Windows

A window is the embodiment of an application object within the Presentation Manager application model. To the end user, a window appears as a rectangular display area on the screen. From an application viewpoint however, the concept of a window is far more powerful than this. Windows may be of two basic types:

  • Display windows have a physical identity represented by a rectangular area on a screen; in this case the window represents a view of a conceptual display surface known as a presentation space, which is in fact a data entity being represented on the screen. This is the average end user's concept of a window.

  • Object windows have no physical manifestation, and are merely addresses or "handles" to which messages may be directed. An object window is typically associated with an entity such as a file or database, and is used to access this entity and perform actions upon it. The object window concept is very important to the implementation of object-oriented principles under Presentation Manager, since it enables the creation of non-visual objects within the Presentation Manager application model. See Parent/Child RelationShip (Object Windows) for further information.

    Windows respond to events, communicated by way of messages, which may originate from Presentation Manager as a result of user interaction, or from other windows existing in the system. Messages are routed to and between windows by Presentation Manager on behalf of the application. Windows may interpret and process messages in different ways, in accordance with the concept of polymorphism.

    Each window existing in the system has a unique identifier known as its window handle, which is assigned by Presentation Manager when the window is created. This handle is used to identify a window in all communication between that window and other parties, such as Presentation Manager or the user. The window handle is specified as the destination address used when passing messages to a window. See Messages for further discussion of Presentation Manager messages. A window is always aware of its own handle, since the handle is part of every message passed to the window. Presentation Manager provides a number of mechanisms by which a window may determine the handles of other windows in the system in order to pass messages to those windows; these mechanisms are described in Window Communication.

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