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{{TutorialBrowser|
series=[[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial|Qt4 Ruby Tutorial]]|
name=With Cannon You Can|
pre=[[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial/Chapter_08|Tutorial 8 - Preparing for Battle]]|
next=[[Special:myLanguage/Development/Tutorials/Qt4_Ruby_Tutorial/Chapter_10|Tutorial 10 - Smooth as Silk]]
}}
== With Cannon You Can ==
[[Image:Qt4_Ruby_Tutorial_Screenshot_9.png|center]]
Files:
* [http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t9/lcdrange.rb lcdrange.rb]
* [http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t9/cannon.rb cannon.rb]
* [http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t9/t9.rb t9.rb]

=== Overview ===

In this example we become graphic by drawing a cute little blue cannon. Only cannon.rb differs from the previous chapter.

=== Line by Line Walkthrough ===
'''[http://www.darshancomputing.com/qt4-qtruby-tutorial/tutorial/t9/cannon.rb cannon.rb]'''

<syntaxhighlight lang="ruby">
def paintEvent(event)
  painter = Qt::Painter.new(self)
</syntaxhighlight>

We'll now start to use [http://doc.qt.nokia.com/latest/qpainter.html Qt::Painter] in earnest. We create a painter that operates on this widget.

<syntaxhighlight lang="ruby">
    painter.setPen(Qt::NoPen)
</syntaxhighlight>

The edges of what [http://doc.qt.nokia.com/latest/qpainter.html Qt::Painter] draws are drawn using the pen. Here we set it to [http://doc.qt.nokia.com/latest/qt.html#PenStyle-enum Qt::NoPen], meaning that there will be no special edge when we draw something.

<syntaxhighlight lang="ruby">
    painter.setBrush(Qt::Brush.new(Qt::blue))
</syntaxhighlight>

When [http://doc.qt.nokia.com/latest/qpainter.html Qt::Painter] fills a rectangle, a circle, or whatever, it fills the shape using its brush. Here we set it to use a solid blue brush. (We could also use a pattern.) The blue brush will go all the way to the edges of the things we draw.

<syntaxhighlight lang="ruby">
    painter.translate(0, rect().height())
</syntaxhighlight>

The [http://doc.qt.nokia.com/latest/qpainter.html#translate Qt::Painter::translate()] function translates the coordinate system of the [http://doc.qt.nokia.com/latest/qpainter.html Qt::Painter] (i.e., it moves it by an offset). Here we set the (0, 0) point to the bottom-left corner of the widget. The x and y directions remain unchanged, i.e., all the y coordinates inside the widget are now negative. (See [http://doc.qt.nokia.com/latest/coordsys.html The Coordinate System] for more information about Qt's coordinate system.)

<syntaxhighlight lang="ruby">
    painter.drawPie(Qt::Rect.new(-35, -35, 70, 70), 0, 90 * 16)
</syntaxhighlight>

The [http://doc.qt.nokia.com/latest/qpainter.html#drawPie Qt::Painter::drawPie()] function draws a pie shape inside the specified rectangle using a start angle and an arc length. The angles are specified in sixteenths of a degree. Zero degrees is at the 3 o'clock position. The drawing direction is counter-clockwise. Here we draw a quarter of a circle in the bottom-left corner of the widget. The pie is filled with blue and has no outline.

<syntaxhighlight lang="ruby">
    painter.rotate(-@currentAngle)
</syntaxhighlight>

The [http://doc.qt.nokia.com/latest/qpainter.html#rotate Qt::Painter::rotate()] function rotates the coordinate system of the [http://doc.qt.nokia.com/latest/qpainter.html Qt::Painter] around the origin. The rotation argument is given in degrees (not given in sixteenths of a degree as above) and clockwise. Here we rotate the coordinate system '''<tt>@currentAngle</tt>''' degrees counter-clockwise.

<syntaxhighlight lang="ruby">
    painter.drawRect(Qt::Rect.new(30, -5, 20, 10))
</syntaxhighlight>

The [http://doc.qt.nokia.com/latest/qpainter.html#drawRect Qt::Painter::drawRect()] function draws the specified rectangle. Here we draw the barrel of the cannon.

It can often be difficult to envision the resulting drawing when the coordinate system has been transformed (translated, rotated, scaled, or sheared) as above.

In this case the coordinate system is first translated and then rotated. If the rectangle [http://doc.qt.nokia.com/latest/qrect.html Qt::Rect].new(30, -5, 20, 10) had been drawn in the translated coordinate system, it would have looked like this:

[[Image:Qt4_Ruby_Tutorial_Screenshot_9-incorrect.png|center]]

Note that the rectangle is clipped by the border of the '''<tt>CannonField</tt>''' widget. When we rotate the coordinate system, for instance 60 degrees, the rectangle will be rotated around (0, 0), which is the bottom-left corner because we have translated the coordinate system. The result looks like this:

[[Image:Qt4_Ruby_Tutorial_Screenshot_9-correct.png|center]]

=== Running the Application ===

When the slider is operated the angle of the drawn cannon changes accordingly.

=== Exercises ===
Set a different pen instead of [http://doc.qt.nokia.com/latest/qt.html#PenStyle-enum Qt::NoPen]. Set a patterned brush.

[[Category:Ruby]]