Printing Message Terminology(IM:XP)

Inside Macintosh: QuickDraw GX Printing Extensions and Drivers /: Chapter 4 - Printing Messages / About the Printing Messages

Printing Message Terminology
This chapter uses object-oriented programming terminology to describe how the printing messages operate. This section reviews the terms used in this chapter to describe the messages. For a more complete description of these terms and how message-passing works in QuickDraw GX, read the chapter "Message Manager" in Inside Macintosh: QuickDraw GX Environment and Utilities. In this book, the chapter "Introduction to Printing Extensions and Drivers" contains an overview of how message-passing works with printing extensions and printer drivers.
The QuickDraw GX printing architecture uses a number of different object types to provide printing. Each print object is an abstract data type that encapsulates certain properties that you can use QuickDraw GX functions to access. The objects used in the QuickDraw GX printing architecture are described in Inside Macintosh: QuickDraw GX Printing. These objects include job objects, printer objects, format objects, and paper-type objects.
When QuickDraw GX needs to accomplish a printing task, it sends a message. For example, QuickDraw GX sends a message when it needs a driver to establish a communications connection with a printing device, when it wants to print a page, and when it needs a printer driver to convert a bitmap into a data format for a raster printer.
You respond to messages by overriding them. Your override of a message intercepts the message and takes some action. A number of message handlers can respond to each message, including the system software, the application program, a printer driver, and one or more printing extensions.
QuickDraw GX provides a default implementation of each printing messages. You can partially override a message to add to the default implementation's response or to change the results of the default implementation. Depending on the nature of a message, you can forward it to the other handlers and then perform your tasks, or you can perform your tasks and then forward it to other handlers.

Note
The action of the default implementation of each printing message is noted in this chapter. Some of the default implementations provided by QuickDraw GX are empty (all that the function does is return).
You can totally override some messages, which means that your override completely replaces the default implementation and does not foward the message. The "Special Considerations" section for each of the messages described in the section "Printing Messages Reference" in this chapter indicates which printing messages you can totally override. If you do totally override a printing message that requires forwarding, the default implementation is not invoked, which means that a vital operation might be neglected and serious errors might result.