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247 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/layout.doc:285 -->
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Writing your own layout manager</title>
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<font color="#004faf">Home</font></a>
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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>
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<p> Here we present an example in detail. The class CardLayout is inspired
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by the Java layout manager of the same name. It lays out the items
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(widgets or nested layouts) on top of each other, each item offset by
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<a href="qlayout.html#spacing">TQLayout::spacing</a>().
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<p> To write your own layout class, you must define the following:
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<ul>
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<li> A data structure to store the items handled by the layout. Each
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item is a <a href="qlayoutitem.html">TQLayoutItem</a>. We will use a
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<a href="qptrlist.html">TQPtrList</a> in this example.
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<li> <a href="qlayout.html#addItem">addItem()</a>, how to add items to
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the layout.
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<li> <a href="qlayout.html#setGeometry">setGeometry()</a>, how to perform
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the layout.
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<li> <a href="qlayoutitem.html#sizeHint">sizeHint()</a>, the preferred size
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of the layout.
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<li> <a href="qlayout.html#iterator">iterator()</a>, how to iterate over
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the layout.
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</ul>
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<p> In most cases, you will also implement <a href="qlayout.html#minimumSize">minimumSize</a>().
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<p> <h2> card.h
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</h2>
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<a name="1"></a><p> <pre>
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#ifndef CARD_H
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#define CARD_H
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#include <<a href="qlayout-h.html">qlayout.h</a>>
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#include <<a href="qptrlist-h.html">qptrlist.h</a>>
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class CardLayout : public <a href="qlayout.html">TQLayout</a>
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{
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public:
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CardLayout( <a href="qwidget.html">TQWidget</a> *parent, int dist )
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: <a href="qlayout.html">TQLayout</a>( parent, 0, dist ) {}
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CardLayout( <a href="qlayout.html">TQLayout</a>* parent, int dist)
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: <a href="qlayout.html">TQLayout</a>( parent, dist ) { }
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CardLayout( int dist )
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: <a href="qlayout.html">TQLayout</a>( dist ) {}
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~CardLayout();
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void addItem(TQLayoutItem *item);
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<a href="qsize.html">TQSize</a> sizeHint() const;
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<a href="qsize.html">TQSize</a> minimumSize() const;
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<a href="qlayoutiterator.html">TQLayoutIterator</a> iterator();
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void setGeometry(const <a href="qrect.html">TQRect</a> &rect);
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private:
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<a href="qptrlist.html">TQPtrList</a><TQLayoutItem> list;
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};
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#endif
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</pre>
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<p> <h3> card.cpp
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</h3>
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<a name="1-1"></a><p> <pre>
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#include "card.h"
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</pre>
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<p> First we define an iterator over the layout. Layout iterators are used
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internally by the layout system to handle deletion of widgets. They
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are also available for application programmers.
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<p> There are two different classes involved: <a href="qlayoutiterator.html">TQLayoutIterator</a> is the class
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that is visible to application programmers, it is <a href="shclass.html#explicitly-shared">explicitly shared</a>.
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The TQLayoutIterator contains a <a href="qglayoutiterator.html">TQGLayoutIterator</a> that does all the
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work. We must create a subclass of TQGLayoutIterator that knows how to
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iterate over our layout class.
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<p> In this case, we choose a simple implementation: we store an integer
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index into the list and a pointer to the list. Every <a href="qglayoutiterator.html">TQGLayoutIterator</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
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constructor. In our example we do not need a destructor.
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<p> <pre>
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class CardLayoutIterator : public <a href="qglayoutiterator.html">TQGLayoutIterator</a>
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{
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public:
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CardLayoutIterator( <a href="qptrlist.html">TQPtrList</a><TQLayoutItem> *l )
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: idx( 0 ), list( l ) {}
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<a href="qlayoutitem.html">TQLayoutItem</a> *current()
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{ return idx < int(list-><a href="qptrlist.html#count">count</a>()) ? list-><a href="qptrlist.html#at">at</a>(idx) : 0; }
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<a href="qlayoutitem.html">TQLayoutItem</a> *next()
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{ idx++; return current(); }
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<a href="qlayoutitem.html">TQLayoutItem</a> *takeCurrent()
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{ return list-><a href="qptrlist.html#take">take</a>( idx ); }
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private:
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int idx;
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<a href="qptrlist.html">TQPtrList</a><TQLayoutItem> *list;
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};
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</pre>
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<p> We must implement TQLayout:iterator() to return a <a href="qlayoutiterator.html">TQLayoutIterator</a> over
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this layout.
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<p> <pre>
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TQLayoutIterator CardLayout::iterator()
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{
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return TQLayoutIterator( new CardLayoutIterator(&list) );
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}
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</pre>
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<p> addItem() implements the default placement strategy for layout items.
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It must be implemented. It is used by <a href="qlayout.html#add">TQLayout::add</a>(), by the <a href="qlayout.html">TQLayout</a>
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constructor that takes a layout as parent, and it is used to implement
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the <a href="qlayout.html#autoAdd">auto-add</a> feature. If your layout
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has advanced placement options that require parameters, you must
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provide extra access functions such as <a href="qgridlayout.html#addMultiCell">TQGridLayout::addMultiCell</a>().
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<p> <pre>
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void CardLayout::addItem( <a href="qlayoutitem.html">TQLayoutItem</a> *item )
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{
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list.append( item );
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}
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</pre>
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<p> The layout takes over responsibility of the items added. Since
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<a href="qlayoutitem.html">TQLayoutItem</a> does not inherit <a href="qobject.html">TQObject</a>, we must delete the items
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manually. The function <a href="qlayout.html#deleteAllItems">TQLayout::deleteAllItems</a>() uses the iterator we
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defined above to delete all the items in the layout.
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<p> <pre>
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CardLayout::~CardLayout()
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{
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deleteAllItems();
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}
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</pre>
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<p> The setGeometry() function actually performs the layout. The rectangle
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supplied as an argument does not include margin(). If relevant, use
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spacing() as the distance between items.
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<p> <pre>
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void CardLayout::setGeometry( const <a href="qrect.html">TQRect</a> &rect )
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{
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TQLayout::<a href="qlayout.html#setGeometry">setGeometry</a>( rect );
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<a href="qptrlistiterator.html">TQPtrListIterator</a><TQLayoutItem> it( list );
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if (it.<a href="qptrlistiterator.html#count">count</a>() == 0)
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return;
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<a href="qlayoutitem.html">TQLayoutItem</a> *item;
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int i = 0;
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int w = rect.<a href="qrect.html#width">width</a>() - ( list.count() - 1 ) * spacing();
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int h = rect.<a href="qrect.html#height">height</a>() - ( list.count() - 1 ) * spacing();
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while ( (item = it.<a href="qptrlistiterator.html#current">current</a>()) != 0 ) {
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++it;
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<a href="qrect.html">TQRect</a> geom( rect.<a href="qrect.html#x">x</a>() + i * spacing(), rect.<a href="qrect.html#y">y</a>() + i * spacing(),
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w, h );
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item-><a href="qlayoutitem.html#setGeometry">setGeometry</a>( geom );
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++i;
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}
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}
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</pre>
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<p> sizeHint() and minimumSize() are normally very similar in
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implementation. The sizes returned by both functions should include
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spacing(), but not margin().
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<p> <pre>
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TQSize CardLayout::sizeHint() const
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{
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<a href="qsize.html">TQSize</a> s( 0, 0 );
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int n = list.count();
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if ( n > 0 )
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s = TQSize( 100, 70 ); // start with a nice default size
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<a href="qptrlistiterator.html">TQPtrListIterator</a><TQLayoutItem> it( list );
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<a href="qlayoutitem.html">TQLayoutItem</a> *item;
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while ( (item = it.<a href="qptrlistiterator.html#current">current</a>()) != 0 ) {
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++it;
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s = s.<a href="qsize.html#expandedTo">expandedTo</a>( item-><a href="qlayoutitem.html#minimumSize">minimumSize</a>() );
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}
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return s + n * TQSize( spacing(), spacing() );
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}
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TQSize CardLayout::minimumSize() const
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{
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<a href="qsize.html">TQSize</a> s( 0, 0 );
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int n = list.count();
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<a href="qptrlistiterator.html">TQPtrListIterator</a><TQLayoutItem> it( list );
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<a href="qlayoutitem.html">TQLayoutItem</a> *item;
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while ( (item = it.<a href="qptrlistiterator.html#current">current</a>()) != 0 ) {
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++it;
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s = s.<a href="qsize.html#expandedTo">expandedTo</a>( item-><a href="qlayoutitem.html#minimumSize">minimumSize</a>() );
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}
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return s + n * TQSize( spacing(), spacing() );
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}
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</pre>
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<p> <h2> Further Notes
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</h2>
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<a name="2"></a><p> This layout does not implement heightForWidth().
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<p> We ignore <a href="qlayoutitem.html#isEmpty">TQLayoutItem::isEmpty</a>(), this means that the layout will
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treat hidden widgets as visible.
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<p> For complex layouts, speed can be greatly increased by caching
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calculated values. In that case, implement <a href="qlayoutitem.html#invalidate">TQLayoutItem::invalidate</a>()
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to mark the cached data as dirty.
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<p> Calling <a href="qlayoutitem.html#sizeHint">TQLayoutItem::sizeHint</a>(), etc. may be expensive, so you should
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store the value in a local variable if you need it again later in the
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same function.
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<p> You should not call <a href="qlayoutitem.html#setGeometry">TQLayoutItem::setGeometry</a>() twice on the same item
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in the same function. That can be very expensive if the item has
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several child widgets, because it will have to do a complete layout
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every time. Instead, calculate the geometry and then set it. (This
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doesn't only apply to layouts, you should do the same if you implement
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your own resizeEvent().)
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<p>
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<!-- eof -->
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<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>TQt 3.3.8</div>
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</table></div></address></body>
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