Qt Tutorial 12 - Hanging in the Air the Way Bricks Don't

t12/cannonfield.cpp

    #include <stdlib.h>

We include the <stdlib.h> header file because we need the rand() function.

        newTarget();

This line has been added to the constructor. It creates a "random" position for the target. In fact, the newTarget() function will try to paint the target. Because we are in a constructor, the CannonField widget is invisible. Qt guarantees that no harm is done when calling QWidget::update() on a hidden widget.

    void CannonField::newTarget()
    {
        static bool firstTime = true;

        if (firstTime) {
            firstTime = false;
            QTime midnight(0, 0, 0);
            srand(midnight.secsTo(QTime::currentTime()));
        }
        target = QPoint(200 + rand() % 190, 10 + rand() % 255);
        update();
    }

This private function creates a target center point at a new random position.

We use the rand() function to fetch random integers. The rand() function normally returns the same series of numbers each time you run a program. This would make the target appear at the same position every time. To avoid this, we must set a random seed the first time this function is called. The random seed must also be random in order to avoid equal random number series. The solution is to use the number of seconds that have passed since midnight as a pseudo-random value.

First we create a static bool local variable. A static variable like this one is guaranteed to keep its value between calls to the function.

The if test will succeed only the first time this function is called because we set firstTime to false inside the if block.

Then we create the QTime object midnight, which represents the time 00:00:00. Next we fetch the number of seconds from midnight until now and use it as a random seed. See the documentation for QDate, QTime, and QDateTime for more information.

Finally we calculate the target's center point. We keep it within the rectangle (x = 200, y = 35, width = 190, height = 255), i.e., the possible x and y values are 200 to 389 and 35 to 289, respectively) in a coordinate system where we put y position 0 at the bottom edge of the widget and let y values increase upwards x is as normal, with 0 at the left edge and with x values increasing to the right.

By experimentation we have found this to always be in reach of the shot.

    void CannonField::moveShot()
    {
        QRegion region = shotRect();
        ++timerCount;

        QRect shotR = shotRect();

This part of the timer event has not changed from the previous chapter.

        if (shotR.intersects(targetRect())) {
            autoShootTimer->stop();
            emit hit();

This if statement checks whether the shot rectangle intersects the target rectangle. If it does, the shot has hit the target (ouch!). We stop the shoot timer and emit the hit() signal to tell the outside world that a target was destroyed, and return.

Note that we could have created a new target on the spot, but because the CannonField is a component we leave such decisions to the user of the component.

        } else if (shotR.x() > width() || shotR.y() > height()) {
            autoShootTimer->stop();
            emit missed();

This if statement is the same as in the previous chapter, except that it now emits the missed() signal to tell the outside world about the failure.

        } else {
            region = region.unite(shotR);
        }
        update(region);
    }

And the rest of the function is as before.

CannonField::paintEvent() is as before, except that this has been added:

        paintTarget(painter);

This line makes sure that the target is also painted when necessary.

    void CannonField::paintTarget(QPainter &painter)
    {
        painter.setPen(Qt::black);
        painter.setBrush(Qt::red);
        painter.drawRect(targetRect());
    }

This private function paints the target; a rectangle filled with red and with a black outline.

    QRect CannonField::targetRect() const
    {
        QRect result(0, 0, 20, 10);
        result.moveCenter(QPoint(target.x(), height() - 1 - target.y()));
        return result;
    }

This private function returns the enclosing rectangle of the target. Remember from newTarget() that the target point uses y coordinate 0 at the bottom of the widget. We calculate the point in widget coordinates before we call QRect::moveCenter().

The reason we have chosen this coordinate mapping is to fix the distance between the target and the bottom of the widget. Remember that the widget can be resized by the user or the program at any time.

        LCDRange *angle = new LCDRange(tr("ANGLE"));

        LCDRange *force = new LCDRange(tr("FORCE"));