virtual event

[Explain virtual events. Is there a tutorial? Illustrate usage. Put forward references in appropriate places--what are those? "advanced Tk-ing?"]

Important Thread (dead in 2013-01)

Bryan Oakley's wizard mega- (meta?) widget taught me a wonderful use of virtual events. Whereas I had code in my wizard implementation like:

proc doNextThing { args } {
    ...
}
button $w.next -text "Next" -command doNextThing

Bryan had the foresight to indirect the button handling via virtual events. Now I do something like:

proc doNextThing { args } {
}
proc dispatch { event } {
    switch -- $event {
        <<Next>> {
            doNextThing
        } 
        ...
    }
}
...
button $w.next -text "Next" \
    -command [list event generate $w <<Next>>]
bind $w <<Next>> [list dispatch <<Next>>]

The wonder of this is that my testing harness can do:

bind $w <<Next>> {+doTestingThing}

and trap the button presses without making invasive changes to the wizard code!

--Chris Nelson

MC (5 May 2003): Until today I hadn't realized that you could have Virtual Events that weren't associated with a real physical event. Then while googling on event add I came across virtual events (was Re: Adding new event types to Tcl/Tk) , comp.lang.tcl , Bryan Oakley ,2003-01-10.

DKF - Most virtual events are generated in response to a physical event of one kind or another (since they all have to have a window) but there's no need for them to be directly associated with anything. Various events dealing with those changes to Tk's internal data models that are not bound to variables (e.g. the <<ListboxSelect>> and <<MenuSelect>> events) are produced to make tracking things easier.

Category Concept