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{{TutorialBrowser|
series=[[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial|Qt4 Ruby Tutorial]]|
name=Let There Be Widgets|
pre=[[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial/Chapter_3|Tutorial 3 - Family Values]]|
next=[[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial/Chapter_05|Tutorial 5 - Building Blocks]]
}}

== Let There Be Widgets ==
[[Image:Qt4_Ruby_Tutorial_Screenshot_4.png|center]]

Files:
* [http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t4/t4.rb 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.

<syntaxhighlight lang="ruby">
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()
</syntaxhighlight>

=== Line by Line Walkthrough ===
<syntaxhighlight lang="ruby">
class MyWidget < Qt::Widget
</syntaxhighlight>

Here we create a new class. Because this class inherits from [http://doc.qt.nokia.com/latest/qwidget.html Qt::Widget], the new class is a widget and may be a top-level window or a child widget (like the [http://doc.qt.nokia.com/latest/qpushbutton.html Qt::PushButton] in the previous chapter).

<syntaxhighlight lang="ruby">
def initialize(parent = nil)
  super
</syntaxhighlight>

This class has only one member, a constructor (in addition to the members it inherits from [http://doc.qt.nokia.com/latest/qwidget.html 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 '''<tt>nil</tt>''' as the parent. As you can see, this widget defaults to be a top-level window.

Like most widgets, it just passes on the '''<tt>parent</tt>''' parameter to the [http://doc.qt.nokia.com/latest/qwidget.html Qt::Widget] constructor. (If you're new to Ruby, calling '''<tt>super</tt>''' 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 '''<tt>super()</tt>''' will call the superclass's method of the same name with an empty parameter list.)

<syntaxhighlight lang="ruby">
    setFixedSize(200, 120)
</syntaxhighlight>

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.

<syntaxhighlight lang="ruby">
quit = Qt::PushButton.new(tr('Quit'), self)
quit.setGeometry(62, 40, 75, 30)
quit.setFont(Qt::Font.new('Times', 18, Qt::Font::Bold))
</syntaxhighlight>

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

The [http://doc.qt.nokia.com/latest/qobject.html#tr 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 [http://doc.qt.nokia.com/latest/qobject.html#tr QObject::tr()] around all user-visible strings, in case you decide later to translate your application to other languages.

The [http://doc.qt.nokia.com/latest/qwidget.html#geometry-prop Qt::Widget::setGeometry()] call sets both the widget's screen position and the size. It is equivalent to calling [http://doc.qt.nokia.com/latest/qwidget.html#pos-prop Qt::Widget::move()] followed by [http://doc.qt.nokia.com/latest/qwidget.html#size-prop Qt::Widget::resize()].

<syntaxhighlight lang="ruby">
connect(quit, SIGNAL('clicked()'), $qApp, SLOT('quit()'))
</syntaxhighlight>

The '''<tt>$qApp</tt>''' pointer is a global variable created when you '''<tt>require 'Qt'</tt>'''. It points to the application's unique [http://doc.qt.nokia.com/latest/qapplication.html Qt::Application] instance.

<syntaxhighlight lang="ruby">
app = Qt::Application.new(ARGV)

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

app.exec()
</syntaxhighlight>

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 '''<tt>MyWidget</tt>''' object. What happens?

Try to add more buttons or put in widgets other than [http://doc.qt.nokia.com/latest/qpushbutton.html Qt::PushButton].

[[Category:Ruby]]