FormControl's valueChanges doesn't trigger when new value set-CodePudding

It is very weird that valueChanges doesn't trigger in below example. Is this a bug ? Do i have to unsubscribe from valueChanges?

subscription = Subscription.EMPTY;

constructor() {

    this.form = new FormGroup({
      id: new FormControl(0, { validators: [] })
    });
  }

  ngOnInit(): void {
    let sub = this.tokenId.valueChanges.subscribe((value: number) => this.tokenSelected(value));

    //Value changes doesn't trigger because of this line 
    //but it works if i remove it. 
    this.subscription.add(sub);
  }

  get id() { return this.form.get('id')!; }

  setId() {
    this.id.setValue(1);
  }

CodePudding user response：

It is very weird that valueChanges doesn't trigger in below example. Is this a bug ?

It's not a bug and valueChanges doesn't trigger because it's already unsubscribed within ngOnInit.

The issue is with the below assignment:

subscription = Subscription.EMPTY;

It creates a new Subscription and sets it's closed property to true. The closed flag is used to indicate whether the Subscription has already been unsubscribed.

When you add a new subscription using add(sub), it internally checks if the subscription is already closed and since the closed property was set to true, the newly added subscription sub is automatically unsubscribed. Hence the valueChanges didn't triggered in your case.

Instead you can have the initialization as below:

subscription = new Subscription();