I'm curious about the proper use of behaviorSubject.value. In a discussion on Stack Overflow, it was mentioned that values should ONLY be obtained through subscription.
One scenario where it makes sense to me is when I need to determine the next value to push through the stream, such as toggling a boolean:
myBoolSubject = new BehaviorSubject(false);
toggle() {
this.myBoolSubject.next(!this.myBoolSubject.value);
}
An alternative using subscribe() would look like this:
toggle() {
this.myBoolSubject.pipe(take(1)).subscribe(
val => this.myBoolSubject.next(!val)
);
}
By examining the source code for RxJS and the referenced answer, it appears that using .value may result in an error only if:
- the subject has been completed,
- there is an error.
In this simple case, completing the subject or errors are not a concern since I am dealing with straightforward boolean values.
Is this a valid use of behaviorSubject.value? Are there any other scenarios?
Another situation where using .value seems appropriate is when creating a new object based on the previously emitted value:
private state = new BehaviorSubject<State>(INITIAL_STATE);
public state$ = this.state.asObservable();
public updateState(changes: Partial<State>){
const newState = {...this.state.value, ...changes};
this.state.next(newState);
}
The alternate approach would involve caching the latest state emission in a separate variable, like so:
private _state = INITIAL_STATE;
private state = new BehaviorSubject<State>(INITIAL_STATE);
public state$ = this.state.asObservable();
public updateState(changes: Partial<State>){
const newState = {...this._state, ...changes};
this.state.next(this._state = newState);
}
Am I overlooking any potential issues here?