When programming an XI application, there will be three or more tool kits underneath your code on each machine you are supporting. For example, if you are writing an XI application for Microsoft Windows, you will encounter the XI tool kit for Windows, XVT for Windows and the Windows SDK. These tool kits are layered, with the “native” tool kit on the bottom of the stack, XVT for that platform on top of the native tool kit and XI on top of XVT.

When we say that a tool kit “sits on top” of another tool kit, such as XI on top of XVT, we mean that the tool kit on top uses the functions of the tool kit below it, but not vice-versa. XI can call XVT functions, but XVT doesn’t use any of the symbols defined by XI. In addition, we often say that a programmer can “drop down a level” to access the functionality of a tool kit beneath it. Since your code is on top of all of the tool kits in a layered API, you can access any layer anywhere in your program.

XI Programming 

An XVT window can have an XI interface which contains XI objects. (When using XI, the XI interface is XI's object for the XVT window. An XI interface can be thought of as the XVT window.) The objects in a window are defined using functions and are instantiated at runtime (as opposed to being part of a resource file). In addition to defining and creating an interface, you will need to write event handlers to process the events generated by the operation of the interface. Using XI functions, you will manipulate XI objects in response to those events. You may also use XI’s application data for objects and XI’s tree memory management.

XI is event-driven since it “sits on top” of XVT and responds to XVT events. In addition, XI generates its own events to which you will need to respond in an event handler function. 

 XVT Programming

To create any application with XI, you will need to do some XVT programming. As with any XVT programming effort, it is important to go back and forth between the hardware platforms you’ll be supporting. Follow the rule, “port early and often.”
For the most basic application using XI, you will need to do several things in XVT. These are explained in detail in Creating an Interface, but for now, here is a summary of what you’ll need to do:

1. Include xi.h instead of xvt.h in your C module.
2. Write a main function.
3. Write a task window event handler. In this function you will need to place a call to xi_init which tells XI to initialize itself.