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<title>Writing your own layout manager</title>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Writing your own layout manager</h1>

  
<p> Here we present 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
<a href="qlayout.html#spacing">QLayout::spacing</a>().
<p> To write your own layout class, you must define the following:
<ul>
<li> A data structure to store the items handled by the layout. Each
item is a <a href="qlayoutitem.html">QLayoutItem</a>. We will use a
<a href="qptrlist.html">QPtrList</a> in this example.
<li> <a href="qlayout.html#addItem">addItem()</a>, how to add items to
the layout.
<li> <a href="qlayout.html#setGeometry">setGeometry()</a>, how to perform
the layout.
<li> <a href="qlayoutitem.html#sizeHint">sizeHint()</a>, the preferred size
of the layout.
<li> <a href="qlayout.html#iterator">iterator()</a>, how to iterate over
the layout.
</ul>
<p> In most cases, you will also implement <a href="qlayout.html#minimumSize">minimumSize</a>().
<p> <h2> card.h
</h2>
<a name="1"></a><p> <pre>
#ifndef CARD_H
#define CARD_H

#include &lt;<a href="qlayout-h.html">qlayout.h</a>&gt;
#include &lt;<a href="qptrlist-h.html">qptrlist.h</a>&gt;

class CardLayout : public <a href="qlayout.html">QLayout</a>
{
public:
    CardLayout( <a href="qwidget.html">QWidget</a> *parent, int dist )
        : <a href="qlayout.html">QLayout</a>( parent, 0, dist ) {}
    CardLayout( <a href="qlayout.html">QLayout</a>* parent, int dist)
        : <a href="qlayout.html">QLayout</a>( parent, dist ) { }
    CardLayout( int dist )
        : <a href="qlayout.html">QLayout</a>( dist ) {}
    ~CardLayout();

    void addItem(QLayoutItem *item);
    <a href="qsize.html">QSize</a> sizeHint() const;
    <a href="qsize.html">QSize</a> minimumSize() const;
    <a href="qlayoutiterator.html">QLayoutIterator</a> iterator();
    void setGeometry(const <a href="qrect.html">QRect</a> &amp;rect);

private:
    <a href="qptrlist.html">QPtrList</a>&lt;QLayoutItem&gt; list;
};

#endif
</pre>
 
<p> <h3> card.cpp
</h3>
<a name="1-1"></a><p> <pre>
#include "card.h"
</pre>
 
<p> 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.
<p> There are two different classes involved: <a href="qlayoutiterator.html">QLayoutIterator</a> is the class
that is visible to application programmers, it is <a href="shclass.html#explicitly-shared">explicitly shared</a>.
The QLayoutIterator contains a <a href="qglayoutiterator.html">QGLayoutIterator</a> that does all the
work. We must create a subclass of QGLayoutIterator that knows how to
iterate over our layout class.
<p> In this case, we choose a simple implementation: we store an integer
index into the list and a pointer to the list. Every <a href="qglayoutiterator.html">QGLayoutIterator</a> subclass must implement <a href="qglayoutiterator.html#current">current</a>(), <a href="qglayoutiterator.html#next">next</a>() and <a href="qglayoutiterator.html#takeCurrent">takeCurrent</a>(), as well as a
constructor. In our example we do not need a destructor.
<p> <pre>
class CardLayoutIterator : public <a href="qglayoutiterator.html">QGLayoutIterator</a>
{
public:
    CardLayoutIterator( <a href="qptrlist.html">QPtrList</a>&lt;QLayoutItem&gt; *l )
        : idx( 0 ), list( l ) {}

    <a href="qlayoutitem.html">QLayoutItem</a> *current()
    { return idx &lt; int(list-&gt;<a href="qptrlist.html#count">count</a>()) ? list-&gt;<a href="qptrlist.html#at">at</a>(idx) : 0;  }

    <a href="qlayoutitem.html">QLayoutItem</a> *next()
    { idx++; return current(); }

    <a href="qlayoutitem.html">QLayoutItem</a> *takeCurrent()
    { return list-&gt;<a href="qptrlist.html#take">take</a>( idx ); }

private:
    int idx;
    <a href="qptrlist.html">QPtrList</a>&lt;QLayoutItem&gt; *list;
};
</pre>
 
<p> We must implement QLayout:iterator() to return a <a href="qlayoutiterator.html">QLayoutIterator</a> over
this layout.
<p> <pre>
QLayoutIterator CardLayout::iterator()
{       
    return QLayoutIterator( new CardLayoutIterator(&amp;list) );
}
</pre>
 
<p> addItem() implements the default placement strategy for layout items.
It must be implemented. It is used by <a href="qlayout.html#add">QLayout::add</a>(), by the <a href="qlayout.html">QLayout</a>
constructor that takes a layout as parent, and it is used to implement
the <a href="qlayout.html#autoAdd">auto-add</a> feature. If your layout
has advanced placement options that require parameters, you must
provide extra access functions such as <a href="qgridlayout.html#addMultiCell">QGridLayout::addMultiCell</a>().
<p> <pre>
void CardLayout::addItem( <a href="qlayoutitem.html">QLayoutItem</a> *item )
{
    list.append( item );
}
</pre>
 
<p> The layout takes over responsibility of the items added. Since
<a href="qlayoutitem.html">QLayoutItem</a> does not inherit <a href="qobject.html">QObject</a>, we must delete the items
manually. The function <a href="qlayout.html#deleteAllItems">QLayout::deleteAllItems</a>() uses the iterator we
defined above to delete all the items in the layout.
<p> <pre>
CardLayout::~CardLayout()
{
    deleteAllItems();
}
</pre>
 
<p> 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.
<p> <pre>
void CardLayout::setGeometry( const <a href="qrect.html">QRect</a> &amp;rect )
{
    QLayout::<a href="qlayout.html#setGeometry">setGeometry</a>( rect );

    <a href="qptrlistiterator.html">QPtrListIterator</a>&lt;QLayoutItem&gt; it( list );
    if (it.<a href="qptrlistiterator.html#count">count</a>() == 0)
        return;

    <a href="qlayoutitem.html">QLayoutItem</a> *item;

    int i = 0;

    int w = rect.<a href="qrect.html#width">width</a>() - ( list.count() - 1 ) * spacing();
    int h = rect.<a href="qrect.html#height">height</a>() - ( list.count() - 1 ) * spacing();

    while ( (item = it.<a href="qptrlistiterator.html#current">current</a>()) != 0 ) {
        ++it;
        <a href="qrect.html">QRect</a> geom( rect.<a href="qrect.html#x">x</a>() + i * spacing(), rect.<a href="qrect.html#y">y</a>() + i * spacing(),
                    w, h );
        item-&gt;<a href="qlayoutitem.html#setGeometry">setGeometry</a>( geom );
        ++i;
    }
}
</pre>
 
<p> sizeHint() and minimumSize() are normally very similar in
implementation. The sizes returned by both functions should include
spacing(), but not margin().
<p> <pre>
QSize CardLayout::sizeHint() const
{
    <a href="qsize.html">QSize</a> s( 0, 0 );
    int n = list.count();
    if ( n &gt; 0 )
        s = QSize( 100, 70 ); // start with a nice default size
    <a href="qptrlistiterator.html">QPtrListIterator</a>&lt;QLayoutItem&gt; it( list );
    <a href="qlayoutitem.html">QLayoutItem</a> *item;
    while ( (item = it.<a href="qptrlistiterator.html#current">current</a>()) != 0 ) {
        ++it;
        s = s.<a href="qsize.html#expandedTo">expandedTo</a>( item-&gt;<a href="qlayoutitem.html#minimumSize">minimumSize</a>() );
    }
    return s + n * QSize( spacing(), spacing() );
}

QSize CardLayout::minimumSize() const
{
    <a href="qsize.html">QSize</a> s( 0, 0 );
    int n = list.count();
    <a href="qptrlistiterator.html">QPtrListIterator</a>&lt;QLayoutItem&gt; it( list );
    <a href="qlayoutitem.html">QLayoutItem</a> *item;
    while ( (item = it.<a href="qptrlistiterator.html#current">current</a>()) != 0 ) {
        ++it;
        s = s.<a href="qsize.html#expandedTo">expandedTo</a>( item-&gt;<a href="qlayoutitem.html#minimumSize">minimumSize</a>() );
    }
    return s + n * QSize( spacing(), spacing() );
}
</pre>
 
<p> <h2> Further Notes
</h2>
<a name="2"></a><p> This layout does not implement heightForWidth().
<p> We ignore <a href="qlayoutitem.html#isEmpty">QLayoutItem::isEmpty</a>(), this means that the layout will
treat hidden widgets as visible.
<p> For complex layouts, speed can be greatly increased by caching
calculated values. In that case, implement <a href="qlayoutitem.html#invalidate">QLayoutItem::invalidate</a>()
to mark the cached data as dirty.
<p> Calling <a href="qlayoutitem.html#sizeHint">QLayoutItem::sizeHint</a>(), etc. may be expensive, so you should
store the value in a local variable if you need it again later in the
same function.
<p> You should not call <a href="qlayoutitem.html#setGeometry">QLayoutItem::setGeometry</a>() 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
doesn't only apply to layouts, you should do the same if you implement
your own resizeEvent().)
<p> 
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