clock_gui.rb

require "observer"
require "tk"

class WatchModel
    include Observable
    def initialize
        @running = false
        @time = 0
        @last = 0.0
        Thread.start do
            loop do
                sleep 0.01
                if @running
                    now = Time.now.to_f
                    @time += now - @last
                    @last = now
                    changed
                    notify_observers(@time)
                end
            end
        end
    end
    
    def start_stop
        @last = Time.now.to_f
        @running = ! @running
    end

    def time
        @time
    end
end

class WatchWindow
    def initialize
        model = WatchModel.new
        model.add_observer(self)

        @label = TkLabel.new(nil).pack('fill'=>'x')
        self.update(0)
        btn = TkButton.new
        btn.text('start/stop')
        btn.command(proc{model.start_stop})
        btn.pack('fill'=>'x')
        btn = TkButton.new
        btn.text('quit')
        btn.command(proc{exit})
        btn.pack('fill'=>'x')
        Tk.mainloop
    end

    def update(time)
        @label.text format("%02d:%02d", time.to_i, (time - time.to_i)*100)
    end
end

WatchWindow.new