Qt Tutorial 8 - Preparing for Battle

t8/main.cpp

    #include "cannonfield.h"

We include the definition of our new class.

    class MyWidget : public QWidget
    {
    public:
        MyWidget(QWidget *parent = 0);
    };

The MyWidget class will include a single LCDRange and a CannonField.

        LCDRange *angle = new LCDRange;

In the constructor, we create and set up the LCDRange widget.

        angle->setRange(5, 70);

We set the LCDRange to accept angles from 5 to 70 degrees.

        CannonField *cannonField = new CannonField;

We create our CannonField widget.

        connect(angle, SIGNAL(valueChanged(int)),
                cannonField, SLOT(setAngle(int)));
        connect(cannonField, SIGNAL(angleChanged(int)),
                angle, SLOT(setValue(int)));

Here we connect the valueChanged() signal of the LCDRange to the setAngle() slot of the CannonField. This will update CannonField's angle value whenever the user operates the LCDRange. We also make the reverse connection so that changing the angle in the CannonField will update the LCDRange value. In our example we never change the angle of the CannonField directly; but by doing the last connect() we ensure that no future changes will disrupt the synchronization between those two values.

This illustrates the power of component programming and proper encapsulation.

Notice how important it is to emit the angleChanged() signal only when the angle actually changes. If both the LCDRange and the CannonField had omitted this check, the program would have entered an infinite loop upon the first change of one of the values.

        QGridLayout *gridLayout = new QGridLayout;

So far, we have used QVBoxLayout for geometry management. Now, however, we want to have a little more control over the layout, and we switch to the more powerful QGridLayout class. QGridLayout isn't a widget; it is a different class that can manage the children of any widget.

We don't need to specify any dimensions to the QGridLayout constructor. The QGridLayout will determine the number of rows and columns based on the grid cells we populate.

The diagram above shows the layout we're trying to achieve. The left side shows a schematic view of the layout; the right side is an actual screenshot of the program.

        gridLayout->addWidget(quit, 0, 0);

We add the Quit button in the top-left cell of the grid, i.e., the cell with coordinates (0, 0).

        gridLayout->addWidget(angle, 1, 0);

We put the angle LCDRange cell (1, 0).

        gridLayout->addWidget(cannonField, 1, 1, 2, 1);

We let the CannonField object occupy cells (1, 1) and (2, 1).

        gridLayout->setColumnStretch(1, 10);

We tell QGridLayout that the right column (column 1) is stretchable, with a stretch factor of 10. Because the left column isn't (its stretch factor is 0, the default value), QGridLayout will try to let the left-hand widgets' sizes be unchanged and will resize just the CannonField when the MyWidget is resized.

In this particular example, any stretch factor greater than 0 for column 1 would have the same effect. In more complex layouts, you can use the stretch factors to tell that a particular column or row should stretch twice as fast as another by assigning appropriate stretch factors.

        angle->setValue(60);

We set an initial angle value. Note that this will trigger the connection from LCDRange to CannonField.

        angle->setFocus();

Our last action is to set angle to have keyboard focus so that keyboard input will go to the LCDRange widget by default.

LCDRange does not contain any keyPressEvent(), so that would seem not to be terribly useful. However, its constructor just got a new line:

        setFocusProxy(slider);

The LCDRange sets the slider to be its focus proxy. That means that when someone (the program or the user) wants to give the LCDRange keyboard focus, the slider should take care of it. QSlider has a decent keyboard interface, so with just one line of code we've given LCDRange one.