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 (e.g. Qt's widgets), 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 (e.g. Qt layouts), abstract data model types and structures (e.g. Qt models), 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 web browser backend. 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.

Dynamic programming 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 (e.g. wxWidgets), 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 application development.

The only downside to coding with the dynamic languages is that application performance will usually suffer [1], but even for large applications this may be acceptable if the application does not require a particularly high frame rate under heavy load. Additionally, logistically speaking, in some cases there are problems with the level of support and maintenance in a particular binding project, including delays between the release of a new native toolkit implementation, and accompanying bindings. It is always useful to look at when the last version control check-in 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 required to read C++ API documentation, and potentially source code for the binding 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 [2]. Unfortunately none of these features are very advanced, and they're generally quirky to use once you've worked with either a wxWidgets-based project, or a Qt project. 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.

Example

Hello World

This is a simple hello world application. It demonstrates how to create a root-level frame, and add a single label to it with the given text.

require 'tk'

root = TkRoot.new { title "Hello, World!" }
TkLabel.new(root) do
   text 'Hello, World!'
   pack { padx 15 ; pady 15; side 'left' }
end
Tk.mainloop

When run, this program will display the following:

Hello World button

This short example replaces the label from above with a TkButton, and connects the button to the "printHello" function via the "command" argument. It looks similar to the example above, but with a button instead of a label, and it will print "Hello, world!" to the terminal when the button is pressed.

require 'tk'

def printHello
    puts "Hello, world!"
end

root = TkRoot.new { title "Hello, World!" }
helpButton = TkButton.new(root) do
   text "Hello, world!"
   command proc { printHello }
   pack { padx 15 ; pady 15; side 'left' }   
end
Tk.mainloop

Output from clicking the "Hello, World!" button:

Hello, world!

Notice that the "command" argument to the TkButton allows a generic codeblock, and any callable may be returned from this block. That is useful to do something like in the following example.

Pros and Cons

Pros

• Supported across many operating systems.
• Tk has an implementation in a great many languages

Cons

• Very old
• Does not use native OS widgets
• Generally considered quirky and hard to use.

QtRuby

Qt is a 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 (e.g. clicked(), or selectionChanged()), but can be emitted from anywhere in the code. This allows a very powerful method for many widgets to "observe" the behavior of any other widget, commonly referred to simply as the observer pattern.

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

Examples

Hello World!

This is a very easy way to get started, and to make sure you've installed QtRuby correctly.[3] 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()

When run, this code shows this:

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:

Output from clicking each of the three buttons:

Button 1 pressed!"
Button 2 pressed!"
Button 3 pressed!"

As you may expect, it prints out "Button X pressed!" whenever the button X is pressed. Unfortunately, QtRuby forces you declare your slots upfront as a list of strings, instead of allowing any method to be used as a slot. This prevents you from executing code when creating the slot, which means that closures are very difficult, if not impossible.

QtRuby slider

This is a more complex QtRuby application.[4]

require 'Qt4'

class MyWidget < Qt::Widget
  def initialize()
    super()
    quit = Qt::PushButton.new('Quit')
    quit.setFont(Qt::Font.new('Times', 18, Qt::Font::Bold))
    
    lcd = Qt::LCDNumber.new(2)

    slider = Qt::Slider.new(Qt::Horizontal)
    slider.setRange(0, 99)
    slider.setValue(0)

    connect(quit, SIGNAL('clicked()'), $qApp, SLOT('quit()'))
    connect(slider, SIGNAL('valueChanged(int)'), lcd, SLOT('display(int)'))

    layout = Qt::VBoxLayout.new()
    layout.addWidget(quit)
    layout.addWidget(lcd)
    layout.addWidget(slider)
    setLayout(layout)
  end
end

app = Qt::Application.new(ARGV)

widget = MyWidget.new()

widget.show()
app.exec()

When executed, this code looks like this:

This example illustrates how to create an LCDNumber widget, and a slider widget, add them to a vertical layout, then connect the slider "valueChanged" signal to the lcd "display" slot. When the slider is moved, it fires a "valueChanged(int)" signal, which includes the integer that the value was changed to. The "connect" method is used to link this signal, with the LCDNumber widget's "display(int)" slot, which changes the display to given integer. The layout is a very interesting widget container. The VBoxLayout will arrange all widgets vertically. Inside the layout, each widget will have sizeHints and a sizePolicy, which the layout will query in order to size and resize the widget properly.

Pros and Cons

Pros

• Excellent C++ documentation [5]
• Easy to use, relatively intuitive
• Extensive widget collection and default signals/slots
• Very good native OS look and feel reproduction
• Qt Designer for rapid layout creation.[6]
• Latest release date as of this writing: July 29, 2010.[7]

Cons

• Requires you to declare slots upfront, instead of being able to use any method, or generic callable as a slot.

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.

Examples

Hello World

This is a simple "hello world" application in wxRuby. It demonstrates a way to create an "App" instance, and display a frame.

require "wx"
include Wx
# a new class which derives from the Wx::App class
class HelloWorld < App
  # we're defining what the application is going to do when it starts
  def on_init  
    # it's going to make a frame entitled "Hello World"
    helloframe = Frame.new(nil, -1, "Hello World")
    # it's going to put the text "Hello World" in that frame
    StaticText.new(helloframe,-1,"Hello World")
    # and then it's going to make the window appear
    helloframe.show()
  end
end
# and this line makes it actually do it!
HelloWorld.new.main_loop 

wxRuby requires you to inherit and instantiate a WX::App, and create widgetry from there. A "Frame" is a generic top-level container for more widgets, and here it is being made to be the parent of the StaticText widget, which is just a label, or sorts.

More Complex

require 'wx'
include Wx

class MyFrame < Frame
  def initialize()
        super(nil, -1, 'My Frame Title')
        # First create the controls
        @my_panel = Panel.new(self)
        @my_label = StaticText.new(@my_panel, -1, 'My Label Text', DEFAULT_POSITION, DEFAULT_SIZE, ALIGN_CENTER)
        @my_textbox = TextCtrl.new(@my_panel, -1, 'Default Textbox Value')
        @my_combo = ComboBox.new(@my_panel, -1, 'Default Combo Text', DEFAULT_POSITION, DEFAULT_SIZE, ['Item 1', 'Item 2', 'Item 3'])
        @my_button = Button.new(@my_panel, -1, 'My Button Text')
        # Bind controls to functions
        evt_button(@my_button.get_id()) { |event| my_button_click(event)}
        # Now do the layout
        @my_panel_sizer = BoxSizer.new(VERTICAL)
        @my_panel.set_sizer(@my_panel_sizer)
        @my_panel_sizer.add(@my_label, 0, GROW|ALL, 2)
        @my_panel_sizer.add(@my_textbox, 0, GROW|ALL, 2)
        @my_panel_sizer.add(@my_combo, 0, GROW|ALL, 2)
        @my_panel_sizer.add(@my_button, 0, GROW|ALL, 2)        
        show()
    end

    def my_button_click(event)
        puts "Button clicked!"
    end

end

class MyApp < App
  def on_init
    MyFrame.new
  end
end

MyApp.new.main_loop()

This example looks like this:

You can see one of the easier ways to connect the button to a randomly specified function, by using a code block, in the "evt_button" method call. This feature greatly facilitates rapid GUI development, as you do not have to keep up with event numbers, or slots.

Pros and cons

Pros

• Very good C++ documentation
• Well-maintained.
• Longer time under the LGPL means it's generally used more often in the industry for rapid prototyping [8]
• Works on Windows, Linux, Mac, and BSD.[9]
• Generic code blocks may be connected to default widget event emitters.
• wxGlade for rapid layout creation

Cons

• Proper event management and usage can get tricky with large applications, though see above about connections with generic code blocks.
• Not as many native widgets as Qt. [10]
• Generally not as intuitive as Qt. [11]
• wxGlade is relatively outdated.[12]

Other GUI toolkits

• FXRuby
• Ruby-GNOME2
• Shoe

Dynamic Language Techniques with GUI toolkits

Buttons with closures

Sometimes it is desirable to connect multiple buttons to the same exact method, but to provide that method with slightly different arguments. However, in many toolkits the button's "click" event does not provide any useful information about which button was actually clicked (i.e., it does not call the connected function with any parameters), so you may end up tediously creating one function for each button, and having that function call the much more desirable function with the parameter that you really wanted. Here is a quick example of three buttons hooked to three functions:

Naive

require 'tk'

def buttonPressed(number)
    puts "Button #{number} pressed!"
end

def buttonPress1()
    buttonPressed(1)
end

def buttonPress2()
    buttonPressed(2)
end

def buttonPress3()
    buttonPressed(3)
end

root = TkRoot.new { title "Hello, World!" }
button1 = TkButton.new(root) do
   text "Button 1"
   command proc { buttonPress1 }
   pack { padx 15 ; pady 15; side 'left' }   
end
button2 = TkButton.new(root) do
   text "Button 2"
   command proc { buttonPress2 }
   pack { padx 15 ; pady 15; side 'left' }   
end
button3 = TkButton.new(root) do
   text "Button 3"
   command proc { buttonPress3 }
   pack { padx 15 ; pady 15; side 'left' }   
end
Tk.mainloop

This application will look like this, and will print the expected "Button X pressed!" message from above:

Output from clicking each of the three buttons:

Button 1 pressed!
Button 2 pressed!
Button 3 pressed!

The thing to note here is that "buttonPress*" is repeated 10 times in various contexts. There is the "buttonPressed(number)" function that we really wanted, three "buttonPressX" functions that all call "buttonPressed(X)," and then each of those functions is passed in as the command to its corresponding button. That's a lot of seemingly repeated code. Doesn't that go against one of our main coding principles, DRY? Can't we do better than that?

Better

It would be far better to make use of Ruby's ability to create closures, if possible. The button will only ever call a method that has zero required parameters, but Ruby can save a method context with parameters in it to be called later, like this:

require 'tk'

def buttonPressed(number)
    lambda {puts "Button #{number} pressed!"}
end

root = TkRoot.new { title "Hello, World!" }
button1 = TkButton.new(root) do
   text "Button 1"
   command buttonPressed(1)
   pack { padx 15 ; pady 15; side 'left' }   
end
button2 = TkButton.new(root) do
   text "Button 2"
   command buttonPressed(2)
   pack { padx 15 ; pady 15; side 'left' }   
end
button3 = TkButton.new(root) do
   text "Button 3"
   command buttonPressed(3)
   pack { padx 15 ; pady 15; side 'left' }   
end
Tk.mainloop

From this, you will get the same look, and the same output as above.

Notice "buttonPressed(number)" returns an anonymous function that takes no parameters. The "command buttonPressed(X)" lines actually execute the buttonPressed method, which then stores "X" in the anonymous method's context so that it can be used later. This is a common idea when mixing GUI toolkits and dynamic languages. The toolkit may provide rigid callback facilities that, at best, can be used in the first "naive" manner above. However, since Ruby (and Python) offer the flexibility of closures, we can augment a callback with some context when we pass it to the widget that will eventually call it. You will notice below that QtRuby, unfortunately, makes this hard by requiring each "slot" to be registered as a string at startup (unless the author has missed something), but both Ruby/Tk and wxRuby offer this facility. The ability to do this can quickly become addictive when building out a quick GUI application.

It is worth noting that you are not limited to just buttons to make use of this idea. You are not limited to one extra parameter, either, and this method can be used to augment callbacks that will be called with one or more arguments by the widget to which it was passed, as well. You are also not limited to static arguments; you could pass a variable, or even another method, or a whole object instance as the argument to buttonPressed, and that will stick around to be used later when the widget executes its command. As long as the method signature matches what the widget will call, you are fine.

Best...or maybe just different

The closure method above is useful to know, however in that situation we created a function that just returns another function. What if you really wanted the more desirable function to just go ahead and do what it says it will do? Maybe you would like to use the "buttonPressed" functionality where there is no button involved? If you tried that with the function above, you would have to do something funny like this: "buttonPressed(1).call()". That is not very desirable, so a slight better way to accomplish the exact same thing as above is to do something like this:

require 'tk'

def buttonPressed(number)
    puts "Button #{number} pressed!"
end

root = TkRoot.new { title "Hello, World!" }
button1 = TkButton.new(root) do
   text "Button 1"
   command proc { buttonPressed(1) }
   pack { padx 15 ; pady 15; side 'left' }   
end
button2 = TkButton.new(root) do
   text "Button 2"
   command proc { buttonPressed(2) }
   pack { padx 15 ; pady 15; side 'left' }   
end
button3 = TkButton.new(root) do
   text "Button 3"
   command proc { buttonPressed(3) }
   pack { padx 15 ; pady 15; side 'left' }   
end
Tk.mainloop

Once again, you will get the same look, and the same output as the first "buttons" example.

Notice that now the "buttonPressed(number)" function simply prints a message, and it is actually the anonymous "proc { buttonPressed(X) }" function that is storing the parameter in its context to use later. It's probably worthwhile to point out again that Ruby/Tk is not the best GUI toolkit available for Ruby. However, this technique can generally be used with any toolkit in Ruby, or any toolkit in any dynamic language, or just any dynamic language application period.

Conclusion

Dynamic languages are very good for quick GUI prototyping, and Ruby is one shining example of that. While Qt may be a more robust GUI toolkit on its own, it seems like QtRuby has some restrictions that would make rapid prototyping slower than it really should be, since slots have to be managed in a long list, and cannot be dynamic instance methods or code blocks. wxRuby appears to be well-supported, well-documented, and easy to use for prototyping. wxRuby may even be good enough for large team usage, if not for production release software. Ruby/Tk is only suitable for individual or small team usage. Nevertheless, if Ruby is a well-understood language on a given team, any of these three Ruby toolkit implementations would be better for rapid prototyping, or for small helper applications than heavier languages like C++ and Java.

References

1. The Computer Language Benchmarks Game [13]

2. QtRuby Wikipedia [14]

External Links

Language Bindings

Ruby/Tk book chapter

Ruby/Tk

Getting Started with wxRuby

wxRuby

wxRuby Hello World

Qt Homepage

Qt Documentation

QtRuby

QtRuby Tutorials