Qt Tutorial - Chapter 8: Preparing for Battle

    #include "cannon.h"

We include our new class.

    class MyWidget: public QWidget
    {
    public:
        MyWidget( QWidget *parent=0, const char *name=0 );
    };

This time we include a single LCDRange and a CannonField in our top-level widget.

        LCDRange *angle = new LCDRange( this, "angle" );

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

        angle->setRange( 5, 70 );

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

        CannonField *cannonField
            = new CannonField( this, "cannonField" );

We create our CannonField.

        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 *grid = new QGridLayout( this, 2, 2, 10 );
        //2x2, 10 pixel border

So far we have used the no-assembly-required QVBox and QGrid widgets 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.

As the comment indicates, we create a two-by-two array with ten pixel borders. (The constructor for QGridLayout can be a little cryptic, so it's good to put in such comments.)

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

We add the Quit button in the top-left cell of the grid: 0, 0.

        grid->addWidget( angle, 1, 0, Qt::AlignTop );

We put the angle LCDRange in the bottom-left cell, aligned to the top of its cell. (This alignment is one of the things QGridLayout allows but QGrid does not allow.)

        grid->addWidget( cannonField, 1, 1 );

We put the CannonField object in the bottom-right cell. (The top- right cell is empty.)

        grid->setColStretch( 1, 10 );

We tell QGridLayout that the right column (column 1) is stretchable. Because the left column isn't (it has stretch factor 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.

        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.

Behavior