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== Building Blocks ==
[[Image:Qt4_Ruby_Tutorial_Screenshot_5.png|center]]

Files:
* [http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t5/t5.rb t5.rb]

=== Overview ===

This example shows how to create and connect together several widgets by using signals and slots, and how to handle resizes.

<syntaxhighlight lang="ruby">
require 'Qt4'

class MyWidget < Qt::Widget
  def initialize()
    super()
    quit = Qt::PushButton.new('Quit')
    quit.setFont(Qt::Font.new('Times', 18, Qt::Font::Bold))
    
    lcd = Qt::LCDNumber.new(2)

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

    connect(quit, SIGNAL('clicked()'), $qApp, SLOT('quit()'))
    connect(slider, SIGNAL('valueChanged(int)'), lcd, SLOT('display(int)'))

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

app = Qt::Application.new(ARGV)

widget = MyWidget.new()

widget.show()
app.exec()
</syntaxhighlight>

=== Line by Line Walkthrough ===
<syntaxhighlight lang="ruby">
    lcd = Qt::LCDNumber.new(2)
</syntaxhighlight>

'''<tt>lcd</tt>''' is a [http://doc.qt.nokia.com/latest/qlcdnumber.html Qt::LCDNumber], a widget that displays numbers in an LCD-like fashion. This instance is set up to display two digits. 

<syntaxhighlight lang="ruby">
slider = Qt::Slider.new(Qt::Horizontal)
slider.setRange(0, 99)
slider.setValue(0)
</syntaxhighlight>

The user can use the  [http://doc.qt.nokia.com/latest/qslider.html Qt::Slider] widget to adjust an integer value in a range. Here we create a horizontal one, set its minimum value to 0, its maximum value to 99, and its initial value to 0. 

<syntaxhighlight lang="ruby">
    connect(slider, SIGNAL('valueChanged(int)'), lcd, SLOT('display(int)'))
</syntaxhighlight>

Here we use the [http://doc.qt.nokia.com/latest/signalsandslots.html signals and slots] mechanism to connect the slider's  [http://doc.qt.nokia.com/latest/qabstractslider.html#valueChanged QAbstractSlider::valueChanged()] signal to the LCD number's display() slot. 

Whenever the slider's value changes it broadcasts the new value by emitting the [http://doc.qt.nokia.com/latest/qabstractslider.html#valueChanged QAbstractSlider::valueChanged()] signal. Because that signal is connected to the LCD number's [http://doc.qt.nokia.com/latest/qlcdnumber.html#intValue-prop QLCDNumber::display()] slot, the slot is called when the signal is broadcast. Neither of the objects knows about the other. This is essential in component programming. 

<syntaxhighlight lang="ruby">
layout = Qt::VBoxLayout.new()
layout.addWidget(quit)
layout.addWidget(lcd)
layout.addWidget(slider)
setLayout(layout)
</syntaxhighlight>

'''<tt>MyWidget</tt>''' now uses a [http://doc.qt.nokia.com/latest/qvboxlayout.html Qt::VBoxLayout] to manage the geometry of its child widgets. For that reason, we don't need to specify the screen coordinates for each widget like we did in Chapter 4. In addition, using a layout ensures that the child widgets are resized when the window is resized. Then we add the '''<tt>quit</tt>''', '''<tt>lcd</tt>''', and '''<tt>slider</tt>''' widgets to the layout using [http://doc.qt.nokia.com/latest/qboxlayout.html#addWidget Qt::BoxLayout::addWidget()]. 

The [http://doc.qt.nokia.com/latest/qwidget.html#setLayout Qt::Widget::setLayout()] function installs the layout on '''<tt>MyWidget</tt>'''. This makes the layout a child widget of '''<tt>MyWidget</tt>''' so we don't have to worry about deleting it; it will be deleted together with '''<tt>MyWidget</tt>'''. Also, the call to [http://doc.qt.nokia.com/latest/qwidget.html#setLayout Qt::Widget::setLayout()] automatically reparents the widgets in the layout so that they are children of '''<tt>MyWidget</tt>'''. Because of this, we didn't need to specify '''<tt>self</tt>''' as the parent for the '''<tt>quit</tt>''', '''<tt>lcd</tt>''', and '''<tt>slider</tt>''' widgets. 

In Qt, widgets are either children of other widgets (e.g. '''<tt>self</tt>'''), or they have no parent. A widget can be ''added'' to a layout, in which case the layout becomes responsible for managing the geometry of that widget, but the layout can never act as a parent itself. Indeed, [http://doc.qt.nokia.com/latest/qwidget.html Qt::Widget]'s constructor takes a [http://doc.qt.nokia.com/latest/qwidget.html Qt::Widget] pointer for the parent, and [http://doc.qt.nokia.com/latest/qlayout.html Qt::Layout] doesn't inherit from [http://doc.qt.nokia.com/latest/qwidget.html Qt::Widget]. 

=== Running the Application ===

The LCD number reflects everything you do to the slider, and the widget handles resizing well. Notice that the LCD number widget changes in size when the window is resized (because it can), but the others stay about the same (because otherwise they would look strange). 

=== Exercises ===

Try changing the LCD number to add more digits or to change mode ([http://doc.qt.nokia.com/latest/qlcdnumber.html#mode-prop Qt::LCDNumber::setMode()]). You can even add four push buttons to set the number base. 

You can also change the slider's range. 

Perhaps it would have been better to use [http://doc.qt.nokia.com/latest/qspinbox.html Qt::SpinBox] than a slider? 

Try to make the application quit when the LCD number overflows.

[[Category:Ruby]]