Programming Visual Basic for the Palm OS

Architecture Overview

Piedmont is AppForge’s SDK for extending its flagship product by creating component-based objects. As with COM on Windows, one or more Piedmont components can be housed within a module. On Windows, a module typically consists of an in-process dynamic link library (DLL) or an out-of-process executable (EXE). A Piedmont module consists of an in-process DLL on Windows, and a shared library on the Palm.

From a developer’s perspective, Piedmont’s architecture resembles the approach taken with ATL COM development. Much of the low-level work associated with COM (IUnknown implementation, reference counting, class factories, object creation, and module registration) is delegated to Piedmont-generated subclasses, which inherit from your class. In this way, Piedmont uses your class as a base class, and never instantiates it directly.^[61] This keeps the focus on implementing interfaces in your class, while delegating the housekeeping portions of COM to the derived classes. The same is true for the module code. A virtual module class allows you to override some virtual methods if desired. Otherwise, all object creation, module registration, and other housekeeping is performed by the derived class implementation.

We will see the details of this architecture unfold as we proceed with an example of our own.

^[61] We will see later how Piedmont deviates from this approach, using the PUSH_IUNKNOWN_IMPL_UP directive, but this is the default implementation.