Qt^®4 Tutorial for the Ruby
Programming Language

Chapter 6: Building Blocks Galore!

Files:

t6.rb

Overview

This example shows how to encapsulate two widgets into a new component and how easy it is to use many widgets. For the first time, we use a custom widget as a child widget.

require 'Qt4'

class LCDRange < Qt::Widget
  def initialize(parent = nil)
    super()
    lcd = Qt::LCDNumber.new(2)

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

    connect(slider, SIGNAL('valueChanged(int)'), lcd, SLOT('display(int)'))

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

class MyWidget < Qt::Widget
  def initialize(parent = nil)
    super()
    quit = Qt::PushButton.new(tr('Quit'))
    quit.setFont(Qt::Font.new('Times', 18, Qt::Font::Bold))
    connect(quit, SIGNAL('clicked()'), $qApp, SLOT('quit()'))

    grid = Qt::GridLayout.new()
    
    for row in 0..2
      for column in 0..2
        grid.addWidget(LCDRange.new(), row, column)
      end
    end

    layout = Qt::VBoxLayout.new()
    layout.addWidget(quit)
    layout.addLayout(grid)
    setLayout(layout)
  end
end    

app = Qt::Application.new(ARGV)

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

app.exec()

Line by Line Walkthrough

class LCDRange < Qt::Widget

The LCDRange widget is a widget without any API. It just has a constructor. This sort of widget is not very useful, so we'll add some API later.

  def initialize(parent = nil)
    super()
    lcd = Qt::LCDNumber.new(2)
    slider = Qt::Slider.new(Qt::Horizontal)
    slider.setRange(0, 99)
    slider.setValue(0)

    connect(slider, SIGNAL('valueChanged(int)'), lcd, SLOT('display(int)'))

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

This is lifted straight from the MyWidget constructor in Chapter 5. The only differences are that the Quit button is left out and the class is renamed.

class MyWidget < Qt::Widget

MyWidget, too, contains no API except a constructor.

  def initialize(parent = nil)
    super()
    quit = Qt::PushButton.new(tr('Quit'))
    quit.setFont(Qt::Font.new('Times', 18, Qt::Font::Bold))
    connect(quit, SIGNAL('clicked()'), $qApp, SLOT('quit()'))

The push button that used to be in what is now LCDRange has been separated so that we can have one Quit button and many LCDRange objects.

    grid = Qt::GridLayout.new()

We create a Qt::Widget with a Qt::GridLayout that will contain three columns. The Qt::GridLayout automatically arranges its widgets in rows and columns; you can specify the row and column numbers when adding widgets to the layout, and Qt::GridLayout will fit them into the grid.

    for row in 0..2
      for column in 0..2
        grid.addWidget(LCDRange.new(), row, column)
      end
    end

We create nine LCDRange widgets, all of which are children of the grid object, and we arrange them in three rows and three columns.

Running the Application

This program shows how easy it is to use many widgets at a time. Each one behaves like the slider and LCD number in the previous chapter. Again, the difference lies in the implementation.

Exercises

Initialize each slider with a different/random value on startup.

Qt®4 Tutorial for the Ruby Programming Language