Microwindows - Microwindows API

Microwindows API

The fundamental communications mechanism in the Microwindows API is the message. A message consists of a well-known message number, and two parameters, known as wParam and lParam. Messages are stored in an application's message-queue, and retrieved via the GetMessage function. The application blocks while waiting for a message. There are messages that correspond to hardware events, like WM_CHAR for keyboard input or WM_LBUTTONDOWN for mouse button down. In addition, events signaling window creation and destruction WM_CREATE and WM_DESTROY are sent. In most cases, a message is associated with a window, identified as an HWND. After retrieving the message, the application sends the message to the associated window's handling procedure using DispatchMessage. When a window class is created, its associated message handling procedure is specified, so the system knows where to send the message.

The message-passing architecture allows the core API to manage many system functions by sending messages on all sorts of events, like window creation, painting needed, moving, etc. By default, the associated window handling function gets a "first pass" at the message, and then calls the DefWindowProc function, which handles default actions for all the messages. In this way, all windows can behave the same way when dragged, etc., unless specifically overridden by the user. Major window management policies can be redefined by merely re-implementing DefWindowProc, rather than making changes throughout the system.