How to bind 'Control-Control' event in Tcl/Tk - events

Question

I would like to bind a very specific event, which is 'Control Key' followed by another 'Control Key', similar to KVM operation, to a widget.
I know how to bind various event but not this one, which comprises two sequential events.
Any idea how to implement it?

Answer 1

In theory, you'd be able to use a binding event sequence to do this:
# The keyboard often has two control keys; they have different names
bind $w <KeyPress-Control_L><KeyPress-Control_L> {puts "double-control (left)"}
bind $w <KeyPress-Control_R><KeyPress-Control_R> {puts "double-control (right)"}
However, the control keys (as with the other modifiers) are especially complicated to handle this way because they are repeated by the OS for you; they're a special case! Because of that, you instead need to build your own queue of keyboard events:
bind $w <KeyPress> {queueEvent %K}
set queue {no-such-key}
proc queueEvent {key} {
global queue
if {[llength [lappend queue $key]] > 2} {
set queue [lrange $queue end-1 end]
}
lassign $queue last this
if {$last eq $this && [string match Control* $this]} {
puts "double ctrl"
}
}
This can interact badly with other bindings. How to fix that depends on the wider context of your application.

Answer 2

Now with taking care of the quick delay required for "double-click"
bind . <KeyPress> {queueEvent %K}
set queue {}
set times {}
proc queueEvent {key} {
global queue times
if {![string match Control* $key]} {set queue {}; set times {}; return}
if {$key eq $queue} {
if {[expr [clock milliseconds] - $times] <= 500} {
puts "double ctrl $key"
set queue {}; set times {}
return
}
}
set queue $key
set times [clock milliseconds]
}