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Ruby Programming
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Scrolling

Unless you plan on drawing very small pictures, the previous example may not be all that useful. TkCanvas, TkListbox, and TkText can be set up to use scrollbars, so you can work on a smaller subset of the ``big picture.''

Communication between a scrollbar and a widget is bidirectional. Moving the scrollbar means that the widget's view has to change; but when the widget's view is changed by some other means, the scrollbar has to change as well to reflect the new position.

Since we haven't done much with lists yet, our scrolling example will use a scrolling list of text. In the following code fragment, we'll start off by creating a plain old TkListbox. Then, we'll make a TkScrollbar. The scrollbar's callback (set with command) will call the list widget's yview method, which will change the value of the visible portion of the list in the y-direction.

After that callback is set up, we make the inverse association: when the list feels the need to scroll, we'll set the appropriate range in the scrollbar using TkScrollbar#set. We'll use this same fragment in a fully functional program in the next section.

list_w = TkListbox.new(frame, 'selectmode' => 'single')

scroll_bar = TkScrollbar.new(frame,                   '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) })
Ruby Programming
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