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Inside Macintosh: QuickDraw GX Environment and Utilities /

Chapter 6 - Message Manager / About the Message Manager

Message Terminology

In working with the Message Manager there are a number of terms that are useful to describe the software components and their interactions.

A message object is the loose equivalent of an object in a fully object-oriented system. It is the recipient of messages. A message object may also send messages to itself or to another message object.

A message is a form of notification passed to a message object in order to have that message object perform some operation.

A message handler is a component of a message class. A message class may consist of one or more handlers, each of which overrides zero or more messages. Each message handler may override some portion of the functionality of the handler below it in the message class. Message classes are built up from message handlers, in a manner similar to that in which a class in an object-oriented language is derived from other classes. To forward is to invoke the override of the next handler in the chain for the current message.

A message override is the loose equivalent of a method. It is the implementation, in actual code, of a given message. The override performs the operation requested by sending a message to a message object.

A message class is the loose equivalent of a class in a fully object-oriented system. It defines the set of messages that message objects instantiated from it understand and encapsulates the message handlers that implement the overrides corresponding to those messages. Message classes define the acceptable set of messages for all handlers that they encapsulate.

An instance is one copy of a message handler in memory.