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{{DISPLAYTITLE:Development/Tutorials/Qt4 Ruby Tutorial/Chapter 07}}<languages />

{{TutorialBrowser|
series=[[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial|Qt4 Ruby Tutorial]]|
name=One Thing Leads to Another|
pre=[[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial/Chapter_06|Tutorial 6 - Building Blocks Galore!]]|
next=[[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial/Chapter_08|Tutorial 8 - Preparing for Battle]]
}}
== One Thing Leads to Another ==
[[Image:Qt4_Ruby_Tutorial_Screenshot_7.png|center]]

Files:
* [http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t7/lcdrange.rb lcdrange.rb]
* [http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t7/t7.rb t7.rb]

=== Overview ===

This example shows how to create custom widgets with signals and slots, and how to connect them together in more complex ways. For the first time, the source is split among several files.

=== Line by Line Walkthrough ===
'''[http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t7/lcdrange.rb lcdrange.rb]'''

This file is mainly lifted from [[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial/Chapter_06|Chapter 6]]; only the non-trivial changes are noted here.

<syntaxhighlight lang="ruby">
signals 'valueChanged(int)'
slots 'setValue(int)'
</syntaxhighlight>
<syntaxhighlight lang="ruby">
def value()
  @slider.value()
end

def setValue(value)
  @slider.setValue(value)
end
</syntaxhighlight>

These make up an interface between this widget and other components in a program. Until now, '''<tt>LCDRange</tt>''' didn't really have an API at all.

'''<tt>value()</tt>''' is a public function for accessing the value of the '''<tt>LCDRange</tt>''', '''<tt>setValue()</tt>''' is our first custom slot, and '''<tt>valueChanged()</tt>''' is our first custom signal.

Slots must be implemented in the normal way (a slot is also a Ruby member function). Signals are automatically implemented. Signals follow the access rules of protected Ruby functions (i.e., they can be emitted only by the class they are defined in or by classes inheriting from it).

The '''<tt>valueChanged()</tt>''' signal is used when the '''<tt>LCDRange's</tt>''' value has changed.

The implementation of '''<tt>value()</tt>''' is straightforward. It simply returns the slider's value.

The implementation of '''<tt>setValue()</tt>''' is equally straightforward. Note that because the slider and LCD number are connected, setting the slider's value automatically updates the LCD number as well. In addition, the slider will automatically adjust the value if it is outside its legal range.

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

The first [http://doc.qt.nokia.com/latest/qobject.html#connect QObject::connect()] call is the same that you have seen in the previous chapter. The second is new; it connects slider's [http://doc.qt.nokia.com/latest/qabstractslider.html#valueChanged QAbstractSlider::valueChanged()] signal to this object's '''<tt>valueChanged()</tt>''' signal. Yes, that's right. Signals can be connected to other signals. When the first is emitted, the second signal is also emitted.

Let's look at what happens when the user operates the slider. The slider sees that its value has changed and emits the [http://doc.qt.nokia.com/latest/qabstractslider.html#valueChanged QAbstractSlider::valueChanged()] signal. That signal is connected both to the [http://doc.qt.nokia.com/latest/qlcdnumber.html#intValue-prop QLCDNumber::display()] slot of the [http://doc.qt.nokia.com/latest/qlcdnumber.html Qt::LCDNumber] and to the '''<tt>valueChanged()</tt>''' signal of the '''<tt>LCDRange</tt>'''.

Thus, when the signal is emitted, '''<tt>LCDRange</tt>''' emits its own '''<tt>valueChanged()</tt>''' signal. In addition, [http://doc.qt.nokia.com/latest/qlcdnumber.html#intValue-prop QLCDNumber::display()] is called and shows the new number.

Note that you're not guaranteed any particular order of execution; '''<tt>LCDRange::valueChanged()</tt>''' may be emitted before or after [http://doc.qt.nokia.com/latest/qlcdnumber.html#intValue-prop QLCDNumber::display()] is called.

'''[http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t7/t7.rb t7.rb]'''
<syntaxhighlight lang="ruby">
previousRange = nil

for row in 0..2
  for column in 0..2
    lcdRange = LCDRange.new()
    grid.addWidget(lcdRange, row, column)
    unless previousRange.nil?
      connect(lcdRange, SIGNAL('valueChanged(int)'),
              previousRange, SLOT('setValue(int)'))
    end
    previousRange = lcdRange
  end
end
</syntaxhighlight>

When we create the nine '''<tt>LCDRange</tt>''' objects, we connect them using the [http://doc.qt.nokia.com/latest/signalsandslots.html signals and slots] mechanism. Each has its '''<tt>valueChanged()</tt>''' signal connected to the previous one's '''<tt>setValue()</tt>''' slot. Because '''<tt>LCDRange</tt>''' emits the '''<tt>valueChanged()</tt>''' signal when its value changes, we are here creating a chain of signals and slots.

=== Running the Application ===

On startup, the program's appearance is identical to the previous one. Try operating the slider to the bottom-right.

=== Exercises ===

Use the bottom-right slider to set all LCDs to 50. Then set all but the last LCD to 40 by clicking once to the left of the bottom-middle slider handle. Now, use the bottom-left slider to set the first seven LCDs back to 50.

Click to the left of the handle on the bottom-right slider. What happens? Why is this the correct behavior?

[[Category:Ruby]]