Writing your own layout manager

#include "card.h"

First we define an iterator over the layout. Layout iterators are used internally by the layout system to handle deletion of widgets. They are also available for application programmers.

There are two different classes involved: QLayoutIterator is the class that is visible to application programmers, it is explicitly shared. The QLayoutIterator contains a QGLayoutIterator that does all the work. We must create a subclass of QGLayoutIterator that knows how to iterate over our layout class.

In this case, we choose a simple implementation: we store an integer index into the list and a pointer to the list. Every QGLayoutIterator subclass must implement current(), next() and takeCurrent(), as well as a constructor. In our example we do not need a destructor.

class CardLayoutIterator : public QGLayoutIterator
{
public:
    CardLayoutIterator( QPtrList<QLayoutItem> *l )
        : idx( 0 ), list( l ) {}

    QLayoutItem *current()
    { return idx < int(list->count()) ? list->at(idx) : 0;  }

    QLayoutItem *next()
    { idx++; return current(); }

    QLayoutItem *takeCurrent()
    { return list->take( idx ); }

private:
    int idx;
    QPtrList<QLayoutItem> *list;
};

We must implement QLayout:iterator() to return a QLayoutIterator over this layout.

QLayoutIterator CardLayout::iterator()
{       
    return QLayoutIterator( new CardLayoutIterator(&list) );
}

addItem() implements the default placement strategy for layout items. It must be implemented. It is used by QLayout::add(), by the QLayout constructor that takes a layout as parent, and it is used to implement the auto-add feature. If your layout has advanced placement options that require parameters, you must provide extra access functions such as QGridLayout::addMultiCell().

void CardLayout::addItem( QLayoutItem *item )
{
    list.append( item );
}

The layout takes over responsibility of the items added. Since QLayoutItem does not inherit QObject, we must delete the items manually. The function QLayout::deleteAllItems() uses the iterator we defined above to delete all the items in the layout.

CardLayout::~CardLayout()
{
    deleteAllItems();
}

The setGeometry() function actually performs the layout. The rectangle supplied as an argument does not include margin(). If relevant, use spacing() as the distance between items.

void CardLayout::setGeometry( const QRect &rect )
{
    QLayout::setGeometry( rect );

    QPtrListIterator<QLayoutItem> it( list );
    if (it.count() == 0)
        return;

    QLayoutItem *item;

    int i = 0;

    int w = rect.width() - ( list.count() - 1 ) * spacing();
    int h = rect.height() - ( list.count() - 1 ) * spacing();

    while ( (item = it.current()) != 0 ) {
        ++it;
        QRect geom( rect.x() + i * spacing(), rect.y() + i * spacing(),
                    w, h );
        item->setGeometry( geom );
        ++i;
    }
}

sizeHint() and minimumSize() are normally very similar in implementation. The sizes returned by both functions should include spacing(), but not margin().

QSize CardLayout::sizeHint() const
{
    QSize s( 0, 0 );
    int n = list.count();
    if ( n > 0 )
        s = QSize( 100, 70 ); // start with a nice default size
    QPtrListIterator<QLayoutItem> it( list );
    QLayoutItem *item;
    while ( (item = it.current()) != 0 ) {
        ++it;
        s = s.expandedTo( item->minimumSize() );
    }
    return s + n * QSize( spacing(), spacing() );
}

QSize CardLayout::minimumSize() const
{
    QSize s( 0, 0 );
    int n = list.count();
    QPtrListIterator<QLayoutItem> it( list );
    QLayoutItem *item;
    while ( (item = it.current()) != 0 ) {
        ++it;
        s = s.expandedTo( item->minimumSize() );
    }
    return s + n * QSize( spacing(), spacing() );
}

Further Notes