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Ruby Programming
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Just One More Thing

We could go on about Tk for another few hundred pages, but that's another book. The following program is our final Tk example---a simple GIF image viewer. You can select a GIF filename from the scrolling list, and a thumb nail version of the image will be displayed. There are just a few more things we'd like to point out.

Have you ever seen an application that creates a ``busy cursor'' and then forgets to reset it to normal? There's a neat trick in Ruby that will prevent this from happening. Remember how uses a block to ensure that the file is closed after it is used? We can do a similar thing with the method busy, as shown in the next example.

This program also demonstrates some simple TkListbox manipulations---adding elements to the list, setting up a callback on a mouse button release,[You probably want the button release, not the press, as the widget gets selected on the button press.] and retrieving the current selection.

So far, we've used TkPhotoImage to only display icons directly, but you can also zoom, subsample, and show portions of images as well. Here we use the subsample feature to scale down the image for viewing.

require 'tk'

def busy   begin     $root.cursor "watch" # Set a watch cursor     $root.update # Make sure it updates  the screen     yield # Call the associated block   ensure     $root.cursor "" # Back to original     $root.update   end end

$root = {title 'Scroll List'} frame =$root)

list_w =, 'selectmode' => 'single')

scroll_bar =,                   'command' => proc { |*args| list_w.yview *args })

scroll_bar.pack('side' => 'left', 'fill' => 'y')

list_w.yscrollcommand(proc { |first,last|                              scroll_bar.set(first,last) }) list_w.pack('side'=>'left')

image_w =, 'image' => image_w).pack('side'=>'left') frame.pack

list_contents = Dir["screenshots/gifs/*.gif"] list_contents.each {|x|   list_w.insert('end',x) # Insert each file name into the list } list_w.bind("ButtonRelease-1") {   index = list_w.curselection[0]   busy {     tmp_img ='file'=> list_contents[index])     scale   = tmp_img.height / 100     scale   = 1 if scale < 1     image_w.copy(tmp_img, 'subsample' => [scale,scale])     tmp_img = nil # Be sure to remove it, the     GC.start      # image may have been large   } }


Finally, a word about garbage collection---we happened to have a few very large GIF files lying about[They were technical documents! Really!] while testing this code. We didn't want to carry these huge images around in memory any longer then necessary, so we set the image reference to nil and call the garbage collector immediately to remove the trash.
Ruby Programming
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  Published under the terms of the Open Publication License Design by Interspire