path: root/doc/layout.doc

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** Explanation of the layout subsystem
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/*!
\page layout.html

\title Layout Classes

The Qt layout system provides a simple and powerful way of specifying
the layout of child widgets.

By specifying the logical layout once, you get the following benefits:
\list
\i Positioning of child widgets.
\i Sensible default sizes for top-level widgets.
\i Sensible minimum sizes for top-level widgets.
\i Resize handling.
\i Automatic update when contents change:
	\list
	\i Font size, text or other contents of subwidgets.
	\i Hiding or showing a subwidget.
	\i Removal of subwidget.
	\endlist
\endlist

Qt's layout classes were designed for hand-written C++ code, so
they're easy to understand and use. 

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 \link designer-manual.book Qt Designer\endlink, a GUI
visual design tool which makes it fast and easy to experiment with
layouts and which generates the C++ layout code for you.

\section1 Layout Widgets

The easiest way to give your widgets a good layout is to use the
layout widgets: \l QHBox, \l QVBox and \l QGrid. 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 \l QWidget does not have a
layout by default, you must add one if you want to lay out widgets
inside a \l QWidget.)

\list
\i A \l QHBox lays out its child widgets in a horizontal row, left to right.

   \img qhbox-m.png Horizontal box with five child widgets

\i A \l QVBox lays out its child widgets in a vertical column, top to bottom.

   \img qvbox-m.png Vertical box with five child widgets

\i A \l QGrid 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.
\endlist

   \img qgrid-m.png Two-column grid with five child widgets

The grid shown above can be produced by the following code:
\code
    QGrid *mainGrid = new QGrid( 2 ); // a 2 x n grid
    new QLabel( "One", mainGrid );
    new QLabel( "Two", mainGrid );
    new QLabel( "Three", mainGrid );
    new QLabel( "Four", mainGrid );
    new QLabel( "Five", mainGrid );
\endcode

You can adjust the layout to some extent by calling
QWidget::setMinimumSize() or QWidget::setFixedSize() on the child widgets.

\section1 Adding Widgets to a Layout

When you add widgets to a layout the layout process works as follows:
\list 1
\i All the widgets will initially be allocated an amount of space in
accordance with their QWidget::sizePolicy(). 
\i If any of the widgets have stretch factors set, with a value
greater than zero, then they are allocated space in proportion to
their \link #stretch stretch factor\endlink. 
\i 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 \c Expanding size policy first.
\i 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.)
\i 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.)
\endlist

\target stretch
\section2 Stretch Factors
\keyword stretch factor

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 QWidget::sizePolicy() 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.

If we have three widgets laid out using a QHBox with no stretch
factors set we will get a layout like this:

\img layout1.png 3 widgets in a row

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.

\img layout2.png 3 stretch factored widgets in a row


\section1 QLayout subclassing

If you need more control over the layout, use a \link QLayout
QLayout\endlink subclass. The layout classes included in Qt are \link
QGridLayout QGridLayout\endlink and \link QBoxLayout
QBoxLayout\endlink. (\link QHBoxLayout  QHBoxLayout\endlink and \link
QVBoxLayout  QVBoxLayout\endlink are trivial subclasses of QBoxLayout,
that are simpler to use and make the code easier to read.)

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.

The following code makes a grid like the one above, with a couple of
improvements:
\code
    QWidget *main = new QWidget;

    // make a 1x1 grid; it will auto-expand
    QGridLayout *grid = new QGridLayout( main, 1, 1 );

    // add the first four widgets with (row, column) addressing
    grid->addWidget( new QLabel( "One", main ),   0, 0 );
    grid->addWidget( new QLabel( "Two", main ),   0, 1 );
    grid->addWidget( new QLabel( "Three", main ), 1, 0 );
    grid->addWidget( new QLabel( "Four", main ),  1, 1 );

    // add the last widget on row 2, spanning from column 0 to
    // column 1, and center aligned 
    grid->addMultiCellWidget( new QLabel( "Five", main ), 2, 2, 0, 1,
			      Qt::AlignCenter );

    // let the ratio between the widths of columns 0 and 1 be 2:3
    grid->setColStretch( 0, 2 );
    grid->setColStretch( 1, 3 );
\endcode

You can insert layouts inside a layout by giving the parent layout as
a parameter in the constructor.
\code
    QWidget *main = new QWidget;
    QLineEdit *field = new QLineEdit( main );
    QPushButton *ok = new QPushButton( "OK", main );
    QPushButton *cancel = new QPushButton( "Cancel", main );
    QLabel *label = new QLabel( "Write once, compile everywhere.", main );

    // a layout on a widget
    QVBoxLayout *vbox = new QVBoxLayout( main );
    vbox->addWidget( label );
    vbox->addWidget( field );

    // a layout inside a layout
    QHBoxLayout *buttons = new QHBoxLayout( vbox );
    buttons->addWidget( ok );
    buttons->addWidget( cancel );
\endcode
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.

\section1 Custom Layouts

If the built-in layout classes are not sufficient, you can define your
own. You must make a subclass of \l QLayout that handles resizing and
size calculations, as well as a subclass of \l QGLayoutIterator to
iterate over your layout class.

See the \link customlayout.html Custom Layout \endlink page for an
in-depth description.

\section1 Custom Widgets In Layouts

When you make your own widget class, you should also communicate its
layout properties. If the widget has a QLayout, this is already taken
care of. If the widget does not have any child widgets, or uses manual
layout, you should reimplement the following QWidget member functions:

\list
\i QWidget::sizeHint() returns the preferred size of the widget.
\i QWidget::minimumSizeHint() returns the smallest size the widget can have.
\i QWidget::sizePolicy() returns a \l QSizePolicy; a value describing
   the space requirements of the widget.
\endlist

Call QWidget::updateGeometry() 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.

If the preferred height of your widget depends on its actual width
(e.g. a label with automatic word-breaking), set the \link
QSizePolicy::hasHeightForWidth() hasHeightForWidth\endlink() flag in
\link QWidget::sizePolicy() sizePolicy\endlink(), and reimplement \l
QWidget::heightForWidth().

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 QLayout subclasses that do not
support heightForWidth() (both QGridLayout and QBoxLayout support it).

For further guidance when implementing these functions, see their
implementations in existing Qt classes that have similar layout
requirements to your new widget.

\section1 Manual Layout

If you are making a one-of-a-kind special layout, you can also make a
custom widget as described above. Reimplement QWidget::resizeEvent()
to calculate the required distribution of sizes and call \link
QWidget::setGeometry() setGeometry\endlink() on each child.

The widget will get an event with \link QEvent::type() type \endlink
\c LayoutHint when the layout needs to be recalculated. Reimplement
QWidget::event() to be notified of \c LayoutHint events.

\section1 Layout Issues

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.

*/


/*!  \page customlayout.html

\title Writing your own layout manager

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
QLayout::spacing().

To write your own layout class, you must define the following:
\list
\i A data structure to store the items handled by the layout. Each
item is a \link QLayoutItem QLayoutItem\endlink. We will use a
QPtrList in this example.
\i \link QLayout::addItem() addItem() \endlink, how to add items to
the layout.
\i \link QLayout::setGeometry() setGeometry() \endlink, how to perform
the layout.
\i \link QLayout::sizeHint() sizeHint() \endlink, the preferred size
of the layout.
\i \link QLayout::iterator() iterator() \endlink, how to iterate over
the layout.
\endlist

In most cases, you will also implement \link QLayout::minimumSize()
minimumSize\endlink().

\section1 card.h

\code
#ifndef CARD_H
#define CARD_H

#include <qlayout.h>
#include <qptrlist.h>

class CardLayout : public QLayout
{
public:
    CardLayout( QWidget *parent, int dist )
	: QLayout( parent, 0, dist ) {}
    CardLayout( QLayout* parent, int dist)
	: QLayout( parent, dist ) { }
    CardLayout( int dist )
	: QLayout( dist ) {}
    ~CardLayout();

    void addItem(QLayoutItem *item);
    QSize sizeHint() const;
    QSize minimumSize() const;
    QLayoutIterator iterator();
    void setGeometry(const QRect &rect);

private:
    QPtrList<QLayoutItem> list;
};

#endif
\endcode

\section2 card.cpp

\code
#include "card.h"
\endcode

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.

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.

In this case, we choose a simple implementation: we store an integer
index into the list and a pointer to the list. Every \l
QGLayoutIterator subclass must implement \link
QGLayoutIterator::current() current\endlink(), \link
QGLayoutIterator::next() next\endlink() and \link
QGLayoutIterator::takeCurrent() takeCurrent\endlink(), as well as a
constructor. In our example we do not need a destructor.

\code
class CardLayoutIterator : public QGLayoutIterator
{
public:
    CardLayoutIterator( QPtrList<QLayoutItem> *l )
	: idx( 0 ), list( l ) {}

    QLayoutItem *current()
    { return idx < int(list->count()) ? list->at(idx) : 0;  }

    QLayoutItem *next()
    { idx++; return current(); }

    QLayoutItem *takeCurrent()
    { return list->take( idx ); }

private:
    int idx;
    QPtrList<QLayoutItem> *list;
};
\endcode

We must implement QLayout:iterator() to return a QLayoutIterator over
this layout.

\code
QLayoutIterator CardLayout::iterator()
{	
    return QLayoutIterator( new CardLayoutIterator(&list) );
}
\endcode

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 \link QLayout::autoAdd() auto-add\endlink feature. If your layout
has advanced placement options that require parameters, you must
provide extra access functions such as \l QGridLayout::addMultiCell().

\code
void CardLayout::addItem( QLayoutItem *item )
{
    list.append( item );
}
\endcode

The layout takes over responsibility of the items added. Since
QLayoutItem does not inherit QObject, we must delete the items
manually. The function QLayout::deleteAllItems() uses the iterator we
defined above to delete all the items in the layout.

\code
CardLayout::~CardLayout()
{
    deleteAllItems();
}
\endcode

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.

\code
void CardLayout::setGeometry( const QRect &rect )
{
    QLayout::setGeometry( rect );

    QPtrListIterator<QLayoutItem> it( list );
    if (it.count() == 0)
	return;

    QLayoutItem *item;

    int i = 0;

    int w = rect.width() - ( list.count() - 1 ) * spacing();
    int h = rect.height() - ( list.count() - 1 ) * spacing();

    while ( (item = it.current()) != 0 ) {
	++it;
	QRect geom( rect.x() + i * spacing(), rect.y() + i * spacing(),
		    w, h );
	item->setGeometry( geom );
	++i;
    }
}
\endcode

sizeHint() and minimumSize() are normally very similar in
implementation. The sizes returned by both functions should include
spacing(), but not margin().

\code
QSize CardLayout::sizeHint() const
{
    QSize s( 0, 0 );
    int n = list.count();
    if ( n > 0 )
	s = QSize( 100, 70 ); // start with a nice default size
    QPtrListIterator<QLayoutItem> it( list );
    QLayoutItem *item;
    while ( (item = it.current()) != 0 ) {
	++it;
	s = s.expandedTo( item->minimumSize() );
    }
    return s + n * QSize( spacing(), spacing() );
}

QSize CardLayout::minimumSize() const
{
    QSize s( 0, 0 );
    int n = list.count();
    QPtrListIterator<QLayoutItem> it( list );
    QLayoutItem *item;
    while ( (item = it.current()) != 0 ) {
	++it;
	s = s.expandedTo( item->minimumSize() );
    }
    return s + n * QSize( spacing(), spacing() );
}
\endcode

\section1 Further Notes

This layout does not implement heightForWidth().

We ignore QLayoutItem::isEmpty(), this means that the layout will
treat hidden widgets as visible.

For complex layouts, speed can be greatly increased by caching
calculated values. In that case, implement QLayoutItem::invalidate()
to mark the cached data as dirty.

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.

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
doesn't only apply to layouts, you should do the same if you implement
your own resizeEvent().)

*/