Wednesday, October 28, 2009

Recipe 21.15. Creating a GUI Application with Ruby/GTK










Recipe 21.15. Creating a GUI Application with Ruby/GTK



Problem


You want to write a GUI application that uses the
GTK widget library, perhaps so you can integrate it with the Gnome desktop environment.




Solution


Use the Ruby bindings to Gnome's
GTK widget library, available as a third-party download. Here's a simple Ruby/GTK application (Figure 21-5).



	#!/usr/bin/ruby -w
	#  
gtktrout.rb
	require 'gtk2'

	Gtk.init
	window = Gtk::Window.new 'Tiny Ruby/GTK Application'
	label = Gtk::Label.new 'You are a trout!'
	window.add label
	window.signal_connect('destroy') { Gtk.main_quit }
	window.show_all
	Gtk.main




Figure 21-5. You are a GTK trout






Discussion


Gnome is one of the two most popular Unix desktop suites. The Ruby-Gnome2 project provides and documents Ruby bindings to Gnome's vast array of C libraries. You can write Ruby applications that fully integrate with the Gnome desktop, but in this recipe I'm going to focus on the basics of the Gnome GUI library GTK.


Although the details are different, the sample program above is basically the same as it would be with Tk (Recipe 21.12) or the wxRuby library (Recipe 21.13). You create two widgets (a window and a label), attach the label to the window, and tell the GUI library to display the window. As with Tk and wxRuby, the application goes into a display loop, capturing user events like mouse clicks.


The sample program won't actually respond to any user events, though, so let's create a Ruby/GTK version of the stopwatch program seen in previous GUI recipes.


The core methods, the ones that actually implement the stopwatch, are basically the same as the corresponding methods in the Tk and wxRuby recipes. Since GTK doesn't have a timer widget, I've implemented a simple timer as a separate thread. The other point of interest is the HTML-like markup that GTK uses to customize the font size and weight of the stopwatch text.



	#!/usr/bin/ruby -w
	# gtk_stopwatch.rb
	require 'gtk2'

	class Stopwatch

	  LABEL_MARKUP = '<span font_desc="16" weight="bold">%s</span>'

	  def start
	    @accumulated ||= 0
	    @elapsed = 0
	    @start = Time.now

	    @mybutton.label = 'Stop'
	    set_button_handler('clicked') { stop }
	    @timer_stopped = false
	    @timer = Thread.new do
	      until @timer_stopped do
	        sleep(0.1)
	        tick unless @timer_stopped
	      end
	    end
	  end

	  def stop
	    @mybutton.label = 'Start'
	    set_button_handler('clicked') { start }
	    @timer_stopped = true
	    @accumulated += @elapsed
	  end

	  def reset
	    stop
	    @accumulated, @elapsed = 0, 0
	    @mylabel.set_markup(LABEL_MARKUP % '00:00:00.0')
	  end

	  def tick
	    @elapsed = Time.now - @start
	    time = @accumulated + @elapsed
	    h = sprintf('%02i', (time.to_i / 3600))
	    m = sprintf('%02i', ((time.to_i % 3600) / 60))
	    s = sprintf('%02i', (time.to_i % 60))
	    mt = sprintf('%1i', ((time - time.to_i)*10).to_i)
	    @mylabel.set_markup(LABEL_MARKUP % "#{h}:#{m}:#{s}:#{mt}")
	  end



Now begins the GUI setup. Ruby uses VBox and HBox objects to pack widgets into the display area. The stopwatch application will give its main window a single VBox containing three widgets arranged from top to bottom: a menu bar, a label (displaying the stopwatch time), and a button (to start and stop the stopwatch):



	  def initialize
	     
Gtk.init
	    root = Gtk::Window.new('GTK Stopwatch')

	    accel_group = Gtk::AccelGroup.new
	    root.add_accel_group(accel_group)
	    root.set_border_width 0

	    box = Gtk::VBox.new(false, 0)
	    root.add(box)



The program's menu bar consists of many nested MenuBar, Menu, and MenuItem objects. Rather than create these objects ourselves, we define the parameters of our menu bar in a nested array, and pass it into an ItemFactory object:



	    menu_factory =  
Gtk::ItemFactory.new(Gtk::ItemFactory::TYPE_MENU_BAR,
	                                        '<main>', nil)
	    menu_spec = [
	                 ['/_Program'],
	                  ['/Program/_Start', '<Item>', nil, nil, lambda { start } ],
	                  ['/Program/S_top', '<Item>', nil, nil, lambda { stop } ],
	                  ['/Program/_Exit', '<Item>', nil, nil,
	                   lambda { Gtk.main_quit } ],
	                 ['/_Reset'],
	                  ['/Reset/_Reset Stopwatch', '<Item>', nil, nil,
	                   lambda { reset } ]
	                ]
	    menu_factory.create_items(menu_spec)
	    menu_root = menu_factory.get_widget('<main>')
	    box.pack_start(menu_root)



The label and the button are pretty simple: just define them and pack them into the VBox:



	    @mylabel = Gtk::Label.new
	    @mylabel.set_markup(LABEL_MARKUP % '00:00:00.0')
	    box.pack_start(@mylabel)

	    @mybutton = Gtk::Button.new('Start')
	    set_button_handler('clicked') { start }
	    box.pack_start(@mybutton)

	    root.signal_connect('destroy') { Gtk.main_quit }
	    root.show_all

	    Gtk.main
	  end



I've been calling a nonexistent method Stopwatch#set_button_handler whenever I want to modify the code that runs when the user clicks the button. I close out the Stopwatch class by defining that method (Figure 21-6):



	  def set_button_handler(event, &block)
	    @mybutton.signal_handler_disconnect(@mybutton_handler) if @mybutton_handler
	    @mybutton_handler = @mybutton.signal_connect(event, &block)
	  end
	end

	Stopwatch.new



In the Tk recipe, I simply called a button's command method whenever I needed to change the code block that runs when the user clicks the button. So why this set_ button_handler code? Why not just call signal_connect whenever I need to change what the button does here? I can't do that because GTK lets you associate multiple code blocks with a single event. This doesn't usually come up, but it's a problem here because I'm changing the function of a button.



Figure 21-6. The GTK stopwatch






If the button is set up to call start when you click it, and you call signal_ connect('clicked',proc { stop }), then clicking on the button will call start and then call stop. You've added a second code block to the "clicked" event, when what you want is to replace the old "clicked" code with the new code. To avoid this problem, set_button_handler removes any old handler from the button before installing the new handler. The set_button_handler method tracks the internal ID of the newly installed handler, so that it can be removed if the user clicks the button yet again.




See Also


  • You can download the Ruby bindings to GTK from the project homepage (http://ruby-gnome2.sourceforge.jp/); the GTK homepage itself is at http://www.gtk.org; Debian GNU/Linux users can install the libgtk2-ruby package

  • The Ruby GTK bindings are documented on the Ruby-GNOME2 Wiki at http://ruby-gnome2.sourceforge.jp/hiki.cgi?Ruby%2FGTK; there's also a tutorial at http://ruby-gnome2.sourceforge.jp/hiki.cgi?tut-gtk

  • Don't confuse the Ruby-GNOME2 project with its predecessor, Ruby-GNOME; the documentation for the older project is still online and will mislead you if you go to the wrong web site













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