Rxjs：防止将数据从服务外推送到主题

Question

Within my anguular app , i ve this service :

@Injectable()
export class myService{

  myBehaviouSubject= new BehaviorSubject("");


  setData(){
     this.myBehaviouSubject.next("123");
  }

}

Inside my app.component , im able to get the value , but i want to keep it readonly or editable only inside the service itself , i want to prevent to push any data from component ( .next('DATA') )

    @Component({

    })
    export class AppComponent implements OnInit {

      constructor(public myService : MyService) { }

    getData(){
      // GET VALUE
      this.myService.myBehaviouSubject.value
    }

    unwantedMethodToSetValue(){
           // SET VALUE -> i want to prevent this
           this.myService.myBehaviouSubject.next("unwanted value")

    }

}

Suggestions ?

Answer 1

You can keep the observable inside service only by declaring it as private field of a class.

 @Injectable() export class myService { private myBehaviouSubject = new BehaviorSubject(""); // Use this observable inside the app component class. myBehaviouSubjectObservable = myBehaviouSubject.asObservable(); setData() { this.myBehaviouSubject.next("123"); } } @Component({ }) export class AppComponent implements OnInit { constructor(public myService: MyService) {} getData() { // You can subscribe to observable and can get value here this.myService.myBehaviouSubjectObservable.subscribe((value) => { console.log(value); }) } unwantedMethodToSetValue() { // SET VALUE -> you cannot do this here now. this.myService.myBehaviouSubject.next("unwanted value") } }

Answer 2

Use property access modifiers:

@Injectable()
export class MyService{
  private myValueSubject: BehaviorSubject<string> = new BehaviorSubject<string>("");
  public readonly myValueObservable: Observable<string> = this.myValueSubject.asObservable();

  public setData() {
     this.myValueSubject.next("123");
  }
  public getData(): string {
    return this.myValueSubject.value;
  }
}

Instances of MyService will not have a publicly accessible subject.

I usually try to avoid a method like getData , favoring subscriptions to the related observable. If I ever find myself writing those kinds of methods, it's a warning flag to re-evaluate my architecture. If you just want to store a value and get/set it with methods, use a plain old private property. The entire purpose of the subject is defeated if you are only ever getting the value through a method like getData()

Check out the documentation for typescript classes, which discusses access modifiers: https://www.typescriptlang.org/docs/handbook/classes.html

Answer 3

The traditional answer : If you return the Subject as an observable, you disallow .next() calls.

But in your case, you also want direct access to the current value without subscribing, so you could add a getter for that too.

@Injectable()
export class myService{

  private readonly myBehaviouSubject = new BehaviorSubject("");


  setData(){
     this.myBehaviouSubject.next("123");
  }
  public get myObservable$(): Observable<string>{
    return this.myBehaviourSubject;
  }
  public get currentValue(): string{
    return this.myBehaviourSubject.value;
  }

}

Answer 4

https://stackblitz.com/edit/angular-protected-rxjs-subject

in this solution which I hope meet you needs:

• be aware that there is no subscription
• fetching updates handled manually
• Property 'myBehaviourSubject' is private and only accessible
  within class 'TestService'.