Advanced Perl Programming

Event Loops

MainLoop executes an event loop that picks up events from the underlying windowing system and dispatches it to the appropriate widgets. When a callback procedure is called in response to an event, it is the responsibility of the callback to return (or “yield”) as soon as possible; otherwise, it holds up all events that have since arrived.

For long-running activities that are CPU-intensive, it is your responsibility to chop that activity up into manageable pieces and arrange for a timer to call the processing routine at regular intervals. This gives the event loop a chance to dispatch pending events. This kind of CPU sharing is called cooperative multitasking. Early versions of Microsoft Windows (until Version 3.1) similarly depended on your application being a good citizen; otherwise, it would hang the entire operating system.

For tasks such as ray-tracing and animations that are both CPU- and GUI-intensive, you can use the $widget—>update method to process all events. This method doesn’t return until all pending event messages (including repaint events) in the event queue have been processed.

Blocking system calls are not a very good idea in an event-driven environment, as we discussed in Chapter 12. The most common of these are the read and write calls, especially if they are talking to pipes and sockets. For example, the diamond operator (<>) blocks until it can get a line of text. Instead of directly calling an I/O call, you must let Perl/Tk tell you that it is ...