CSC/ECE 517 Fall 2010/ch1 S10 MM

Introduction

A GUI toolkit is a set of widgets and APIs that can be used and extended to construct a graphical user interface application. At a minimum a GUI toolkit will provide generic graphical widgets to display to the screen, and an eventing system to tie interactive widget pieces with other widgets, or to a data model. Most toolkits will provide widget containers for easy layouts, abstract data model types and containers, and an easy to use mechanism for implementing an observer pattern for linking widgets and data structures. Advanced toolkits, such as Qt, will even offer convenience widgets for advanced data display, concurrent programming abstractions, or even a whole browser backend (based on Webkit, in Qt's case). Ideally a toolkit would work across several different operating systems with very minimal changes to the application code or compilation process.

Most GUI toolkits are natively composed in C++. Object-oriented language features are all but required for the level of base functionality extension required in a large GUI systems. In many cases, though, bindings for the toolkit will spring up in different languages, and usually the most useful bindings will be in a dynamic scripting language such as Python, or Ruby. This page will talk about several GUI toolkit bindings in the Ruby language.

Dynamically typed languages offer many different advantages for GUI development. Since GUIs typically work with many different simple and composite data types, and the toolkit itself adds a large number of class types on top of that, defining class interfaces and usable data structures, and making them available to the right widgets at the right time within the application can be tedious in statically typed languages. Dynamic languages offer the ability to define, and redefine these things, allowing very rapid development.

The only downside to coding with the dynamic languages is that the speed of the application will suffer due to interpreter overhead, but even for large applications this may be acceptable if the application does not require a particularly high frame rate under heavy load. Logistically, however, in many cases there are problems with the level of support and maintenance in a particular binding project. It is always useful to look at when the last version control checkin was made for a project if that is available, and try to compare the latest binding release date with the latest native implementation release date. Also, it is usually harder to find documentation for the specific dynamic language that you are using, and you may be stuck reading C++ API documentation, and potentially source code for the project when you hit something you did not expect. There are also usually slight complications with deployment, and there may be licensing discrepancies between the C++ and Python/Ruby versions as well.

Overview of Ruby GUI Toolkits

Ruby/Tk

Tk is an older GUI toolkit, originally written in Tcl. Tk offers a good set of generic widgets, widget containers for relatively easy layouts, a Canvas, and event binding. Unfortunately none of these features are very advanced, and they're generally quirky to use once you've used either a wxWidgets base system or a Qt system. For example, Tk appears to only give you a single callback or variable binding for widget output and events, instead of providing a more robust mechanism for observing data changes through events or signals. You could roll your own mechanism with those features, but that could get tedious in a hurry. Also, the widgets are drawn in a distinctly Tk-style, instead of in the native style of the OS. It is highly desirable for a production GUI application to offer the standard idioms that a user expects in a native OS application, so Tk probably should never be used for a globally deployed application.

QtRuby

Qt is well-supported open source GUI toolkit that is available under the LGPL license. QtRuby attempts to offer the vast array of functionality provided by Qt in Ruby form. One useful and distinguishing characteristic of Qt is the signal/slot mechanism, which allows you to register an arbitrary number of "slots" to receive a particular "signal." Signals are generally emitted through interacting with widgets on the screen, but can be emitted from anywhere in the code. This allows a very powerful method of "observing" the behavior of a particular widget.

Qt is very robust, and works across all three major operating systems. QtRuby strives to achieve the same feat.

Unfortunately it appears that QtRuby is not being actively maintained, as the last release was in August of 2009.

Examples

Hello World!

This is a very easy way to get started, and to make sure you've installed QtRuby correctly. Simply paste the code below into a file "hello_world.rb", and execute `ruby hello_world.rb`. It's just that easy.

require 'Qt4'
app = Qt::Application.new(ARGV)

hello = Qt::PushButton.new('Hello World!')
hello.resize(100, 30)
hello.show()

app.exec()

The example is relatively straightforward. The first line includes the Qt4 module. The second line instantiates an Qt "application" instance, for which they will only ever be one instance for the duration of the Qt portion of your application. The application contains the main event loop, some event handling mechanism, and just a few global variables that can be accessed from within your application. However, the application is relatively useless without some widgetry added. This program simply instantiates a "QPushButton" instance, and sets the size, and then tells it to show itself. Here is what it looks like:

Signal/Slot setup

As mentioned, Qt's signal/slot mechanism is relatively unique in the GUI toolkit world, and is extremely easy to use. This is a quick demonstration showing how to create a new custom widget by subclassing the QWidget class, and then creating a vertical box layout, add three buttons to it, and connect those buttons to three different slots:

require 'Qt4'

class MyWidget < Qt::Widget
    slots 'button1()', 'button2()', 'button3()'
    def initialize(parent = nil)
        super
        setFixedSize(200, 120)


        button1 = Qt::PushButton.new(tr('Button1'), self)
        button2 = Qt::PushButton.new(tr('Button2'), self)
        button3 = Qt::PushButton.new(tr('Button3'), self)

        buttonLayout = Qt::VBoxLayout.new()
        buttonLayout.addWidget(button1)
        buttonLayout.addWidget(button2)
        buttonLayout.addWidget(button3)
        setLayout(buttonLayout)

        connect(button1, SIGNAL('clicked()'), self, SLOT('button1()'))
        connect(button2, SIGNAL('clicked()'), self, SLOT('button2()'))
        connect(button3, SIGNAL('clicked()'), self, SLOT('button3()'))
    end

    def button1()
        puts "Button 1 pressed!"
    end
    
    def button2()
        puts "Button 2 pressed!"
    end

    def button3()
        puts "Button 3 pressed!"
    end
end

app = Qt::Application.new(ARGV)

widget = MyWidget.new()
widget.show()

app.exec()

Here is what this application looks like when you run it:

As you may expect, it prints out "Button X pressed!" whenever the button X is pressed.

wxRuby

wxWidgets is a very popular GUI toolkit as it has been under an open license resembling the LGPL for much longer than Qt. It is entirely open source and never developed exclusively by a for-profit company. Documentation for wxWidgets is generally very good, although a lot of documentation for wxPython is generally found when hitting google.

Other GUI toolkits

• FXRuby
• Ruby-GNOME2
• Shoe

External Links

Language Bindings

Ruby/Tk book chapter

Ruby/Tk

wxRuby

Qt Homepage

Qt Documentation

QtRuby

QtRuby Tutorials