Editing XHB (section)

=== The GUI programming model ===

A GUI 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 imperative in the lines of:

  1. Open a connection to the X server.
  2. Either poll or wait for events (an event pumping loop)
     1. Receive the next event from the X server.
     2. Handle the event, possibly generating more events or requests (e.g., sending various drawing requests to the X server)
     3. If the event was a quit message, exit the loop
  3. Close the connection to the X server

Note that we omitted program initialization and other boring stuff.