In terms of BX, they would provide a single XmForm as a child of their class, and then make this a receptor for the specific subclasses derived from WindowApp. So to achieve uniqueness the design would use inheritance and subclassing. This is fine for the usual case where the developer creates the VkWindow derived class as the final subclass. This line must only be called once per VkWindow and the generated code is violating this rule. So inheritance is out in terms of customizing a window, and the alternative is composition.
Composition is argued for in some cases because it reduces the depth of the inheritance tree.
This paper will describe a method of composition that will support the new application-specific functionality yet still retaining the core functionality of the WindowApp class. For the moment do not concern yourself with how we will use BX to support our composition ideas. First we will describe the general class design and philosophy. We will, however, assume that we are still using Viewkit. To start with, let us continue using the WindowApp class as described above.