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We initialize our new private variables and connect the [http://doc.qt.nokia.com/latest/qtimer.html#timeout Qt::Timer::timeout()] signal to our '''<tt>moveShot()</tt>''' slot. We'll move the shot every time the timer times out.‎

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The '''<tt>timerCount</tt>''' keeps track of the time passed since the shot was fired. The '''<tt>shootAngle</tt>''' is the cannon angle and '''<tt>shootForce</tt>''' is the cannon force when the shot was fired.‎

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This function shoots a shot unless a shot is in the air. The '''<tt>timerCount</tt>''' is reset to zero. The '''<tt>shootAngle</tt>''' and '''<tt>shootForce</tt>''' variables are set to the current cannon angle and force. Finally, we start the timer.‎

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'''<tt>moveShot()</tt>''' is the slot that moves the shot, called every 5 milliseconds when the [http://doc.qt.nokia.com/latest/qtimer.html Qt::Timer] fires.‎

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Its tasks are to compute the new position, update the screen with the shot in the new position, and if necessary, stop the timer.‎

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First we make a [http://doc.qt.nokia.com/latest/qregion.html Qt::Region] that holds the old '''<tt>shotRect()</tt>'''. A [http://doc.qt.nokia.com/latest/qregion.html Qt::Region] is capable of holding any sort of region, and we'll use it here to simplify the painting. '''<tt>shotRect()</tt>''' returns the rectangle where the shot is now. It is explained in detail later.‎

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Then we increment the '''<tt>timerCount</tt>''', which has the effect of moving the shot one step along its trajectory.‎

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Next we fetch the new shot rectangle.‎

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If the shot has moved beyond the right or bottom edge of the widget we stop the timer, or we add the new '''<tt>shotRect()</tt>''' to the [http://doc.qt.nokia.com/latest/qregion.html Qt::Region].‎

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Finally, we repaint the [http://doc.qt.nokia.com/latest/qregion.html Qt::Region]. This will send a single paint event for just the one or two rectangles that need updating.‎

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The paint event function has been simplified since the previous chapter. Most of the logic has been moved to the new '''<tt>paintShot()</tt>''' and '''<tt>paintCannon()</tt>''' functions.‎

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This private function paints the shot by drawing a black filled rectangle.‎

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We leave out the implementation of '''<tt>paintCannon()</tt>'''; it is the same as the [http://doc.qt.nokia.com/latest/qwidget.html#paintEvent Qt::Widget::paintEvent()] reimplementation from the previous chapter.‎

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This private function calculates the center point of the shot and returns the enclosing rectangle of the shot. It uses the initial cannon force and angle in addition to '''<tt>timerCount</tt>''', which increases as time passes.‎

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The formula used is the standard Newtonian formula for frictionless movement in a gravity field. For simplicity, we've chosen to disregard any Einsteinian effects.‎

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We calculate the center point in a coordinate system where y coordinates increase upward. After we have calculated the center point, we construct a [http://doc.qt.nokia.com/latest/qrect.html Qt::Rect] with size 6 x 6 and move its center point to the point calculated above. In the same operation we convert the point into the widget's coordinate system (see [http://doc.qt.nokia.com/latest/coordsys.html The Coordinate System]).‎

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The only addition is the Shoot button.‎

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In the constructor we create and set up the Shoot button exactly like we did with the Quit button.‎

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Connects the '''<tt>clicked()</tt>''' signal of the Shoot button to the '''<tt>shoot()</tt>''' slot of the '''<tt>CannonField</tt>'''.‎

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=== Running the Application ===‎

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The cannon can shoot, but there's nothing to shoot at.‎

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=== Exercises ===‎