Errata

I copied the source code for the drawing program example on page 222 and tried to run it but it doesn't let me draw anything.

http://docstore.mik.ua/orelly/perl3/tk/ch09_21.htm
code:

use Tk;

$mw = MainWindow->new;
$mw->title("Quick Draw");

$f = $mw->Frame(-relief => 'groove',
-bd => 2,
-label => "Draw:")->pack(-side => 'left', -fill => 'y');
$draw_item = "rectangle";
$f->Radiobutton(-variable => \$draw_item,
-text => "Rectangle",
-value => "rectangle",
-command => \&bind_start)->pack(-anchor => 'w');
$f->Radiobutton(-variable => \$draw_item,
-text => "Oval",
-value => "oval",
-command => \&bind_start)->pack(-anchor => 'w');
$f->Radiobutton(-variable => \$draw_item,
-text => "Line",
-value => "line",
-command => \&bind_start)->pack(-anchor => 'w');
$f->Label(-text => "Line Width:")->pack(-anchor => 'w');
$thickness = 1;
$f->Entry(-textvariable => \$thickness)->pack(-anchor => 'w');

$c = $mw->Scrolled("Canvas", -cursor => "crosshair")->pack(
-side => "left", -fill => 'both', -expand => 1);
$canvas = $c->Subwidget("canvas");

&bind_start( );

MainLoop;

sub bind_start {
# If there is a "Motion" binding, we need to allow the user
# to finish drawing the item before rebinding Button-1
# this fcn gets called when the finish drawing the item again
@bindings = $canvas->bind("<Motion>");
return if ($#bindings >= 0);

if ($draw_item eq "rectangle"||$draw_item eq "oval"||$draw_item eq "line") {
$canvas->bind("<Button-1>", [\&start_drawing, Ev('x'), Ev('y')]);
}
}

sub start_drawing {
my ($canv, $x, $y) = @_;
$x = $canv->canvasx($x);
$y = $canv->canvasy($y);

# Do a little error checking
$thickness = 1 if ($thickness !~ /[0-9]+/);

if ($draw_item eq "rectangle") {
$canvas->createRectangle($x, $y, $x, $y,
-width => $thickness, -tags => "drawmenow");
} elsif ($draw_item eq "oval") {
$canvas->createOval($x, $y, $x, $y,
-width => $thickness, -tags => "drawmenow");
} elsif ($draw_item eq "line") {
$canvas->createLine($x, $y, $x, $y,
-width => $thickness, -tags => "drawmenow");
}

$startx = $x; $starty = $y;
# Map the Button-1 binding to &end_drawing instead of start drawing
$canvas->bind("<Motion>", [\&size_item, Ev('x'), Ev('y')]);
$canvas->bind("<Button-1>", [\&end_drawing, Ev('x'), Ev('y')]);
}

sub size_item {
my ($canv, $x, $y) = @_;
$x = $canv->canvasx($x);
$y = $canv->canvasy($y);

$canvas->coords("drawmenow", $startx, $starty, $x, $y);
}

sub end_drawing {
my ($canv, $x, $y) = @_;
$x = $canv->canvasx($x);
$y = $canv->canvasy($y);

# finalize the size of the item, and remove the tag from the item
$canvas->coords("drawmenow", $startx, $starty, $x, $y);
$canvas->dtag("drawmenow");

# remove motion binding.
$canvas->CanvasBind("<Motion>", "");
&bind_start( );
}