source: trunk/doc/src/tutorials/widgets-tutorial.qdoc@ 560

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/*!
    \page widgets-tutorial.html

    \title Widgets Tutorial
    \ingroup tutorials

    \brief This tutorial covers basic usage of widgets and layouts, showing how they are used to build GUI applications.

    \section1 Introduction

    Widgets are the basic building blocks of graphical user interface (GUI)
    applications made with Qt. Each GUI component, such as a button, label or
    text editor, is a widget and can be placed within an existing user
    interface or displayed as an independent window. Each type of component
    is provided by a particular subclass of QWidget, which is itself a
    subclass of QObject.

    QWidget is not an abstract class; it can be used as a container for other
    widgets, and can be subclassed with minimal effort to create custom
    widgets. It is most often used to create windows in which other widgets
    are placed.

    As with \l{QObject}s, widgets can be created with parent objects to
    indicate ownership, ensuring that objects are deleted when they are no
    longer used. With widgets, these parent-child relationships have an
    additional meaning: each child is displayed within the screen area
    occupied by its parent. This means that, when a window is deleted, all
    the widgets it contains are automatically deleted.

    \section1 Creating a Window

    If a widget is created without a parent, it is treated as a window, or
    \e{top-level widget}, when it is shown. Since it has no parent object to
    ensure that it is deleted when no longer needed, it is up to the
    developer to keep track of the top-level widgets in an application.

    In the following example, we use QWidget to create and show a window with
    a default size:

    \raw HTML
    <table align="left" width="100%">
    <tr class="qt-code"><td>
    \endraw
    \snippet snippets/widgets-tutorial/toplevel/main.cpp create, resize and show
    \raw HTML
    </td><td align="right">
    \endraw
    \inlineimage widgets-tutorial-toplevel.png
    \raw HTML
    </td></tr>
    </table>
    \endraw

    We can add a child widget to this window by passing \c window as the
    parent to its constructor. In this case, we add a button to the window
    and place it in a specific location:

    \raw HTML
    <table align="left" width="100%">
    <tr class="qt-code"><td>
    \endraw
    \snippet snippets/widgets-tutorial/childwidget/main.cpp create, position and show
    \raw HTML
    </td><td align="right">
    \endraw
    \inlineimage widgets-tutorial-childwidget.png
    \raw HTML
    </td></tr>
    </table>
    \endraw

    The button is now a child of the window and will be deleted when the
    window is destroyed. Note that hiding or closing the window does not
    automatically destroy it.

    \section1 Using Layouts

    Usually, child widgets are arranged inside a window using layout objects
    rather than by specifying positions and sizes explicitly. Here, we
    construct a label and line edit widget that we would like to arrange
    side-by-side.

    \raw HTML
    <table align="left" width="100%">
    <tr class="qt-code"><td>
    \endraw
    \snippet snippets/widgets-tutorial/windowlayout/main.cpp create, lay out widgets and show
    \raw HTML
    </td><td align="right">
    \endraw
    \inlineimage widgets-tutorial-windowlayout.png
    \raw HTML
    </td></tr>
    </table>
    \endraw

    The \c layout object we construct manages the positions and sizes of
    widgets supplied to it with the \l{QHBoxLayout::}{addWidget()} function.
    The layout itself is supplied to the window itself in the call to
    \l{QWidget::}{setLayout()}. Layouts are only visible through the effects
    they have on the widgets (and other layouts) they are responsible for
    managing.

    In the example above, the ownership of each widget is not immediately
    clear. Since we construct the widgets and the layout without parent
    objects, we would expect to see an empty window and two separate windows
    containing a label and a line edit. However, when we tell the layout to
    manage the label and line edit and set the layout on the window, both the
    widgets and the layout itself are ''reparented'' to become children of
    the window.

    Just as widgets can contain other widgets, layouts can be used to provide
    different levels of grouping for widgets. Here, we want to display a
    label alongside a line edit at the top of a window, above a table view
    showing the results of a query.

    \raw HTML
    <table align="left" width="100%">
    <tr class="qt-code"><td>
    \endraw
    \snippet snippets/widgets-tutorial/nestedlayouts/main.cpp create, lay out widgets and show
    \raw HTML
    </td><td align="right">
    \endraw
    \inlineimage widgets-tutorial-nestedlayouts.png
    \raw HTML
    </td></tr>
    </table>
    \endraw

    As well as QHBoxLayout and QVBoxLayout, Qt also provides QGridLayout
    and QFormLayout classes to help with more complex user interfaces.
*/