Angular Observable BehaviourSubject.getValue() returns normal value-CodePudding

I have this syntax:

const pageId = this._someService.currentData$<Data>().getValue().id;

which returns a normal string value with getValue() from the Observable (BehaviourSubject) currentPage$<Page>(). The class BehaviousSubject implements the getValue() method

export declare class BehaviorSubject<T> extends Subject<T> {
getValue(): T;
}

but how is that possible? I thought we have to subscribe to an observable to get the actual value.. ?

CodePudding user response：

   private deviceType = new BehaviorSubject<string>('');
  currentDeviceType = this.deviceType.asObservable();
  changeDeviceType(value: string) {
    this.deviceType.next(value);
  }

This is how you can make a subject an Observable and the function to change the value, so now you can subscribe to the currentDeviceType and get all the value changes.

CodePudding user response：

There are multiple types of Observables, most of them do not hold the value, but some of them do.

The examples that hold values are BehaviorSubject and ReplaySubject.

BehaviorSubject requires a value in the constructor. It implements all methods of Observable as well as Subject, but it has its own special value and getValue properties.

ReplaySubject will reply X (provided in the constructor) number of the last emissions upon subscription. Passing 1 will make it almost into a BehaviorSubject.

Why?

If you need some variable to be observable, but you sometimes need it immediately (synchronously), you make it into a BehaviorSubject. You don't have to expose it as BehaviorSubject, you could expose only the Observable or Subject of it.

class LoginService {
  private readonly _isLoggiedIn$ = new BehaviorSubject(false);
  public readonly isLoggiedIn$ = this._isLoggiedIn$ as Subject<boolean>;
}