Development/Tutorials/Qt4 Ruby Tutorial/Chapter 08: Difference between revisions

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    m (Text replace - "<code ruby>" to "<syntaxhighlight lang="ruby">")
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    We now add the possibility of setting the range of the '''<tt>LCDRange</tt>'''. Until now, it has been fixed at 0 to 99.
    We now add the possibility of setting the range of the '''<tt>LCDRange</tt>'''. Until now, it has been fixed at 0 to 99.


    <code ruby>
    <syntaxhighlight lang="ruby">
    def setRange(minVal, maxVal)
    def setRange(minVal, maxVal)
       if minVal < 0 || maxVal > 99 || minVal > maxVal
       if minVal < 0 || maxVal > 99 || minVal > maxVal
    Line 43: Line 43:
    The '''<tt>setRange()</tt>''' slot sets the range of the slider in the '''<tt>LCDRange</tt>'''. Because we have set up the [http://doc.qt.nokia.com/latest/qlcdnumber.html QLCDNumber] to always display two digits, we want to limit the possible range of '''<tt>minVal</tt>''' and '''<tt>maxVal</tt>''' to avoid overflow of the [http://doc.qt.nokia.com/latest/qlcdnumber.html QLCDNumber]. (We could have allowed values down to -9 but chose not to.) If the arguments are illegal, we use Qt's [http://doc.qt.nokia.com/latest/qtglobal.html#qWarning QtGlobal::qWarning()] function to issue a warning to the user and return immediately. [http://doc.qt.nokia.com/latest/qtglobal.html#qWarning QtGlobal::qWarning()] is a '''<tt>printf</tt>'''-like function that by default sends its output to '''<tt>$stderr</tt>'''. If you want, you can install your own handler function using [http://doc.qt.nokia.com/latest/qtglobal.html#qInstallMsgHandler QtGlobal::qInstallMsgHandler()].
    The '''<tt>setRange()</tt>''' slot sets the range of the slider in the '''<tt>LCDRange</tt>'''. Because we have set up the [http://doc.qt.nokia.com/latest/qlcdnumber.html QLCDNumber] to always display two digits, we want to limit the possible range of '''<tt>minVal</tt>''' and '''<tt>maxVal</tt>''' to avoid overflow of the [http://doc.qt.nokia.com/latest/qlcdnumber.html QLCDNumber]. (We could have allowed values down to -9 but chose not to.) If the arguments are illegal, we use Qt's [http://doc.qt.nokia.com/latest/qtglobal.html#qWarning QtGlobal::qWarning()] function to issue a warning to the user and return immediately. [http://doc.qt.nokia.com/latest/qtglobal.html#qWarning QtGlobal::qWarning()] is a '''<tt>printf</tt>'''-like function that by default sends its output to '''<tt>$stderr</tt>'''. If you want, you can install your own handler function using [http://doc.qt.nokia.com/latest/qtglobal.html#qInstallMsgHandler QtGlobal::qInstallMsgHandler()].


    <code ruby>
    <syntaxhighlight lang="ruby">
    lcd.setSegmentStyle(Qt::LCDNumber::Filled)
    lcd.setSegmentStyle(Qt::LCDNumber::Filled)
    </code>
    </code>
    Line 51: Line 51:
    '''[http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t8/cannon.rb cannon.rb]'''
    '''[http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t8/cannon.rb cannon.rb]'''


    <code ruby>
    <syntaxhighlight lang="ruby">
    @currentAngle = 45
    @currentAngle = 45
    setPalette(Qt::Palette.new(Qt::Color.new(250, 250, 200)))
    setPalette(Qt::Palette.new(Qt::Color.new(250, 250, 200)))
    Line 63: Line 63:
    The [http://doc.qt.nokia.com/latest/qcolor.html Qt::Color] is specified as a RGB (red-green-blue) triplet, where each value is between 0 (dark) and 255 (bright). We could also have used a predefined color such as [http://doc.qt.nokia.com/latest/qt.html#GlobalColor-enum Qt::yellow] instead of specifying an RGB value.
    The [http://doc.qt.nokia.com/latest/qcolor.html Qt::Color] is specified as a RGB (red-green-blue) triplet, where each value is between 0 (dark) and 255 (bright). We could also have used a predefined color such as [http://doc.qt.nokia.com/latest/qt.html#GlobalColor-enum Qt::yellow] instead of specifying an RGB value.


    <code ruby>
    <syntaxhighlight lang="ruby">
    def setAngle(angle)
    def setAngle(angle)
       if angle < 5
       if angle < 5
    Line 89: Line 89:
    Finally, we emit the '''<tt>angleChanged()</tt>''' signal to tell the outside world that the angle has changed. The '''<tt>emit</tt>''' keyword is unique to Qt and not regular Ruby syntax.
    Finally, we emit the '''<tt>angleChanged()</tt>''' signal to tell the outside world that the angle has changed. The '''<tt>emit</tt>''' keyword is unique to Qt and not regular Ruby syntax.


    <code ruby>
    <syntaxhighlight lang="ruby">
    def paintEvent(event)
    def paintEvent(event)
       painter = Qt::Painter.new(self)
       painter = Qt::Painter.new(self)
    Line 103: Line 103:
    '''[http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t8/t8.rb t8.rb]'''
    '''[http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t8/t8.rb t8.rb]'''


    <code ruby>
    <syntaxhighlight lang="ruby">
    angle = LCDRange.new()
    angle = LCDRange.new()
    angle.setRange(5, 70)
    angle.setRange(5, 70)
    Line 110: Line 110:
    In the constructor, we create and set up the LCDRange widget. We set the LCDRange to accept angles from 5 to 70 degrees.
    In the constructor, we create and set up the LCDRange widget. We set the LCDRange to accept angles from 5 to 70 degrees.


    <code ruby>
    <syntaxhighlight lang="ruby">
      cannonField = CannonField.new()
      cannonField = CannonField.new()
    </code>
    </code>
    Line 116: Line 116:
    We create our CannonField widget.
    We create our CannonField widget.


    <code ruby>
    <syntaxhighlight lang="ruby">
    connect(angle, SIGNAL('valueChanged(int)'),
    connect(angle, SIGNAL('valueChanged(int)'),
             cannonField, SLOT('setAngle(int)'))
             cannonField, SLOT('setAngle(int)'))
    Line 129: Line 129:
    Notice how important it is to emit the '''<tt>angleChanged()</tt>''' signal only when the angle actually changes. If both the '''<tt>LCDRange</tt>''' and the '''<tt>CannonField</tt>''' had omitted this check, the program would have entered an infinite loop upon the first change of one of the values.
    Notice how important it is to emit the '''<tt>angleChanged()</tt>''' signal only when the angle actually changes. If both the '''<tt>LCDRange</tt>''' and the '''<tt>CannonField</tt>''' had omitted this check, the program would have entered an infinite loop upon the first change of one of the values.


    <code ruby>
    <syntaxhighlight lang="ruby">
    gridLayout = Qt::GridLayout.new()
    gridLayout = Qt::GridLayout.new()
    </code>
    </code>
    Line 141: Line 141:
    The diagram above shows the layout we're trying to achieve. The left side shows a schematic view of the layout; the right side is an actual screenshot of the program. ''(These two images are copyrighted/owned by Nokia.)''
    The diagram above shows the layout we're trying to achieve. The left side shows a schematic view of the layout; the right side is an actual screenshot of the program. ''(These two images are copyrighted/owned by Nokia.)''


    <code ruby>
    <syntaxhighlight lang="ruby">
         gridLayout.addWidget(quit, 0, 0)
         gridLayout.addWidget(quit, 0, 0)
    </code>
    </code>
    Line 147: Line 147:
    We add the <strong>Quit</strong> button in the top-left cell of the grid, i.e., the cell with coordinates (0, 0).
    We add the <strong>Quit</strong> button in the top-left cell of the grid, i.e., the cell with coordinates (0, 0).


    <code ruby>
    <syntaxhighlight lang="ruby">
         gridLayout.addWidget(angle, 1, 0)
         gridLayout.addWidget(angle, 1, 0)
    </code>
    </code>
    Line 153: Line 153:
    We put the angle '''<tt>LCDRange</tt>''' cell (1, 0).
    We put the angle '''<tt>LCDRange</tt>''' cell (1, 0).


    <code ruby>
    <syntaxhighlight lang="ruby">
    gridLayout.addWidget(cannonField, 1, 1, 2, 1)
    gridLayout.addWidget(cannonField, 1, 1, 2, 1)
    </code>
    </code>
    Line 159: Line 159:
    We let the '''<tt>CannonField</tt>''' object occupy cells (1, 1) and (2, 1).
    We let the '''<tt>CannonField</tt>''' object occupy cells (1, 1) and (2, 1).


    <code ruby>
    <syntaxhighlight lang="ruby">
         gridLayout.setColumnStretch(1, 10)
         gridLayout.setColumnStretch(1, 10)
    </code>
    </code>
    Line 167: Line 167:
    In this particular example, any stretch factor greater than 0 for column 1 would have the same effect. In more complex layouts, you can use the stretch factors to tell that a particular column or row should stretch twice as fast as another by assigning appropriate stretch factors.
    In this particular example, any stretch factor greater than 0 for column 1 would have the same effect. In more complex layouts, you can use the stretch factors to tell that a particular column or row should stretch twice as fast as another by assigning appropriate stretch factors.


    <code ruby>
    <syntaxhighlight lang="ruby">
         angle.setValue(60)
         angle.setValue(60)
    </code>
    </code>
    Line 173: Line 173:
    We set an initial angle value. Note that this will trigger the connection from '''<tt>LCDRange</tt>''' to '''<tt>CannonField</tt>'''.
    We set an initial angle value. Note that this will trigger the connection from '''<tt>LCDRange</tt>''' to '''<tt>CannonField</tt>'''.


    <code ruby>
    <syntaxhighlight lang="ruby">
         angle.setFocus()
         angle.setFocus()
    </code>
    </code>

    Revision as of 20:43, 29 June 2011


    Development/Tutorials/Qt4 Ruby Tutorial/Chapter 08


    Preparing for Battle
    Tutorial Series   Qt4 Ruby Tutorial
    Previous   Tutorial 7 - One Thing Leads to Another
    What's Next   Tutorial 9 - With Cannon You Can
    Further Reading   n/a

    Preparing for Battle

    Files:

    Overview

    In this example, we introduce the first custom widget that can paint itself. We also add a useful keyboard interface (with two lines of code).

    Line by Line Walkthrough

    lcdrange.rb

    This file is very similar to the lcdrange.rb in Chapter 7. We have added one slot: setRange().

    We now add the possibility of setting the range of the LCDRange. Until now, it has been fixed at 0 to 99.

    <syntaxhighlight lang="ruby"> def setRange(minVal, maxVal)

     if minVal < 0 || maxVal > 99 || minVal > maxVal
       qWarning("LCDRange::setRange(#{minVal}, #{maxVal})\n" +
                  "\tRange must be 0..99\n" +
                  "\tand minVal must not be greater than maxVal")
       return
     end
    
     @slider.setRange(minVal, maxVal)
    

    end

    The setRange() slot sets the range of the slider in the LCDRange. Because we have set up the QLCDNumber to always display two digits, we want to limit the possible range of minVal and maxVal to avoid overflow of the QLCDNumber. (We could have allowed values down to -9 but chose not to.) If the arguments are illegal, we use Qt's QtGlobal::qWarning() function to issue a warning to the user and return immediately. QtGlobal::qWarning() is a printf-like function that by default sends its output to $stderr. If you want, you can install your own handler function using QtGlobal::qInstallMsgHandler().

    <syntaxhighlight lang="ruby"> lcd.setSegmentStyle(Qt::LCDNumber::Filled)

    This makes our lcd numbers look way better. I'm not certain, but I believe what makes it possible to do this is setting a palette (see next section). What I do know is that this line has no effect when I tried it in previous chapters, but works here.

    cannon.rb

    <syntaxhighlight lang="ruby"> @currentAngle = 45 setPalette(Qt::Palette.new(Qt::Color.new(250, 250, 200))) setAutoFillBackground(true)

    The constructor initializes the angle value to 45 degrees and sets a custom palette for this widget.

    This palette uses the indicated color as background and picks other colors suitably. (For this widget only the background and text colors will actually be used.) We then call setAutoFillBackground(true) to tell Qt fill the background automatically.

    The Qt::Color is specified as a RGB (red-green-blue) triplet, where each value is between 0 (dark) and 255 (bright). We could also have used a predefined color such as Qt::yellow instead of specifying an RGB value.

    <syntaxhighlight lang="ruby"> def setAngle(angle)

     if angle < 5
       angle = 5
     elsif angle > 70
       angle = 70
     end
    
     if @currentAngle == angle
       return
     end
    
     @currentAngle = angle
     update()
     emit angleChanged(@currentAngle)
    

    end def setAngle(degrees)

    This function sets the angle value. We have chosen a legal range of 5 to 70 and adjust the given number of degrees accordingly. We have chosen not to issue a warning if the new angle is out of range.

    If the new angle equals the old one, we return immediately. It is important to only emit the angleChanged() signal when the angle really has changed.

    Then we set the new angle value and repaint our widget. The Qt::Widget::update() function clears the widget (usually filling it with its background color) and sends a paint event to the widget. This results in a call to the paint event function of the widget.

    Finally, we emit the angleChanged() signal to tell the outside world that the angle has changed. The emit keyword is unique to Qt and not regular Ruby syntax.

    <syntaxhighlight lang="ruby"> def paintEvent(event)

     painter = Qt::Painter.new(self)
     painter.drawText(200, 200, tr("Angle = #{@currentAngle}"))
     painter.end()
    

    end

    This is our first attempt to write a paint event handler. The event argument contains a description of the paint event. Qt::PaintEvent contains the region in the widget that must be updated. For the time being, we will be lazy and just paint everything.

    Our code displays the angle value in the widget at a fixed position. We create a Qt::Painter operating on this widget and use it to paint a string. We'll come back to Qt::Painter later; it can do a great many things.

    t8.rb

    <syntaxhighlight lang="ruby"> angle = LCDRange.new() angle.setRange(5, 70)

    In the constructor, we create and set up the LCDRange widget. We set the LCDRange to accept angles from 5 to 70 degrees.

    <syntaxhighlight lang="ruby">

    cannonField = CannonField.new()
    

    We create our CannonField widget.

    <syntaxhighlight lang="ruby"> connect(angle, SIGNAL('valueChanged(int)'),

           cannonField, SLOT('setAngle(int)'))
    

    connect(cannonField, SIGNAL('angleChanged(int)'),

           angle, SLOT('setValue(int)'))
    

    Here we connect the valueChanged() signal of the LCDRange to the setValue() slot of the CannonField. This will update CannonField's angle value whenever the user operates the LCDRange. We also make the reverse connection so that changing the angle in the CannonField will update the LCDRange value. In our example we never change the angle of the CannonField directly; but by doing the last connect() we ensure that no future changes will disrupt the synchronization between those two values.

    This illustrates the power of component programming and proper encapsulation.

    Notice how important it is to emit the angleChanged() signal only when the angle actually changes. If both the LCDRange and the CannonField had omitted this check, the program would have entered an infinite loop upon the first change of one of the values.

    <syntaxhighlight lang="ruby"> gridLayout = Qt::GridLayout.new()

    So far, we have used Qt::VBoxLayout for geometry management. Now, however, we want to have a little more control over the layout, and we switch to the more powerful Qt::GridLayout class. Qt::GridLayout isn't a widget; it is a different class that can manage the children of any widget.

    We don't need to specify any dimensions to the Qt::GridLayout constructor. The Qt::GridLayout will determine the number of rows and columns based on the grid cells we populate.

    The diagram above shows the layout we're trying to achieve. The left side shows a schematic view of the layout; the right side is an actual screenshot of the program. (These two images are copyrighted/owned by Nokia.)

    <syntaxhighlight lang="ruby">

       gridLayout.addWidget(quit, 0, 0)
    

    We add the Quit button in the top-left cell of the grid, i.e., the cell with coordinates (0, 0).

    <syntaxhighlight lang="ruby">

       gridLayout.addWidget(angle, 1, 0)
    

    We put the angle LCDRange cell (1, 0).

    <syntaxhighlight lang="ruby"> gridLayout.addWidget(cannonField, 1, 1, 2, 1)

    We let the CannonField object occupy cells (1, 1) and (2, 1).

    <syntaxhighlight lang="ruby">

       gridLayout.setColumnStretch(1, 10)
    

    We tell Qt::GridLayout that the right column (column 1) is stretchable, with a stretch factor of 10. Because the left column isn't (its stretch factor is 0, the default value), Qt::GridLayout will try to let the left-hand widgets' sizes be unchanged and will resize just the CannonField when the MyWidget is resized.

    In this particular example, any stretch factor greater than 0 for column 1 would have the same effect. In more complex layouts, you can use the stretch factors to tell that a particular column or row should stretch twice as fast as another by assigning appropriate stretch factors.

    <syntaxhighlight lang="ruby">

       angle.setValue(60)
    

    We set an initial angle value. Note that this will trigger the connection from LCDRange to CannonField.

    <syntaxhighlight lang="ruby">

       angle.setFocus()
    

    Our last action is to set angle to have keyboard focus so that keyboard input will go to the LCDRange widget by default.

    Running the Application

    When the slider is operated, the CannonField displays the new angle value. Upon resizing, CannonField is given as much space as possible.

    Exercises

    Try to resize the window. What happens if you make it really narrow or really squat?

    If you give the left-hand column a non-zero stretch factor, what happens when you resize the window?

    Try to change "Quit" to "&Quit". How does the button's look change? ( Whether it does change or not depends on the platform.) What happens if you press Alt+Q while the program is running?

    Center the text in the CannonField.