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Qt Tutorial 10 - Smooth as Silk


Screenshot of Chapter 10

In this example, we add a force control.

Line by Line Walkthrough


The CannonField now has a force value in addition to the angle.

     int angle() const { return currentAngle; }
     int force() const { return currentForce; }

 public slots:
     void setAngle(int angle);
     void setForce(int force);

     void angleChanged(int newAngle);
     void forceChanged(int newForce);

The interface to the force follows the same practice as for the angle.

     QRect cannonRect() const;

We have put the definition of the cannon's enclosing rectangle in a separate function.

     int currentAngle;
     int currentForce;

The force is stored in the currentForce variable.


 CannonField::CannonField(QWidget *parent)
     : QWidget(parent)
     currentAngle = 45;
     currentForce = 0;
     setPalette(QPalette(QColor(250, 250, 200)));

The force is initialized to zero.

 void CannonField::setAngle(int angle)
     if (angle < 5)
         angle = 5;
     if (angle > 70)
         angle = 70;
     if (currentAngle == angle)
     currentAngle = angle;
     emit angleChanged(currentAngle);

We have made a slight change in the setAngle() function. It repaints only the portion of the widget that contains the cannon.

 void CannonField::setForce(int force)
     if (force < 0)
         force = 0;
     if (currentForce == force)
     currentForce = force;
     emit forceChanged(currentForce);

The implementation of setForce() is quite similar to that of setAngle(). The only difference is that because we don't show the force value, we don't need to repaint the widget.

 void CannonField::paintEvent(QPaintEvent * /* event */)
     QPainter painter(this);


     painter.translate(0, height());
     painter.drawPie(QRect(-35, -35, 70, 70), 0, 90 * 16);
     painter.drawRect(QRect(30, -5, 20, 10));

We paint as in Chapter 9.

 QRect CannonField::cannonRect() const
     QRect result(0, 0, 50, 50);
     return result;

The cannonRect() function returns the rectangle enclosing the cannon in widget coordinates. First we create a rectangle with the size 50 x 50 and then move it so its bottom-left corner is equal to the widget's own bottom-left corner.

The QWidget::rect() function returns the widget's enclosing rectangle in the widget's own coordinates. The top-left corner of the rectangle is always (0, 0).


 MyWidget::MyWidget(QWidget *parent)
     : QWidget(parent)

The constructor is mostly the same, but some new bits have been added.

     LCDRange *force = new LCDRange;
     force->setRange(10, 50);

We add a second LCDRange, which will be used to set the force.

     connect(force, SIGNAL(valueChanged(int)),
             cannonField, SLOT(setForce(int)));
     connect(cannonField, SIGNAL(forceChanged(int)),
             force, SLOT(setValue(int)));

We connect the force widget and the cannonField widget, just like we did for the angle widget.

     QVBoxLayout *leftLayout = new QVBoxLayout;

     QGridLayout *gridLayout = new QGridLayout;
     gridLayout->addWidget(quit, 0, 0);
     gridLayout->addLayout(leftLayout, 1, 0);
     gridLayout->addWidget(cannonField, 1, 1, 2, 1);
     gridLayout->setColumnStretch(1, 10);

In Chapter 9, we put angle in the lower-left cell of the layout. Now we want to have two widgets in that cell, so we make a vertical box, put the vertical box in the grid cell, and put each of angle and range in the vertical box.


We initialize the force value to 25.

Running the Application

We now have a force control.


Make the size of the cannon barrel be dependent on the force.

Put the cannon in the bottom-right corner.

Try adding a better keyboard interface. For example, make + and - increase and decrease the force and enter shoot. If you're bothered by the way the Left and Right keys work, change that too. [Hint: Reimplement QWidget::keyPressEvent().]

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