source: trunk/doc/src/tutorials/widgets-tutorial.qdoc@ 561

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/*!
    \page widgets-tutorial.html
    \title Widgets Tutorial
    \brief This tutorial covers basic usage of widgets and layouts, showing how
    they are used to build GUI applications.

    \startpage {index.html}{Qt Reference Documentation}
    \contentspage Tutorials
    \nextpage {tutorials/widgets/toplevel}{Creating a Window}


    \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 Writing a main Function

    Many of the GUI examples in Qt follow the pattern of having a \c{main.cpp}
    file containing code to initialize the application, and a number of other
    source and header files containing the application logic and custom GUI
    components.

    A typical \c main() function, written in \c{main.cpp}, looks like this:

    \snippet doc/src/snippets/widgets-tutorial/template.cpp main.cpp body

    We first construct a QApplication object which is configured using any
    arguments passed in from the command line. After any widgets have been
    created and shown, we call QApplication::exec() to start Qt's event loop.
    Control passes to Qt until this function returns, at which point we return
    the value we obtain from this function.

    In each part of this tutorial, we provide an example that is written
    entirely within a \c main() function. In more sophisticated examples, the
    code to set up widgets and layouts is written in other parts of the
    example. For example, the GUI for a main window may be set up in the
    constructor of a QMainWindow subclass.

    The \l{Widgets examples} are a good place to look for
    more complex and complete examples and applications.

    \section1 Building Examples and Tutorials

    If you obtained a binary package of Qt or compiled it yourself, the
    examples described in this tutorial should already be ready to run.
    However, if you may wish to modify them and recompile them, you need to
    perform the following steps:

    \list 1
    \o At the command line, enter the directory containing the example you
       wish to recompile.
    \o Type \c qmake and press \key{Return}. If this doesn't work, make sure
       that the executable is on your path, or enter its full location.
    \o On Linux/Unix and Mac OS X, type \c make and press \key{Return};
       on Windows with Visual Studio, type \c nmake and press \key{Return}.
    \endlist

    An executable file should have been created within the current directory.
    On Windows, this file may be located within a \c debug or \c release
    subdirectory. You can run this file to see the example code at work.
*/

/*!
    \page widgets-tutorial-toplevel.html
    \contentspage {Widgets Tutorial}{Contents}
    \previouspage {Widgets Tutorial}
    \nextpage {Widgets Tutorial - Child Widgets}
    \example tutorials/widgets/toplevel
    \title Widgets Tutorial - 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 tutorials/widgets/toplevel/main.cpp main program
    \raw HTML
    </td><td align="right">
    \endraw
    \inlineimage widgets-tutorial-toplevel.png
    \raw HTML
    </td></tr>
    </table>
    \endraw

    To create a real GUI, we need to place widgets inside the window. To do
    this, we pass a QWidget instance to a widget's constructor, as we will
    demonstrate in the next part of this tutorial.
*/

/*!
    \page widgets-tutorial-childwidget.html
    \contentspage {Widgets Tutorial}{Contents}
    \previouspage {Widgets Tutorial - Creating a Window}
    \nextpage {Widgets Tutorial - Using Layouts}
    \example tutorials/widgets/childwidget
    \title Widgets Tutorial - Child Widgets

    We can add a child widget to the window created in the previous example 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 tutorials/widgets/childwidget/main.cpp main program
    \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. It will be destroyed when the example exits.
*/

/*!
    \page widgets-tutorial-windowlayout.html
    \contentspage {Widgets Tutorial}{Contents}
    \previouspage {Widgets Tutorial - Child Widgets}
    \nextpage {Widgets Tutorial - Nested Layouts}
    \example tutorials/widgets/windowlayout
    \title Widgets Tutorial - 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 tutorials/widgets/windowlayout/main.cpp main program
    \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.
*/

/*!
    \page widgets-tutorial-nestedlayouts.html
    \contentspage {Widgets Tutorial}{Contents}
    \previouspage {Widgets Tutorial - Using Layouts}
    \example tutorials/widgets/nestedlayouts
    \title Widgets Tutorial - Nested Layouts

    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.

    We achieve this by creating two layouts: \c{queryLayout} is a QHBoxLayout
    that contains QLabel and QLineEdit widgets placed side-by-side;
    \c{mainLayout} is a QVBoxLayout that contains \c{queryLayout} and a
    QTableView arranged vertically.

    \raw HTML
    <table align="left" width="100%">
    <tr class="qt-code"><td>
    \endraw
    \snippet tutorials/widgets/nestedlayouts/main.cpp first part
    \snippet tutorials/widgets/nestedlayouts/main.cpp last part
    \raw HTML
    </td><td align="right">
    \endraw
    \inlineimage widgets-tutorial-nestedlayouts.png
    \raw HTML
    </td></tr>
    </table>
    \endraw

    Note that we call the \c{mainLayout}'s \l{QBoxLayout::}{addLayout()}
    function to insert the \c{queryLayout} above the \c{resultView} table.

    We have omitted the code that sets up the model containing the data shown
    by the QTableView widget, \c resultView. For completeness, we show this below.

    As well as QHBoxLayout and QVBoxLayout, Qt also provides QGridLayout
    and QFormLayout classes to help with more complex user interfaces.
    These can be seen if you run \l{Qt Designer}.

    \section1 Setting up the Model

    In the code above, we did not show where the table's data came from
    because we wanted to concentrate on the use of layouts. Here, we see
    that the model holds a number of items corresponding to rows, each of
    which is set up to contain data for two columns.

    \snippet tutorials/widgets/nestedlayouts/main.cpp set up the model

    The use of models and views is covered in the
    \l{Item Views Examples} and in the \l{Model/View Programming} overview.
*/