GUI programming: the asynchronous model

Unlike conventional computer programs, that carry some serial nature, a GUI program usually uses an asynchronous programming model, also known as "event-driven programming". This means that that program mostly sits idle, waiting for events sent by the X server, and then acts upon these events. An event may say "The user pressed the 1st button mouse in spot (x,y)", or "The window you control needs to be redrawn". In order for the program to be responsive to the user input, as well as to refresh requests, it needs to handle each event in a rather short period of time (e.g. less that 200 milliseconds, as a rule of thumb).

This also implies that the program may not perform operations that might take a long time while handling an event (such as opening a network connection to some remote server, or connecting to a database server, or even performing a long file copy operation). Instead, it needs to perform all these operations in an asynchronous manner. This may be done by using various asynchronous models to perform the longish operations, or by performing them in a different process or thread.

So the way a GUI program looks is something like that:

1. Perform initialization routines.
2. Connect to the X server.
3. Perform X-related initialization.
4. While not finished:
   1. Receive the next event from the X server.
   2. Handle the event, possibly sending various drawing requests to the X server.
   3. If the event was a quit message, exit the loop.
5. Close down the connection to the X server.
6. Perform cleanup operations.