Purpose: This page attempts to provide a simple working example of an event-driven program. (also see Keep a GUI alive during a long calculation)
KBK:
Many times, users ask how to keep a Tk user interface "live" while some long-running calculation is being performed, or some I/O is proceeding in the background -- in general, how to keep a Tk application running while it's waiting for something. Often people, point to the update command in reply. The update command is not the Tcl Way.
Let's try writing a script that counts a label down from 10 to 1. Here's a version of the program that uses update:
# Create a simple GUI to monitor the clock
label .counter -font {Helvetica 72} -width 3 -textvariable count
grid .counter -padx 100 -pady 100
# Run the countdown
for { set count 10 } { $count >= 0 } { incr count -1 } {
# Make sure that the GUI stays up to date
update
# Wait one second between time ticks
after 1000
}
exitIf you run this program, you'll see that it displays numbers counting down from 10 to 0 and then exits. The problem, though, is that it isn't really live. During the 'after', it isn't interacting with the user. Updating your UI this way is a really bad idea.
Consider, rather, structuring your application like the program below. Like the one above, it counts down from 10 to 0 and then exits.
To understand it, it's best to look first at the main program (at the bottom of the file, below the countdown procedure. It does two things: it initiates the countdown by calling countdown for the first time, and it creates a trivial user interface, consisting of just a label widget, to display the result.
Everything interesting happens within the countdown procedure. On the first trip through, it finds that the count variable does not exist, and sets it to 10. It then executes the
after 1000 countdown
statement, which causes the event loop to call countdown again one second later.
At this point, the GUI gets created; the label widget finds that the value of its text variable is 10, and displays it.
One second later, countdown enters the second time. This time, it finds that count exists, and decrements it from 10 to 9. The magic of Tk (Tcl variable traces, if you must know) causes the label widget to update automatically. The countdown procedure then executes that after statement again, so that it will wake up one second later.
[ ... ]
On its final trip, countdown enters with the value of count at 0. It decrements it to -1, discovers that it has gone negative, and unceremoniously exits.
# Chain of events that manages the countdown
proc countdown {} {
variable count
# The first time through, 'count' is 10; thereafter, it
# decrements on each trip
if { ![info exists count] } {
set count 10
} else {
incr count -1
}
# When the count goes negative, exit
if { $count < 0 } {
exit
}
# Schedule the next tick of the clock
after 1000 countdown
return
}
# Start the clock
countdown
# Create a simple GUI to monitor the clock
label .counter -font {Helvetica 72} -width 3 -textvariable count
grid .counter -padx 100 -pady 100The more concise version of the countdown procedure can be written as:
proc countdown2 {{cnt 10}} {
set ::count $cnt
if {$cnt < 0} exit
incr cnt -1
after 1000 [list countdown2 $cnt]
}Exercises