Dynamic form validation in ionic2

I am creating a form in ionic2 in which fields changes as per categories changed by user dynamically.

if a field category is A is selected

then some fields such as start_date and time are required

but for category B only start_date is required

and for C both are not required.

I am using FormBuilder in ionic2 and validating like:

this.logForm = formBuilder.group({
        'start_date': ['', Validators.compose([Validators.maxLength(30),
                              Validators.required])],
        'time': ['', Validators.compose([Validators.maxLength(30),
                              Validators.required])],
    });

I am getting the selected category in an alert result like this:

category(){
   var options = {
      title: 'Choose a task category',
      inputs: [],
      message: 'Which category do you like?',
      buttons: [
        {
          text: 'Cancel',
          role: 'cancel',
          handler: () => {
            console.log('Cancel clicked');
          }
        },
        {
          text: 'Ok',
          handler: data => {}
        }
      ]
   }
}

In the handler of 'Ok' I can successfully retrieve the selected category.

So my question is how can i apply validation dynamically as per the category selected by user in app.