Richard Suchenwirth 2005-12-26: Tk's [[text] widget has undo/redo functionality, so one might ask how to have that for [[canvas] widgets as well. As someone asked for it, here's an experimental script with just a canvas, on which you can doodle with left mouse-button down, and Undo and Redo buttons.
Seems to work nice and robustly in my tests. However, in my simple ways I usually prefer to just code
bind $canvas <3> [list $canvas delete current]
so I'm not tied to the creation order. Then again, this approach has no Redo.. :)
#-- Doodling: start a line, and continue it on Motion
proc newLine {w x y} {
push ::Undo [set ::ID [$w create line $x $y $x $y]]
}
proc drawLine {w x y} {
$w coords $::ID [concat [$w coords $::ID] $x $y]
}
#-- What this page is all about :-)
proc undo w {
set id [pop ::Undo]
if {$id ne ""} {
push ::Redo [item'dump $w $id]
$w delete $id
} ;# else Undo stack was empty
}
proc redo w {
set data [pop ::Redo]
if {[llength $data]} {
push ::Undo [eval [linsert $data 0 $w create]]
} ;# else Redo stack was empty
}
#-- description of a canvas item: type, coords, -key val... switches
proc item'dump {w id} {
set res [$w type $id]
lappend res [$w coords $id]
foreach i [$w itemconfigure $id] {
lappend res [lindex $i 0] [lindex $i end]
}
set res
}
#-- Stack routines (see [Stacks and queues]):
interp alias {} push {} lappend
proc pop {_stack} {
upvar 1 $_stack stack
K [lindex $stack end] [set stack [lrange $stack 0 end-1]]
}
proc K {a b} {set a}
#-- Testing the whole thing:
pack [frame .f] -fill x -expand 1
button .f.undo -text Undo -command {undo .c}
button .f.redo -text Redo -command {redo .c}
eval pack [winfo children .f] -side left
set Undo {}; set Redo {}
pack [canvas .c -background white] -fill both -expand 1
bind .c <1> {newLine .c %x %y}
bind .c <B1-Motion> {drawLine .c %x %y}