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Qt Tutorial - Chapter 1: Hello, World!

This first program is a simple hello-world example. It contains only the bare minimum you need to get a Qt application up and running. The picture above is a snapshot of this program.

#****************************************************************
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#** Qt tutorial 1
#**
#****************************************************************


require 'Qt'

a = Qt::Application.new(ARGV)
hello = Qt::PushButton.new('Hello World!', nil)
hello.resize(100, 30)
a.setMainWidget(hello)
hello.show()
a.exec()

Line-by-line Walkthrough

    require 'Qt'

This line loads the QtRuby extension. There has to be exactly one Qt::Application object in every application that uses Qt. Qt::Application manages various application-wide resources, such as the default font and cursor.

Qt::PushButton is a classical GUI push button that the user can press and release. It manages its own look and feel, like every other Qt::Widget. A widget is a user interface object that can process user input and draw graphics. The programmer can change both the overall look and feel and many minor properties of it (such as color), as well as the widget's content. A QPushButton can show either a text or a Qt::Pixmap.

The top level in QtRuby only needs to perform some kind of initialization before passing the control to the Qt library, which then tells the program about the user's actions via events.

ARGV is the array of command-line arguments. This is a ruby feature. It is not specific to Qt; however, Qt needs to process these arguments (see following).

        a = Qt::Application.new( ARGV )

a is this program's Qt::Application. Here it is created and processes some of the command-line arguments (such as -display under X Window). Note that all command-line arguments recognized by Qt are removed from ARGV. See the Qt::Application.ARGV() documentation for details.

Note: It is essential that the Qt::Application object be created before any window-system parts of Qt are used.

        hello = Qt::PushButton.new( "Hello world!", nil )

Here, after the Qt::Application, comes the first window-system code: A push button is created.

The button is set up to display the text "Hello world!" and be a window of its own (because the constructor specifies nil for the parent window, inside which the button should be located).

        hello.resize( 100, 30 )

The button is set up to be 100 pixels wide and 30 pixels high (plus the window system frame). In this case we don't care about the button's position, and we accept the default value.

        a.setMainWidget( hello )

The push button is chosen as the main widget for the application. If the user closes a main widget, the application exits.

You don't have to have a main widget, but most programs do have one.

        hello.show()

A widget is never visible when you create it. You must call show() to make it visible.

        a.exec()

This is where main() passes control to Qt, and exec() will return when the application exits.

In exec(), Qt receives and processes user and system events and passes these on to the appropriate widgets.

You should now try to run this program.

Behavior

When you run it, you will see a small window filled with a single button, and on it you can read the famous words, Hello World!

Exercises

Try to resize the window. Press the button. If you're running X Window, try running the program with the -geometry option (for example, -geometry 100x200+10+20).

You're now ready for Chapter 2.

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