponent implements OnInit {
    public isA: any;


  constructor(public authService: AuthService,){

  }

  ngOnInit() {
    this.isA = this.authService.admin; // this is undefined
  }
}

组件编辑*****

  ngOnInit() {
    this.msgService.getPermission();
    this.msgService.receiveMessage();
    this.message = this.msgService.currentMessage;
      this.authService.currentUser.take(1).subscribe(user => {
        if (user) {
          console.log(user.uid);
          this.db.object(`/users/${user.uid}`).subscribe(usr => this.setAdmin(usr))
        }
      });
  }

  setAdmin(usr) {
    this.admin = usr.admin;
  }

Answer 1

首先，理想情况下，您的服务需要一个 get 方法。其次，在不知道什么时候会存在的情况下访问一个值是很棘手的（ASYNC，不会立即 return 也不会为它停止代码。像 PHP ）。所以你的选择是将你想用 .subscribe () 方法中的值做的事情包装起来，或者让 ngDoCheck (){} 不断地监视变化，当值存在时它会拾取（我不推荐这种方法因为它运行任何更改）。或者我更喜欢把东西做成 BehaviourSubject 这样它就可以作为 Observable 并且你可以 Subscribe 它，这样当值发生变化时组件会自动拾取它。

@Injectable()
export class AuthService {

  public admin: BehaviourSubject < any > = new BehaviourSubject(null);

  constructor() {
    this.currentUser.take(1).subscribe(user => {
      if (user) {
        this.db.object(`/users/${user.uid}`).subscribe(usr => this.setAdmin(usr));
      }
    });
  }
}


setAdmin(usr) {
  console.log(usr.admin); // returns value from database
  this.admin.next (usr.admin);
  console.log(this.admin); // returns usr.admin value
}

getAdmin(): Observable < any > {
  return this.admin.asObservable();
}

export class AppComponent implements OnInit {
  public isA: any;


  constructor(public authService: AuthService, ) {

  }

  ngOnInit() {
    this.authService.getAdmin.subscribe((admin: any) => {
      //Anything in here get executed every time the `admin` variable's value chnages
    });
  }

  // If you want to see how the ngDocheck() behave
  ngDoCheck() {
    console.log("DoCheck");
  }
}

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