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in reply to I don't understand a piece of Perl/Tk code

What does Ev mean in this context ?..........Ev is never defined nowhere in the source.....

Those are X window events. They are mentioned in perldoc Tk::bind under the Events section, but they are built so deep into the Tk windowing system, that it is probably in the Tcl c code, from which Perl/Tk is derived. If you look at perldoc Tk::Widget, you will see that every Tk widget has a $widget->pointerx (y) attribute. That is the x and y mouse pointer location relative to the widget itself...i.e. 0,0 is the widget's upper left corner. Tk bind has built in events, for the mouse and keypress activity, called Ev('x'), Ev('y'), Ev('k') and you can bind to those events. Additionally, most widgets will have a method to determine which widget element is located at any of it's pointerx and pointery locations. Here is a simple example: 

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $dx;
my $dy;

my $mw = MainWindow->new;
$mw->geometry("700x600");

my $canvas = $mw->Canvas(-width => 700, -height => 565,
                -bg => 'black',
        -borderwidth => 3, 
        -relief => 'sunken',
           )->pack;

my $closebutton = $mw->Button(-text => 'Exit', -command => sub{Tk::exi
+t(0)})
               ->pack; 

my $dragster = $canvas->createRectangle(0, 20, 50, 75, 
                             -fill => 'red',
                 -tags => ['move'],
                 
                 );

$canvas->bind('move', '<1>', sub {&mobileStart();});
$canvas->bind('move', '<B1-Motion>', sub {&mobileMove();});
$canvas->bind('move', '<ButtonRelease>', sub {&mobileStop();});

MainLoop;

sub mobileStart {
      my $ev = $canvas->XEvent;
      ($dx, $dy) = (0 - $ev->x, 0 - $ev->y);
      $canvas->raise('current');
      print "START MOVE->  $dx  $dy\n";
}


sub mobileMove {
      my $ev = $canvas->XEvent;
      $canvas->move('current', $ev->x + $dx, $ev->y +$dy);
      ($dx, $dy) = (0 - $ev->x, 0 - $ev->y);
      print "MOVING->  $dx  $dy\n";
      my $color = $canvas->itemcget($dragster,'-fill');
      print "color -> $color\n";
      
      #or use bbox here for odd shapes.. bounding box
      my @coords = $canvas->coords($dragster);
      print "coords-> @coords\n";
     
}


sub mobileStop{&mobileMove;}

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Re^2: I don't understand a piece of Perl/Tk code
by spx2 (Deacon) on Aug 17, 2007 at 08:36 UTC

    Hi! Very nice piece of code. Altough i still have questions regarding it. In particular,you apply method move to $canvas altough you want to move $dragster. There is a hint about this in the fact that move() method has a parameter called "current",is this indicating that $dragster is the current object that has focus or... ?

    Now i have a second question. Is there first parameter in bind user-defined ,or is it from some list of possible values ? is the first argument to bind('move') in any way linked to the method move wich is applied to canvas ?

[reply]
      You need to understand how the canvas tag system works. The dragster IS NOT an object, so it cannot have methods. I could make it into object, with some package code, but in general, "items" on a canvas, are elements of the canvas widget.

      Your second question is about canvas tags also. There is a difference between the canvas's bind and the normal Tk::bind. The canvas's bind will let you bind to a tag. If you look at each canvas item's creation, you will see it is given a tag(s). This is the basis of the canvas's power, because you can do all sorts of trickery with addtag and deltag to change an item's tags dynamically. So you can do things like setup a callback when a mouse enters any item with a certain tag. Like this: 

      #!/usr/bin/perl
use Tk;

my $mw = MainWindow->new();
$mw->geometry("600x400+100+100");
$canvas = $mw->Canvas(width => 600, height => 400)->pack();
$canvas->Tk::bind("<Button-1>", [ \&print_xy, Ev('x'), Ev('y') ]);

# This draws half of an oval
$oval = $canvas->createOval(100,100,200,250,-fill => 'white',-tags => 
+"blue");

$canvas->Tk::bind("<Button-3>", sub {$canvas->itemconfigure($oval,-fil
+l => "red")});

$canvas->bind('blue', '<Enter>', 
            sub { $canvas->itemconfigure("blue", -fill => "blue"); } )
+;
    # sub {exit});
# When the mouse is not over, color it black.
$canvas->bind("blue", "<Leave>",
         sub { $canvas->itemconfigure("blue", -fill => "black"); });

#$tw->tagBind('tag','<Enter>',sub{$overtag=1});

MainLoop;

sub print_xy {
  print "@_\n";
  my ($canv, $x, $y) = @_;
  print "(x,y) = ", $canv->canvasx($x), ", ", $canv->canvasy($y), "\n"
+;
}

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