source: trunk/doc/src/layout.qdoc@ 341

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/*!
    \page layout.html

    \title Layout Classes
    \ingroup architecture
    \ingroup classlists
    \brief A tour of the standard layout managers and an introduction to custom
    layouts.

    The Qt layout system provides a simple and powerful way of automatically
    arranging child widgets within a widget to ensure that they make good use
    of the available space.

    \tableofcontents

    \section1 Introduction

    Qt includes a set of layout management classes that are used to describe
    how widgets are laid out in an application's user interface. These layouts
    automatically position and resize widgets when the amount of space
    available for them changes, ensuring that they are consistently arranged
    and that the user interface as a whole remains usable.

    All QWidget subclasses can use layouts to manage their children. The
    QWidget::setLayout() function applies a layout to a widget. When a layout
    is set on a widget in this way, it takes charge of the following tasks:

    \list
    \o Positioning of child widgets.
    \o Sensible default sizes for windows.
    \o Sensible minimum sizes for windows.
    \o Resize handling.
    \o Automatic updates when contents change:
        \list
        \o Font size, text or other contents of child widgets.
        \o Hiding or showing a child widget.
        \o Removal of child widgets.
        \endlist
    \endlist

    Qt's layout classes were designed for hand-written C++ code, allowing
    measurements to be specified in pixels for simplicity, so they are easy to
    understand and use. The code generated for forms created using \QD also
    uses the layout classes. \QD is useful to use when experimenting with the
    design of a form since it avoids the compile, link and run cycle usually
    involved in user interface development.


    \section1 Horizontal, Vertical, Grid, and Form Layouts

    The easiest way to give your widgets a good layout is to use the built-in
    layout managers: QHBoxLayout, QVBoxLayout, QGridLayout, and QFormLayout.
    These classes inherit from QLayout, which in turn derives from QObject (not
    QWidget). They take care of geometry  management for a set of widgets. To
    create more complex layouts, you can nest layout managers inside each other.

    \list
        \o  A QHBoxLayout lays out widgets in a horizontal row, from left to
            right (or right to left for right-to-left languages).
            \image qhboxlayout-with-5-children.png

        \o  A QVBoxLayout lays out widgets in a vertical column, from top to
            bottom.
            \image qvboxlayout-with-5-children.png

        \o  A QGridLayout lays out widgets in a two-dimensional grid. Widgets
            can occupy multiple cells.
            \image qgridlayout-with-5-children.png

        \o  A QFormLayout lays out widgets in a 2-column descriptive label-
            field style.
            \image qformlayout-with-6-children.png
    \endlist


    \section2 Laying Out Widgets in Code

    The following code creates a QHBoxLayout that manages the geometry of five
    \l{QPushButton}s, as shown on the first screenshot above:

    \snippet doc/src/snippets/layouts/layouts.cpp 0
    \snippet doc/src/snippets/layouts/layouts.cpp 1
    \snippet doc/src/snippets/layouts/layouts.cpp 2
    \codeline
    \snippet doc/src/snippets/layouts/layouts.cpp 3
    \snippet doc/src/snippets/layouts/layouts.cpp 4
    \snippet doc/src/snippets/layouts/layouts.cpp 5

    The code for QVBoxLayout is identical, except the line where the layout is
    created. The code for QGridLayout is a bit different, because we need to
    specify the row and column position of the child widget:

    \snippet doc/src/snippets/layouts/layouts.cpp 12
    \snippet doc/src/snippets/layouts/layouts.cpp 13
    \snippet doc/src/snippets/layouts/layouts.cpp 14
    \codeline
    \snippet doc/src/snippets/layouts/layouts.cpp 15
    \snippet doc/src/snippets/layouts/layouts.cpp 16
    \snippet doc/src/snippets/layouts/layouts.cpp 17

    The third QPushButton spans 2 columns. This is possible by specifying 2 as
    the fifth argument to QGridLayout::addWidget().

    Finally, the code for QFormLayout is ..


    \section2 Tips for Using Layouts

    When you use a layout, you do not need to pass a parent when constructing
    the child widgets. The layout will automatically reparent the widgets
    (using QWidget::setParent()) so that they are children of the widget on
    which the layout is installed.

    \note Widgets in a layout are children of the widget on which the layout
    is installed, \e not of the layout itself. Widgets can only have other
    widgets as parent, not layouts.

    You can nest layouts using \c addLayout() on a layout; the inner layout
    then becomes a child of the layout it is inserted into.


    \section1 Adding Widgets to a Layout

    When you add widgets to a layout, the layout process works as follows:

    \list 1
        \o  All the widgets will initially be allocated an amount of space in
            accordance with their QWidget::sizePolicy() and
            QWidget::sizeHint().

        \o  If any of the widgets have stretch factors set, with a value
            greater than zero, then they are allocated space in proportion to
            their stretch factor (explained below).

        \o  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
            \l{QSizePolicy::Expanding}{Expanding} size policy first.

        \o  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.)

        \o  Any widgets that are allocated more space than their maximum size
            are allocated the maximum size space they require. (Widgets do not
            have to have a maximum size in which case the strech factor is
            their determining factor.)
    \endlist


    \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 QHBoxLayout with no stretch
    factors set we will get a layout like this:

    \img layout1.png Three 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 Three widgets with different stretch factors in a row


    \section1 Custom Widgets in Layouts

    When you make your own widget class, you should also communicate its layout
    properties. If the widget has a one of Qt's layouts, this is already taken
    care of. If the widget does not have any child widgets, or uses manual
    layout, you can change the behavior of the widget using any or all of the
    following mechanisms:

    \list
        \o  Reimplement QWidget::sizeHint() to return the preferred size of the
            widget.
        \o  Reimplement QWidget::minimumSizeHint() to return the smallest size
            the widget can have.
        \o  Call QWidget::setSizePolicy() to specify 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
    consecutive calls to QWidget::updateGeometry() will only cause one layout
    recalculation.

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

    Even if you implement QWidget::heightForWidth(), it is still a good idea to
    provide a reasonable sizeHint().

    For further guidance when implementing these functions, see the
    \e{Qt Quarterly} article
    \l{http://doc.trolltech.com/qq/qq04-height-for-width.html}
    {Trading Height for Width}.


    \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 \l{QWidget::}{sizeHint()} and
    \l{QWidget::}{minimumSizeHint()} functions.

    In some cases, it is relevant when a layout is added to a widget. When
    you set the widget of a QDockWidget or a QScrollArea (with
    QDockWidget::setWidget() and QScrollArea::setWidget()), the layout must
    already have been set on the widget. If not, the widget will not be
    visible.


    \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
    \l{QWidget::}{setGeometry()} on each child.

    The widget will get an event of type QEvent::LayoutRequest when the
    layout needs to be recalculated. Reimplement QWidget::event() to handle
    QEvent::LayoutRequest events.


    \section1 How to Write A Custom Layout Manager

    An alternative to manual layout is to write your own layout manager by
    subclassing QLayout. The \l{layouts/borderlayout}{Border Layout} and
    \l{layouts/flowlayout}{Flow Layout} examples show how to do this.

    Here we present an example in detail. The \c CardLayout class 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
        \o  A data structure to store the items handled by the layout. Each
            item is a \link QLayoutItem QLayoutItem\endlink. We will use a
            QList in this example.
        \o  \l{QLayout::}{addItem()}, how to add items to the layout.
        \o  \l{QLayout::}{setGeometry()}, how to perform the layout.
        \o  \l{QLayout::}{sizeHint()}, the preferred size of the layout.
        \o  \l{QLayout::}{itemAt()}, how to iterate over the layout.
        \o  \l{QLayout::}{takeAt()}, how to remove items from the layout.
    \endlist

    In most cases, you will also implement \l{QLayout::}{minimumSize()}.


    \section2 The Header File (\c card.h)

    \snippet doc/src/snippets/code/doc_src_layout.qdoc 0


    \section2 The Implementation File (\c card.cpp)

    \snippet doc/src/snippets/code/doc_src_layout.qdoc 1

    First we define two functions that iterate over the layout: \c{itemAt()}
    and \c{takeAt()}. These functions are used internally by the layout system
    to handle deletion of widgets. They are also available for application
    programmers. 

    \c{itemAt()} returns the item at the given index. \c{takeAt()} removes the
    item at the given index, and returns it. In this case we use the list index
    as the layout index. In other cases where we have a more complex data
    structure, we may have to spend more effort defining a linear order for the
    items.

    \snippet doc/src/snippets/code/doc_src_layout.qdoc 2

    \c{addItem()} implements the default placement strategy for layout items.
    This function must be implemented. It is used by QLayout::add(), by the
    QLayout constructor that takes a layout as parent. If your layout has
    advanced placement options that require parameters, you must provide extra
    access functions such as the row and column spanning overloads of
    QGridLayout::addItem(), QGridLayout::addWidget(), and
    QGridLayout::addLayout().

    \snippet doc/src/snippets/code/doc_src_layout.qdoc 3

    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 \c{takeAt()} defined above to delete all the
    items in the layout.

    \snippet doc/src/snippets/code/doc_src_layout.qdoc 4

    The \c{setGeometry()} function actually performs the layout. The rectangle
    supplied as an argument does not include \c{margin()}. If relevant, use
    \c{spacing()} as the distance between items.

    \snippet doc/src/snippets/code/doc_src_layout.qdoc 5

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

    \snippet doc/src/snippets/code/doc_src_layout.qdoc 6


    \section2 Further Notes

    \list
        \o  This custom layout does not handle height for width.
        \o  We ignore QLayoutItem::isEmpty(); this means that the layout will
            treat hidden widgets as visible.
        \o  For complex layouts, speed can be greatly increased by caching
            calculated values. In that case, implement
            QLayoutItem::invalidate() to mark the cached data is dirty.
        \o  Calling QLayoutItem::sizeHint(), etc. may be expensive. So, you
            should store the value in a local variable if you need it again
            later within in the same function.
        \o  You should not call QLayoutItem::setGeometry() twice on the same
            item in the smae function. This call can be very expensive if the
            item has several child widgets, because the layout manager must 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(), for example.)
    \endlist
*/