Let's take a look at some of the common patterns when working with forms.
Problem: Adding form fields dynamically or changing form fields from what has been declared.
Solution: Add or change fields during initialization of the form.
Forms are usually defined in a declarative style with form fields listed as class fields. However, sometimes we do not know the number or type of these fields in advance. This calls for the form to be dynamically generated. This pattern is sometimes called Dynamic Forms or Runtime form generation.
Imagine a flight passenger check-in system, which allows for the upgrade of economy class tickets to first class. If there are any first-class seats left, there needs to be an additional option to the user if they would like to fly first class. However, this optional field cannot be declared since it will not be shown to all users. Such dynamic forms can be handled by this pattern.