Development/Tutorials/Qt4 Ruby Tutorial/Chapter 3

Development/Tutorials/Qt4 Ruby Tutorial/Chapter 3

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Family Values

Tutorial Series	Qt4 Ruby Tutorial
Previous	Tutorial 2 -Calling it Quits
What's Next	Tutorial 4 - Let There Be Widgets
Further Reading	n/a

Family Values

Files:

• t3.rb

Overview

This example shows how to create parent and child widgets.

We'll keep it simple and use just a single parent and a lone child.

require 'Qt4'

app = Qt::Application.new(ARGV)

window = Qt::Widget.new()
window.resize(200, 120)

quit = Qt::PushButton.new('Quit', window)
quit.font = Qt::Font.new('Times', 18, Qt::Font::Bold)
quit.setGeometry(10, 40, 180, 40)
Qt::Object.connect(quit, SIGNAL('clicked()'), app, SLOT('quit()'))

window.show()
app.exec()

Line by Line Walkthrough

<syntaxhighlight lang="ruby"> window = Qt::Widget.new()

Here we simply create a plain widget object. The Qt::Widget class is the base class of all user interface objects. The widget is the atom of the user interface: It receives mouse, keyboard and other events from the window system, and paints a representation of itself on the screen. A widget is clipped by its parent and by the widgets in front of it.

A widget that isn't embedded in a parent widget, like this particular widget, is called a window. Usually, windows have their own window frame and taskbar entry, provided by the window system. A widget without a parent widget is always an independent window. Its initial position on the screen is controlled by the window system.

<syntaxhighlight lang="ruby"> window.resize(200, 120)

We set the window's width to 200 pixels and its height to 120 pixels.

<syntaxhighlight lang="ruby"> quit = Qt::PushButton.new('Quit', window)

A child is born. This Qt::PushButton is created with a parent widget (window). A child widget is always displayed within its parent's area. When displayed, it is clipped by its parent's bounds. By default, it is rooted at the top-left corner of its parent, at position (0, 0).

<syntaxhighlight lang="ruby"> quit.setGeometry(10, 40, 180, 40)

The Qt::Widget::setGeometry() function takes four arguments: The first two arguments are the x and y coordinates of the button's top-left corner. The coordinates are relative to the parent widget. The last two arguments are the button's width and height. The result is a button that extends from position (10, 40) to position (190, 80).

<syntaxhighlight lang="ruby"> window.show()

When a parent widget is shown, it will call show for all its children (except those that were explicitly hidden using Qt::Widget::hide()).

Running the Application

The button no longer fills the entire window. Instead, it stays at position (10, 40) within the window and with a size of (180, 40), because of the Qt::Widget::setGeometry() call.

Exercises

Try resizing the window. How does the button change? What happens to the button's height if you run the program with a bigger font? What happens if you try to make the window really small?