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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>Layout Classes</h1>
<p> The Qt layout system provides a simple and powerful way of specifying
the layout of child widgets.
<p> By specifying the logical layout once, you get the following benefits:
<ul>
<li> Positioning of child widgets.
<li> Sensible default sizes for top-level widgets.
<li> Sensible minimum sizes for top-level widgets.
<li> Resize handling.
<li> Automatic update when contents change:
<ul>
<li> Font size, text or other contents of subwidgets.
<li> Hiding or showing a subwidget.
<li> Removal of subwidget.
</ul>
</ul>
<p> Qt's layout classes were designed for hand-written C++ code, so
they're easy to understand and use.
<p> The disadvantage of hand-written layout code is that it isn't very
convenient when you're experimenting with the design of a form and you
have to go through the compile, link and run cycle for each change.
Our solution is <a href="designer-manual.html">Qt Designer</a>, a GUI
visual design tool which makes it fast and easy to experiment with
layouts and which generates the C++ layout code for you.
<p> <h2> Layout Widgets
</h2>
<a name="1"></a><p> The easiest way to give your widgets a good layout is to use the
layout widgets: <a href="qhbox.html">QHBox</a>, <a href="qvbox.html">QVBox</a> and <a href="qgrid.html">QGrid</a>. A layout widget
automatically lays out its child widgets in the order they are
constructed. To create more complex layouts, you can nest layout
widgets inside each other. (Note that <a href="qwidget.html">QWidget</a> does not have a
layout by default, you must add one if you want to lay out widgets
inside a <a href="qwidget.html">QWidget</a>.)
<p> <ul>
<li> A <a href="qhbox.html">QHBox</a> lays out its child widgets in a horizontal row, left to right.
<p> <center><img src="qhbox-m.png" alt="Horizontal box with five child widgets"></center>
<p> <li> A <a href="qvbox.html">QVBox</a> lays out its child widgets in a vertical column, top to bottom.
<p> <center><img src="qvbox-m.png" alt="Vertical box with five child widgets"></center>
<p> <li> A <a href="qgrid.html">QGrid</a> lays out its child widgets in a two dimensional grid.
You can specify how many columns the grid has, and it is populated left to
right, beginning a new row when the previous row is full. The grid is
fixed; the child widgets will not flow to other rows as the widget is
resized.
</ul>
<p> <center><img src="qgrid-m.png" alt="Two-column grid with five child widgets"></center>
<p> The grid shown above can be produced by the following code:
<pre>
<a href="qgrid.html">QGrid</a> *mainGrid = new <a href="qgrid.html">QGrid</a>( 2 ); // a 2 x n grid
new <a href="qlabel.html">QLabel</a>( "One", mainGrid );
new <a href="qlabel.html">QLabel</a>( "Two", mainGrid );
new <a href="qlabel.html">QLabel</a>( "Three", mainGrid );
new <a href="qlabel.html">QLabel</a>( "Four", mainGrid );
new <a href="qlabel.html">QLabel</a>( "Five", mainGrid );
</pre>
<p> You can adjust the layout to some extent by calling
<a href="qwidget.html#setMinimumSize">QWidget::setMinimumSize</a>() or <a href="qwidget.html#setFixedSize">QWidget::setFixedSize</a>() on the child widgets.
<p> <h2> Adding Widgets to a Layout
</h2>
<a name="2"></a><p> When you add widgets to a layout the layout process works as follows:
<ol type=1>
<li> All the widgets will initially be allocated an amount of space in
accordance with their <a href="qwidget.html#sizePolicy">QWidget::sizePolicy</a>().
<li> If any of the widgets have stretch factors set, with a value
greater than zero, then they are allocated space in proportion to
their <a href="#stretch">stretch factor</a>.
<li> If any of the widgets have stretch factors set to zero they will
only get more space if no other widgets want the space. Of these,
space is allocated to widgets with an <tt>Expanding</tt> size policy first.
<li> Any widgets that are allocated less space than their minimum size
(or minimum size hint if no minimum size is specified) are allocated
this minimum size they require. (Widgets don't have to have a minimum
size or minimum size hint in which case the strech factor is their
determining factor.)
<li> Any widgets that are allocated more space than their maximum size
are allocated the maximum size space they require. (Widgets don't have
to have a maximum size in which case the strech factor is their
determining factor.)
</ol>
<p> <a name="stretch"></a>
<h3> Stretch Factors
<!-- index stretch factor --><a name="stretch-factor"></a>
</h3>
<a name="2-1"></a><p> Widgets are normally created without any stretch factor set. When they
are laid out in a layout the widgets are given a share of space in
accordance with their <a href="qwidget.html#sizePolicy">QWidget::sizePolicy</a>() or their minimum size hint
whichever is the greater. Stretch factors are used to change how much
space widgets are given in proportion to one another.
<p> If we have three widgets laid out using a <a href="qhbox.html">QHBox</a> with no stretch
factors set we will get a layout like this:
<p> <center><img src="layout1.png" alt="3 widgets in a row"></center>
<p> If we apply stretch factors to each widget, they will be laid out in
proportion (but never less than their minimum size hint), e.g.
<p> <center><img src="layout2.png" alt="3 stretch factored widgets in a row"></center>
<p> <h2> <a href="qlayout.html">QLayout</a> subclassing
</h2>
<a name="3"></a><p> If you need more control over the layout, use a <a href="qlayout.html">QLayout</a> subclass. The layout classes included in Qt are <a href="qgridlayout.html">QGridLayout</a> and <a href="qboxlayout.html">QBoxLayout</a>. (<a href="qhboxlayout.html">QHBoxLayout</a> and <a href="qvboxlayout.html">QVBoxLayout</a> are trivial subclasses of <a href="qboxlayout.html">QBoxLayout</a>,
that are simpler to use and make the code easier to read.)
<p> When you use a layout, you must insert each child both into its parent
widget (done in the constructor) and into its layout (typically done
with a function called addWidget()). This way, you can give layout
parameters for each widget, specifying properties like alignment,
stretch, and placement.
<p> The following code makes a grid like the one above, with a couple of
improvements:
<pre>
<a href="qwidget.html">QWidget</a> *main = new <a href="qwidget.html">QWidget</a>;
// make a 1x1 grid; it will auto-expand
<a href="qgridlayout.html">QGridLayout</a> *grid = new <a href="qgridlayout.html">QGridLayout</a>( main, 1, 1 );
// add the first four widgets with (row, column) addressing
grid-><a href="qgridlayout.html#addWidget">addWidget</a>( new <a href="qlabel.html">QLabel</a>( "One", main ), 0, 0 );
grid-><a href="qgridlayout.html#addWidget">addWidget</a>( new <a href="qlabel.html">QLabel</a>( "Two", main ), 0, 1 );
grid-><a href="qgridlayout.html#addWidget">addWidget</a>( new <a href="qlabel.html">QLabel</a>( "Three", main ), 1, 0 );
grid-><a href="qgridlayout.html#addWidget">addWidget</a>( new <a href="qlabel.html">QLabel</a>( "Four", main ), 1, 1 );
// add the last widget on row 2, spanning from column 0 to
// column 1, and center aligned
grid-><a href="qgridlayout.html#addMultiCellWidget">addMultiCellWidget</a>( new <a href="qlabel.html">QLabel</a>( "Five", main ), 2, 2, 0, 1,
Qt::AlignCenter );
// let the ratio between the widths of columns 0 and 1 be 2:3
grid-><a href="qgridlayout.html#setColStretch">setColStretch</a>( 0, 2 );
grid-><a href="qgridlayout.html#setColStretch">setColStretch</a>( 1, 3 );
</pre>
<p> You can insert layouts inside a layout by giving the parent layout as
a parameter in the constructor.
<pre>
<a href="qwidget.html">QWidget</a> *main = new <a href="qwidget.html">QWidget</a>;
<a href="qlineedit.html">QLineEdit</a> *field = new <a href="qlineedit.html">QLineEdit</a>( main );
<a href="qpushbutton.html">QPushButton</a> *ok = new <a href="qpushbutton.html">QPushButton</a>( "OK", main );
<a href="qpushbutton.html">QPushButton</a> *cancel = new <a href="qpushbutton.html">QPushButton</a>( "Cancel", main );
<a href="qlabel.html">QLabel</a> *label = new <a href="qlabel.html">QLabel</a>( "Write once, compile everywhere.", main );
// a layout on a widget
<a href="qvboxlayout.html">QVBoxLayout</a> *vbox = new <a href="qvboxlayout.html">QVBoxLayout</a>( main );
vbox-><a href="qboxlayout.html#addWidget">addWidget</a>( label );
vbox-><a href="qboxlayout.html#addWidget">addWidget</a>( field );
// a layout inside a layout
<a href="qhboxlayout.html">QHBoxLayout</a> *buttons = new <a href="qhboxlayout.html">QHBoxLayout</a>( vbox );
buttons-><a href="qboxlayout.html#addWidget">addWidget</a>( ok );
buttons-><a href="qboxlayout.html#addWidget">addWidget</a>( cancel );
</pre>
If you are not satisfied with the default placement, you can create
the layout without a parent and then insert it with addLayout().
The inner layout then becomes a child of the layout it is inserted
into.
<p> <h2> Custom Layouts
</h2>
<a name="4"></a><p> If the built-in layout classes are not sufficient, you can define your
own. You must make a subclass of <a href="qlayout.html">QLayout</a> that handles resizing and
size calculations, as well as a subclass of <a href="qglayoutiterator.html">QGLayoutIterator</a> to
iterate over your layout class.
<p> See the <a href="customlayout.html">Custom Layout</a> page for an
in-depth description.
<p> <h2> Custom Widgets In Layouts
</h2>
<a name="5"></a><p> When you make your own widget class, you should also communicate its
layout properties. If the widget has a <a href="qlayout.html">QLayout</a>, this is already taken
care of. If the widget does not have any child widgets, or uses manual
layout, you should reimplement the following <a href="qwidget.html">QWidget</a> member functions:
<p> <ul>
<li> <a href="qwidget.html#sizeHint">QWidget::sizeHint</a>() returns the preferred size of the widget.
<li> <a href="qwidget.html#minimumSizeHint">QWidget::minimumSizeHint</a>() returns the smallest size the widget can have.
<li> <a href="qwidget.html#sizePolicy">QWidget::sizePolicy</a>() returns a <a href="qsizepolicy.html">QSizePolicy</a>; a value describing
the space requirements of the widget.
</ul>
<p> Call <a href="qwidget.html#updateGeometry">QWidget::updateGeometry</a>() whenever the size hint, minimum size
hint or size policy changes. This will cause a layout recalculation.
Multiple calls to updateGeometry() will only cause one recalculation.
<p> If the preferred height of your widget depends on its actual width
(e.g. a label with automatic word-breaking), set the <a href="qsizepolicy.html#hasHeightForWidth">hasHeightForWidth</a>() flag in
<a href="qwidget.html#sizePolicy">sizePolicy</a>(), and reimplement <a href="qwidget.html#heightForWidth">QWidget::heightForWidth</a>().
<p> Even if you implement heightForWidth(), it is still necessary to
provide a good sizeHint(). The sizeHint() provides the preferred width
of the widget, and it is used by <a href="qlayout.html">QLayout</a> subclasses that do not
support heightForWidth() (both <a href="qgridlayout.html">QGridLayout</a> and <a href="qboxlayout.html">QBoxLayout</a> support it).
<p> For further guidance when implementing these functions, see their
implementations in existing Qt classes that have similar layout
requirements to your new widget.
<p> <h2> Manual Layout
</h2>
<a name="6"></a><p> If you are making a one-of-a-kind special layout, you can also make a
custom widget as described above. Reimplement <a href="qwidget.html#resizeEvent">QWidget::resizeEvent</a>()
to calculate the required distribution of sizes and call <a href="qwidget.html#setGeometry">setGeometry</a>() on each child.
<p> The widget will get an event with <a href="qevent.html#type">type</a>
<tt>LayoutHint</tt> when the layout needs to be recalculated. Reimplement
<a href="qwidget.html#event">QWidget::event</a>() to be notified of <tt>LayoutHint</tt> events.
<p> <h2> Layout Issues
</h2>
<a name="7"></a><p> The use of rich text in a label widget can introduce some problems to
the layout of its parent widget. Problems occur due to the way rich text
is handled by Qt's layout managers when the label is word wrapped.
In certain cases the parent layout is put into QLayout::FreeResize mode,
meaning that it will not adapt the layout of its contents to fit inside
small sized windows, or even prevent the user from making the
window too small to be usable. This can be overcome by subclassing
the problematic widgets, and implementing suitable sizeHint() and
minimumSizeHint() functions.
<p>
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<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>
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