Development/Tutorials/Qt4 Ruby Tutorial/Chapter 04

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Let There Be Widgets

Tutorial Series	Qt4 Ruby Tutorial
Previous	Tutorial 3 - Family Values
What's Next	Tutorial 5 - Building Blocks
Further Reading	n/a

Let There Be Widgets

Files:

t4.rb

Overview

This example shows how to create your own widget, and describes how to control the minimum and maximum sizes of a widget.

require 'Qt4'

class MyWidget < Qt::Widget
  def initialize(parent = nil)
    super
    setFixedSize(200, 120)

    quit = Qt::PushButton.new(tr('Quit'), self)
    quit.setGeometry(62, 40, 75, 30)
    quit.setFont(Qt::Font.new('Times', 18, Qt::Font::Bold))

    connect(quit, SIGNAL('clicked()'), $qApp, SLOT('quit()'))
  end
end

app = Qt::Application.new(ARGV)

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

app.exec()

Line by Line Walkthrough

class MyWidget < Qt::Widget

Here we create a new class. Because this class inherits from Qt::Widget, the new class is a widget and may be a top-level window or a child widget (like the Qt::PushButton in the previous chapter).

def initialize(parent = nil)
  super

This class has only one member, a constructor (in addition to the members it inherits from Qt::Widget). The constructor is a standard Qt widget constructor; you should always include a similar constructor when you create widgets.

The argument is its parent widget. To create a top-level window you specify nil as the parent. As you can see, this widget defaults to be a top-level window.

Like most widgets, it just passes on the parent parameter to the Qt::Widget constructor. (If you're new to Ruby, calling super without any parameters will automatically pass on any parameters a method received to the superclass's method of the same name. Update: Apparently, it's also important to leave off the parentheses — calling super() will call the superclass's method of the same name with an empty parameter list.)

    setFixedSize(200, 120)

Because this widget doesn't know how to handle resizing, we fix its size. In the next chapter, we will show how a widget can respond to resize event from the user.

quit = Qt::PushButton.new(tr('Quit'), self)
quit.setGeometry(62, 40, 75, 30)
quit.setFont(Qt::Font.new('Times', 18, Qt::Font::Bold))

Here we create and set up a child widget of this widget (the new widget's parent is self, i.e. the MyWidget instance).

The QObject::tr() function call around the string literal 'Quit' marks the string for translation, making it possible to change it at run-time based on the contents of a translation file. It is a good habit to use QObject::tr() around all user-visible strings, in case you decide later to translate your application to other languages.

The Qt::Widget::setGeometry() call sets both the widget's screen position and the size. It is equivalent to calling Qt::Widget::move() followed by Qt::Widget::resize().

connect(quit, SIGNAL('clicked()'), $qApp, SLOT('quit()'))

The $qApp pointer is a global variable created when you require 'Qt'. It points to the application's unique Qt::Application instance.

app = Qt::Application.new(ARGV)

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

app.exec()

Here we instantiate our new child, set it to be the main widget, and execute the application.

Running the Application

This program is very similar in behavior to the previous one. The difference lies in the way we have implemented it. It does behave slightly differently, however. Just try to resize it to see.

Exercises

Try to create another MyWidget object. What happens?

Try to add more buttons or put in widgets other than Qt::PushButton.