Writing your own layout manager

Here we go through an example in detail. The class CardLayout is inspired by the Java layout manager of the same name. It lays out the items (widgets or nested layouts) on top of each other, each item offset by spacing().

To write your own layout class, you must define the following:

• A data structure to store the items handle by the layout. Each item is a QLayoutItem. We will just use a QList in this example.
• addItem(), how to add items to the layout.
• setGeometry(), how to do the layout.
• sizeHint(), the preferred size of the layout.
• iterator(), how to iterate over the layout.

In most cases, you will also implement minimumSize().

card.h

class CardLayout : public QLayout
{
public:
    CardLayout( QWidget *parent, int dist )
        : QLayout( parent, 0, dist ) {}
    CardLayout( QLayout* parent, int dist)
        : QLayout( parent, dist ) {}
    CardLayout( int dist )
        : QLayout( dist ) {}
    ~CardLayout();

    void addItem(QLayoutItem *item);
    QSize sizeHint() const;
    QSize minimumSize() const;
    QLayoutIterator iterator();
    void setGeometry(const QRect &rect);

private:
    QList<QLayoutItem> list;
};

card.cpp

We first 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(QList<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;
  QList<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 will have to provide extra access functions like eg. 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 deleteAllItems() uses the iterator we defined above to delete all 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);

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

    QLayoutItem *o;

    int i = 0;

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

    while ((o=it.current()) != 0)
        {
            ++it;
            QRect geom(rect.x() + i * spacing(), rect.y() + i * spacing(),
                       w, h );
            o->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
    QListIterator<QLayoutItem> it(list);
    QLayoutItem *o;
    while ( (o=it.current()) != 0 ) {
        ++it;
        s = s.expandedTo( o->minimumSize() );
    }
    return s + n*QSize(spacing(),spacing());
}

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

Further comments

This layout does not implement heightForWidth().

We ignore QLayoutItem::isEmpty(), this means that the layout will treat hidden widgets as visible.

For complex layouts, speed can be greatly increased by caching calculated values and/or data structures. In that case, implement invalidate() to mark the cached data as dirty.

Calling QLayoutItem::sizeHint(), etc. may be expensive, so you should store the value in a local variable if you need it again later in the same function.

You should not call QLayoutItem::setGeometry() twice on the same item in the same function. That can be very expensive if the item has several child widgets, because it will have to do a complete layout every time. Instead, calculate the geometry and then set it. (This does not only apply to layouts, you should do the same if you implement your own resizeEvent()).