RequiredIfDirective.ts

@Directive({
  selector: '[requiredIf]',
   providers: [
        {provide: NG_VALIDATORS,useExisting:RequiredIfDirective, multi: true}
    ]
})
export class RequiredIfDirective implements Validator {
    @Input("requiredIf")
    requiredIf: boolean;

    validate(c:AbstractControl) {
  
       let value = c.value;
        if ((value == null || value == undefined || value == "") && this.requiredIf) {
                return {
                    requiredIf: {condition:this.requiredIf}
                };
        }
        return null;
    }

}