<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/layout.doc:285 --> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Writing your own layout manager</title> <style type="text/css"><!-- fn { margin-left: 1cm; text-indent: -1cm; } a:link { color: #004faf; text-decoration: none } a:visited { color: #672967; text-decoration: none } body { background: #ffffff; color: black; } --></style> </head> <body> <table border="0" cellpadding="0" cellspacing="0" width="100%"> <tr bgcolor="#E5E5E5"> <td valign=center> <a href="index.html"> <font color="#004faf">Home</font></a> | <a href="classes.html"> <font color="#004faf">All Classes</font></a> | <a href="mainclasses.html"> <font color="#004faf">Main Classes</font></a> | <a href="annotated.html"> <font color="#004faf">Annotated</font></a> | <a href="groups.html"> <font color="#004faf">Grouped Classes</font></a> | <a href="functions.html"> <font color="#004faf">Functions</font></a> </td> <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 <<a href="qlayout-h.html">qlayout.h</a>> #include <<a href="qptrlist-h.html">qptrlist.h</a>> 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> &rect); private: <a href="qptrlist.html">QPtrList</a><QLayoutItem> 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><QLayoutItem> *l ) : idx( 0 ), list( l ) {} <a href="qlayoutitem.html">QLayoutItem</a> *current() { return idx < int(list-><a href="qptrlist.html#count">count</a>()) ? list-><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-><a href="qptrlist.html#take">take</a>( idx ); } private: int idx; <a href="qptrlist.html">QPtrList</a><QLayoutItem> *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(&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> &rect ) { QLayout::<a href="qlayout.html#setGeometry">setGeometry</a>( rect ); <a href="qptrlistiterator.html">QPtrListIterator</a><QLayoutItem> 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-><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 > 0 ) s = QSize( 100, 70 ); // start with a nice default size <a href="qptrlistiterator.html">QPtrListIterator</a><QLayoutItem> 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-><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><QLayoutItem> 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-><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> <!-- eof --> <p><address><hr><div align=center> <table width=100% cellspacing=0 border=0><tr> <td>Copyright © 2007 <a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a> <td align=right><div align=right>Qt 3.3.8</div> </table></div></address></body> </html>