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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Qt Toolkit - Writing your own layout manager</title><style type="text/css"><!--
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<h1 align=center> Writing your own layout manager</h1><br clear="all">
Here we go through an example in detail. The class <code>CardLayout</code> 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#372777">spacing()</a>.
<p>
To write your own layout class, you must define the following:
<ul>
<li>A data structure to store the items
handle by the layout.
Each item is a <a href="qlayoutitem.html">QLayoutItem</a>.
We will just use a QList in this example.
<li><a href="qlayout.html#db69d5">addItem()</a>, how to add items
to the layout.
<li><a href="qlayout.html#f1f752">setGeometry()</a>, how to do the
layout.
<li><a href="qlayoutitem.html#48b5b2">sizeHint()</a>, the preferred size of the
layout.
<li><a href="qlayout.html#3d3ab8">iterator()</a>, how to iterate over the
layout.
</ul>
<p>
In most cases, you will also implement <a href="qlayout.html#8e1dc2">minimumSize()</a>.
<p>
<h3> card.h </h3>
<p>
<pre>class CardLayout : public QLayout
{
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(<a href="qlayoutitem.html">QLayoutItem</a> *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 QRect &rect);
private:
<a href="qlist.html">QList</a><<a href="qlayoutitem.html">QLayoutItem</a>> list;
};
</pre>
<p>
<h3> card.cpp </h3>
<pre></pre>
<p>
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.
<p>
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.
<p>
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 <a href="qglayoutiterator.html#cdab98">current()</a>,
<a href="qglayoutiterator.html#e03723">next()</a> and
<a href="qglayoutiterator.html#fa72ce">takeCurrent()</a>, as well as
a constructor. In our example we do not need a destructor.
<p>
<pre>class CardLayoutIterator :public QGLayoutIterator
{
public:
CardLayoutIterator(<a href="qlist.html">QList</a><<a href="qlayoutitem.html">QLayoutItem</a>> *l)
: idx(0), list(l) {}
<a href="qlayoutitem.html">QLayoutItem</a> *current()
{ return idx < int(list-><a href="qlist.html#359d9f">count</a>()) ? list-><a href="qlist.html#0e7e42">at</a>(idx) : 0; }
<a href="qlayoutitem.html">QLayoutItem</a> *next()
{ idx++; return current(); }
<a href="qlayoutitem.html">QLayoutItem</a> *takeCurrent()
{ return list-><a href="qlist.html#0639ad">take</a>( idx ); }
private:
int idx;
<a href="qlist.html">QList</a><<a href="qlayoutitem.html">QLayoutItem</a>> *list;
};
</pre>
<p>
We must implement QLayout:iterator() to return a QLayoutIterator over
this layout.
<p>
<pre><a href="qlayoutiterator.html">QLayoutIterator</a> CardLayout::iterator()
{
return QLayoutIterator( new CardLayoutIterator(&list) );
}
</pre>
<p>
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 <a href="qlayout.html#d02056">auto-add</a> feature. If your layout
has advanced placement options that require parameters, you will have to
provide extra access functions like eg. <a href="qgridlayout.html#67545e">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
QLayoutItem does not inherit QObject, we must delete the items
manually. The function <a href="qlayout.html#3349c9">deleteAllItems()</a>
uses the iterator we defined above to delete all 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 QRect &rect)
{
<a href="qlayout.html#f1f752">QLayout::setGeometry</a>(rect);
<a href="qlistiterator.html">QListIterator</a><<a href="qlayoutitem.html">QLayoutItem</a>> it(list);
if (it.<a href="qlistiterator.html#e61e8b">count</a>() == 0)
return;
<a href="qlayoutitem.html">QLayoutItem</a> *o;
int i = 0;
int w = rect.width() - (list.count() - 1) * spacing();
int h = rect.height() - (list.count() - 1) * spacing();
while ((o=it.<a href="qlistiterator.html#e58f06">current</a>()) != 0)
{
++it;
<a href="qrect.html">QRect</a> geom(rect.x() + i * spacing(), rect.y() + i * spacing(),
w, h );
o-><a href="qlayoutitem.html#0c6928">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><a href="qsize.html">QSize</a> CardLayout::sizeHint() const
{
<a href="qsize.html">QSize</a> s(0,0);
int n = list.count();
if ( n > 0 )
s = QSize(100,70); //start with a nice default size
<a href="qlistiterator.html">QListIterator</a><<a href="qlayoutitem.html">QLayoutItem</a>> it(list);
<a href="qlayoutitem.html">QLayoutItem</a> *o;
while ( (o=it.<a href="qlistiterator.html#e58f06">current</a>()) != 0 ) {
++it;
s = s.<a href="qsize.html#a74829">expandedTo</a>( o-><a href="qlayoutitem.html#83838c">minimumSize</a>() );
}
return s + n*QSize(spacing(),spacing());
}
<a href="qsize.html">QSize</a> CardLayout::minimumSize() const
{
<a href="qsize.html">QSize</a> s(0,0);
int n = list.count();
<a href="qlistiterator.html">QListIterator</a><<a href="qlayoutitem.html">QLayoutItem</a>> it(list);
<a href="qlayoutitem.html">QLayoutItem</a> *o;
while ( (o=it.<a href="qlistiterator.html#e58f06">current</a>()) != 0 ) {
++it;
s = s.<a href="qsize.html#a74829">expandedTo</a>( o-><a href="qlayoutitem.html#83838c">minimumSize</a>() );
}
return s + n*QSize(spacing(),spacing());
}
</pre>
<p>
<h2>Further comments</h2>
<p>
This layout does not implement heightForWidth().
<p>
We ignore QLayoutItem::isEmpty(), this means that the layout will treat
hidden widgets as visible.
<p>
For complex layouts, speed can be greatly increased by caching calculated
values and/or data structures. In that case, implement <a href="qlayoutitem.html#91d950">invalidate()</a> to mark the cached
data as dirty.
<p>
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.
<p>
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()).
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<td>Copyright 2001 Trolltech<td><a href="http://www.trolltech.com/trademarks.html">Trademarks</a>
<td align="right"><div align="right">Qt version 2.3.2</div>
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