[MJ] - The following is a work in progress. I couldn't find a good event tutorial starting from basics anywhere. If this exists please include a link.

A Tcl event tutorial

The goal of this page is to give an introduction to the Tcl eventloop suitable for Tcl beginners. If you make changes to this page please keep that in mind.

Assumptions, it is assumed that you are reasonably versed in Tcl. This means that you should know:
    * How Tcl handles quoting and grouping This means you know the difference between $var, "$var", {$var} and [[$var]] if you ddon't know this difference, read man Tcl.
    * You know how to create procedures (see man proc)
    * You know how Tcl handles global variables (see man variable and man global)
    * You know the basic concepts of programming in Tcl (puts, gets, switch etc.)
If you don't know this it is better to start with a generic Tcl tutorial first (links here)

'''What is event based programming'''

   * event queues
   * eventloop
   * why would you use/need events?

When you write an console based application and you don't use events. The commands in your script will be executed in a specific order. For example the script:

 set a 1
 set b 2
 puts $a
 puts $b

will: 
    * set a to 1
    * set b to 2
    * show the value of a
    * show the value of b

For a large class of applications this is not a limitation and you can get a long way without ever needing events.

Now imagine you write an application that takes user input and based on that user input takes a certain action. For instance you have a currency conversion application where the user can change the currency to convert from, the currency to convert to and the amount to convert. One way to write this is like:

However if you want to allow the user to take actions in a more random order this will be fairly limited and you might arrive to something more like:

 while 1 {
       show_menu
       gets stdin input
       switch $input {
	      # do different things based on user input
       }
 }

This show the beginnings of a rudimentary event loop. The system is waiting for an event (the user presses a key) and takes an appropriate action (execute the correct switch case) or to make the correspondance even clearer:

 while 1 {
       show_menu
       gets stdin event
       switch $event {
	      # do different things based on the user input event
       }
 }

When you start building GUIs it is even more clear that you will need to act on events, because the user has a very large number of possible actions he can perform at any given moment in time. Think about:
    * Pressing a button
    * Opening a menu
    * Typing some text in a text box

If all of these actions generate events and we have some way of reacting to these events we can take the correct action based on what the user does without knowing when he will do it (e.g. he might enter some text first and then press a button or vice versa)


'''How does Tcl use event based programming'''
   * file events
   * eventloop
   * callbacks
   * after
   * event sources


'''Events in Tk'''


'''Common pitfalls when using events in Tcl'''

   * event callbacks execute in global scope
   * events should not take long to handle
   * callback `quoting hell` -> use procs