How to create an asynchronous form validator?

Question

Angular

I want to create an asynchronous validator using an observable, but I don't know if this is possible. Now it only turns out to be done through promises, but that's not it.

//вызов 
this.userForm = fb.group({
            firstName : null,
            surname : null,
            username : null,
            email : [null, Validators.compose([Validators.required, Validators.minLength(5)]), ValidationService.uniqueEmail],
            password : '',
            role : '',
            state : '',
            phone : '',
        })

static uniqueEmail(control: FormControl) {
        return Promise.resolve().then(() => {
            setTimeout(() => {
                console.log('ttt');
                if(control.value == '[email protected]') {
                    return null;
                } else {
                    return { 'notUnique' : true };
                }
            }, 2000);
        });
    }

The point is that I want to check if the email is unique, but with promises it won't work because I want to use a delay using debounceTime(500) . The principle should be like this. The user enters data, we have a delay of 0.5 seconds, then there is a request, but if the user continues to enter data back, this request must be canceled and a new one sent. how can this be implemented? I think that only observable will help here, only how to get involved with it