Angular2: Conditional required validation

Question

I am trying to create a conditional required validation on a specific field.I try doing this by return Validators.required back in my function, but this doesn't seem to work. How do I go about doing this? Here's my code:

private _ansat: AbstractControl = new FormControl('', Validators.required);
private _helbred: AbstractControl = new FormControl('', Validators.compose([this.useValidateIfRadio(this._ansat, 0, Validators.required)]) );


constructor(private _fb: FormBuilder) {
    this.myForm = this._fb.group({
            ansat: this._ansat,
            helbred: this._helbred
        });
}

useValidateIfRadio (c: AbstractControl, n: number, v) {
        return function (control) {
            return new Promise(resolve => {
             // this.msg = ansatControl.value;
             console.log(v);
                if (c.value === n) {

                    resolve(v);
                }
                else {
                  resolve(null);

                }
            });
        };
    };

Any help is greatly appreciated.

Answer 1

These answers got me most of the way there, but I found out a pretty big gotcha… in some cases, setValidators only adds to the existing array of validators and does not work well to clear them. In some cases, like when ngOnInit loads twice in a row, the conditions could be first negative and then positive for a passed-in value you're depending on. In such a case, you will set it to required, then later attempt to clear it, but the UI will still behave like it expects it. To fix this, consider the following...

const myControl = this.your_form.get('field_name');
if(some_logic) {
     myControl.clearAsyncValidators();
     myControl.clearValidators();
     myControl.updateValueAndValidity({onlySelf:true});
} else {
     myControl.setValidators([Validators.required, Validators.other…]);
}