Chapter 16. Event-Driven Programming

This chapter describes event-driven programming using timers and asynchronous I/O facilities. The after command causes Tcl commands to occur at a time in the future, and the fileevent command registers a command to occur in response to file input/output (I/O). Tcl commands discussed are: after, fblocked, fconfigure, fileevent, and vwait.

Event-driven programming is used in long-running programs like network servers and graphical user interfaces. This chapter introduces event-driven programming in Tcl. Tcl provides an easy model in which you register Tcl commands, and the system then calls those commands when a particular event occurs. The after command is used to execute Tcl commands at a later time, and the fileevent command is used to execute Tcl commands when the system is ready for I/O. The vwait command is used to wait for events. During the wait, Tcl automatically calls Tcl commands that are associated with different events.

The event model is also used when programming user interfaces using Tk. Originally, event processing was associated only with Tk. The event loop moved from Tk to Tcl in the Tcl 7.5/Tk 4.1 release.