Qt Tutorial - Chapter 14: Facing the Wall

        barrelPressed = FALSE;

This line has been added to the constructor. Initially, the mouse is not pressed on the barrel.

        } else if ( shotR.x() > width() || shotR.y() > height() ||
                    shotR.intersects(barrierRect()) ) {

Now that we have a barrier, there are three ways to miss. We test for the third, too.

    void CannonField::mousePressEvent( QMouseEvent *e )
    {
        if ( e->button() != LeftButton )
            return;
        if ( barrelHit( e->pos() ) )
            barrelPressed = TRUE;
    }

This is a Qt event handler. It is called when the user presses a mouse button when the mouse cursor is over the widget.

If the event was not generated by the left mouse button, we return immediately. Otherwise, we check if the position of the mouse cursor is within the cannon's barrel. If it is, we set barrelPressed to TRUE.

Notice that the pos() function returns a point in the widget's coordinate system.

    void CannonField::mouseMoveEvent( QMouseEvent *e )
    {
        if ( !barrelPressed )
            return;
        QPoint pnt = e->pos();
        if ( pnt.x() <= 0 )
            pnt.setX( 1 );
        if ( pnt.y() >= height() )
            pnt.setY( height() - 1 );
        double rad = atan(((double)rect().bottom()-pnt.y())/pnt.x());
        setAngle( qRound ( rad*180/3.14159265 ) );
    }

This is another Qt event handler. It is called when the user already has pressed the mouse button inside this widget and then moves/drags the mouse. (You can make Qt send mouse move events even when no buttons are pressed. See QWidget::setMouseTracking().)

This handler repositions the cannon's barrel according to the position of the mouse cursor.

First, if the barrel is not pressed, we return. Next, we fetch the mouse cursor's position. If the mouse cursor is to the left or below the widget, we adjust the point to be inside the widget.

Then we calculate the angle between the bottom edge of the widget and the imaginary line between the bottom-left corner of the widget and the cursor position. Finally we set the cannon's angle to the new value converted to degrees.

Remember that setAngle() redraws the cannon.

    void CannonField::mouseReleaseEvent( QMouseEvent *e )
    {
        if ( e->button() == LeftButton )
            barrelPressed = FALSE;
    }

This Qt event handler is called whenever the user releases a mouse button and it was pressed inside this widget.

If the left button is released, we can be sure that the barrel is no longer pressed.

The paint event has two extra lines:

        if ( updateR.intersects( barrierRect() ) )
            paintBarrier( &p );

paintBarrier() does the same sort of thing as paintShot(), paintTarget(), and paintCannon().

    void CannonField::paintBarrier( QPainter *p )
    {
        p->setBrush( yellow );
        p->setPen( black );
        p->drawRect( barrierRect() );
    }

This private function paints the barrier as a rectangle filled with yellow and with a black outline.

    QRect CannonField::barrierRect() const
    {
        return QRect( 145, height() - 100, 15, 100 );
    }

This private function returns the rectangle of the barrier. We fix the bottom edge of the barrier to the bottom edge of the widget.

    bool CannonField::barrelHit( const QPoint &p ) const
    {
        QWMatrix mtx;
        mtx.translate( 0, height() - 1 );
        mtx.rotate( -ang );
        mtx = mtx.invert();
        return barrelRect.contains( mtx.map(p) );
    }

This function returns TRUE if the point is in the barrel; otherwise it returns FALSE.

Here we use the class QWMatrix. It is defined in the header file qwmatrix.h, which is included by qpainter.h.

QWMatrix defines a coordinate system mapping. It can perform the same transformations as the QPainter.

Here we perform the same transformation steps as we do when drawing the barrel in the paintCannon() function. First we translate the coordinate system and then we rotate it.

Now we need to check whether the point p (in widget coordinates) lies inside the barrel. To do this, we invert the transformation matrix. The inverted matrix performs the inverse transformation that we used when drawing the barrel. We map the point p using the inverted matrix and return TRUE if it is inside the original barrel rectangle.

t14/gamebrd.cpp

    #include <qaccel.h>

        QVBox *box = new QVBox( this, "cannonFrame" );
        box->setFrameStyle( QFrame::WinPanel | QFrame::Sunken );
        cannonField = new CannonField( box, "cannonField" );

        QAccel *accel = new QAccel( this );
        accel->connectItem( accel->insertItem( Key_Enter ),
                            this, SLOT(fire()) );
        accel->connectItem( accel->insertItem( Key_Return ),
                            this, SLOT(fire()) );

        accel->connectItem( accel->insertItem( CTRL+Key_Q ),
                            qApp, SLOT(quit()) );

        QGridLayout *grid = new QGridLayout( this, 2, 2, 10 );
        grid->addWidget( quit, 0, 0 );
        grid->addWidget( box, 1, 1 );
        grid->setColStretch( 1, 10 );