Layout Management

The Implementation File (card.cpp)

 //#include "card.h"

First we define count() to fetch the number of items in the list.

 QLayoutItem *CardLayout::count() const
 {
         // QList::size() returns the number of QLayoutItems in the list
     return list.size();
 }

Then we define two functions that iterate over the layout: itemAt() and takeAt(). These functions are used internally by the layout system to handle deletion of widgets. They are also available for application programmers.

itemAt() returns the item at the given index. takeAt() removes the item at the given index, and returns it. In this case we use the list index as the layout index. In other cases where we have a more complex data structure, we may have to spend more effort defining a linear order for the items.

 QLayoutItem *CardLayout::itemAt(int idx) const
 {
     // QList::value() performs index checking, and returns 0 if we are
     // outside the valid range
     return list.value(idx);
 }

 QLayoutItem *CardLayout::takeAt(int idx)
 {
     // QList::take does not do index checking
     return idx >= 0 && idx < list.size() ? list.takeAt(idx) : 0;
 }

addItem() implements the default placement strategy for layout items. This function must be implemented. It is used by QLayout::add(), by the QLayout constructor that takes a layout as parent. If your layout has advanced placement options that require parameters, you must provide extra access functions such as the row and column spanning overloads of QGridLayout::addItem(), QGridLayout::addWidget(), and QGridLayout::addLayout().

 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. In the destructor, we remove each item from the list using takeAt(), and then delete it.

 CardLayout::~CardLayout()
 {
      QLayoutItem *item;
      while ((item = takeAt(0)))
          delete item;
 }

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 &r)
 {
     QLayout::setGeometry(r);

     if (list.size() == 0)
         return;

     int w = r.width() - (list.count() - 1) * spacing();
     int h = r.height() - (list.count() - 1) * spacing();
     int i = 0;
     while (i < list.size()) {
         QLayoutItem *o = list.at(i);
         QRect geom(r.x() + i * spacing(), r.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
     int i = 0;
     while (i < n) {
         QLayoutItem *o = list.at(i);
         s = s.expandedTo(o->sizeHint());
         ++i;
     }
     return s + n*QSize(spacing(), spacing());
 }

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