widgets-tutorial.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page widgets-tutorial.html
    \ingroup tutorials
    \title Widgets Tutorial
    \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 for graphical user interface
    (GUI) applications built with Qt. Each GUI component (e.g.
    buttons, labels, text editors) is a \l{QWidget}{widget} that is
    placed somewhere within a user interface window, or is displayed
    as an independent window. Each type of widget is provided by a
    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 it can be subclassed with minimal effort to
    create new, custom widgets. QWidget is often used to create a
    window inside which other \l{QWidget}s are placed.
 
    As with \l{QObject}s, \l{QWidget}s 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 widget is
    displayed within the screen area occupied by its parent widget.
    This means that when you delete a window widget, all the child
    widgets it contains are also deleted.
 
    \section1 Writing a Main Function
 
    Many of the GUI examples provided with Qt follow the pattern of
    having a \c{main.cpp} file, which contains the standard code to
    initialize the application, plus any number of other source/header
    files that contain the application logic and custom GUI components.
 
    A typical \c main() function in \c{main.cpp} looks like this:
 
    \snippet widgets-tutorial/template.cpp main.cpp body
 
    First, a QApplication object is constructed, which can be
    configured with arguments passed in from the command line. After
    the widgets have been created and shown, QApplication::exec() is
    called to start Qt's event loop.  Control passes to Qt until this
    function returns. Finally, \c{main()} returns the value returned
    by QApplication::exec().
 
    \section1 Simple Widget Examples
 
    Each of these simple widget examples is written entirely within
    the \c main() function.
 
    \list
    \li \l {tutorials/widgets/toplevel} {Creating a window}
 
    \li \l {tutorials/widgets/childwidget} {Creating child widgets}
 
    \li \l {tutorials/widgets/windowlayout} {Using layouts}
 
    \li \l {tutorials/widgets/nestedlayouts} {Nested layouts}
    \endlist
 
    \section1 Real World Widget Examples
 
    In these \l{Qt Widgets Examples} {more advanced examples}, the code
    that creates the widgets and layouts is stored in other files. For
    example, the GUI for a main window may be created in the
    constructor of a QMainWindow subclass.
 
    \section1 Building The Examples
 
    If you installed a binary package to get Qt, or if you compiled Qt
    yourself, the examples described in this tutorial should already
    be built and ready to run. If you wish to modify and recompile
    them, follow these steps:
 
    \list 1
 
    \li From a command prompt, enter the directory containing the
       example you have modified.
 
    \li Type \c qmake and press \uicontrol{Return}. If this doesn't work,
       make sure that the executable is on your path, or enter its
       full location.
 
    \li On Linux/Unix and \macos, type \c make and press
       \uicontrol{Return}; on Windows with Visual Studio, type \c nmake and
       press \uicontrol{Return}.
 
    \endlist
 
    An executable file is created in the current directory.  On
    Windows, this file may be located in a \c debug or \c release
    subdirectory. You can run this executable to see the example code
    at work.
*/
 
/*!
    \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:
 
    \div {class="qt-code"}
    \table
    \row
    \li \snippet tutorials/widgets/toplevel/main.cpp main program
    \li \inlineimage widgets-tutorial-toplevel.png
    \endtable
    \enddiv
 
    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.
 
*/
 
/*!
    \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:
 
    \div {class="qt-code"}
    \table
    \row
    \li \snippet tutorials/widgets/childwidget/main.cpp main program
    \row
    \li \inlineimage widgets-tutorial-childwidget.png
    \endtable
    \enddiv
 
    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.
*/
 
/*!
    \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.
 
    \div {class="qt-code"}
    \table
    \row
    \li \snippet tutorials/widgets/windowlayout/main.cpp main program
    \row
    \li \inlineimage widgets-tutorial-windowlayout.png
    \endtable
    \enddiv
 
    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.
*/
 
/*!
    \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.
 
    \div {class="qt-code"}
    \table
    \row
    \li \snippet tutorials/widgets/nestedlayouts/main.cpp first part
       \snippet tutorials/widgets/nestedlayouts/main.cpp last part
    \li \inlineimage widgets-tutorial-nestedlayouts.png
    \endtable
    \enddiv
 
    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 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.
*/