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29 \page widgets-tutorial.html
31 \title Widgets Tutorial
32 \brief This tutorial covers basic usage of widgets and layouts, showing how
33 they are used to build GUI applications.
35 \section1 Introduction
37 Widgets are the basic building blocks for graphical user interface
38 (GUI) applications built with Qt. Each GUI component (e.g.
39 buttons, labels, text editor) is a \l{QWidget}{widget} that is
40 placed somewhere within a user interface window, or is displayed
41 as an independent window. Each type of widge is provided by a
42 subclass of QWidget, which is itself a subclass of QObject.
44 QWidget is not an abstract class. It can be used as a container
45 for other widgets, and it can be subclassed with minimal effort to
46 create new, custom widgets. QWidget is often used to create a
47 window inside which other \l{QWidget}s are placed.
49 As with \l{QObject}s, \l{QWidget}s can be created with parent
50 objects to indicate ownership, ensuring that objects are deleted
51 when they are no longer used. With widgets, these parent-child
52 relationships have an additional meaning: Each child widget is
53 displayed within the screen area occupied by its parent widget.
54 This means that when you delete a window widget, all the child
55 widgets it contains are also deleted.
57 \section1 Writing a main Function
59 Many of the GUI examples provided with Qt follow the pattern of
60 having a \c{main.cpp} file, which contains the standard code to
61 initialize the application, plus any number of other source/header
62 files that contain the application logic and custom GUI components.
64 A typical \c main() function in \c{main.cpp} looks like this:
66 \snippet doc/src/snippets/widgets-tutorial/template.cpp main.cpp body
68 First, a QApplication object is constructed, which can be
69 configured with arguments passed in from the command line. After
70 the widgets have been created and shown, QApplication::exec() is
71 called to start Qt's event loop. Control passes to Qt until this
72 function returns. Finally, \c{main()} returns the value returned
73 by QApplication::exec().
75 \section1 Simple widget examples
77 Each of theses simple widget examples is written entirely within
78 the \c main() function.
81 \o \l {tutorials/widgets/toplevel} {Creating a window}
83 \o \l {tutorials/widgets/childwidget} {Creating child widgets}
85 \o \l {tutorials/widgets/windowlayout} {Using layouts}
87 \o \l {tutorials/widgets/nestedlayouts} {Nested layouts}
90 \section1 Real world widget examples
92 In these \l{Widget examples} {more advanced examples}, the code
93 that creates the widgets and layouts is stored in other files. For
94 example, the GUI for a main window may be created in the
95 constructor of a QMainWindow subclass.
97 \section1 Building The Examples
99 If you installed a binary package to get Qt, or if you compiled Qt
100 yourself, the examples described in this tutorial should already
101 be built and ready to run. If you wish to modify and recompile
102 them, follow these steps:
106 \o From a command prompt, enter the directory containing the
107 example you have modified.
109 \o Type \c qmake and press \key{Return}. If this doesn't work,
110 make sure that the executable is on your path, or enter its
113 \o On Linux/Unix and Mac OS X, type \c make and press
114 \key{Return}; on Windows with Visual Studio, type \c nmake and
119 An executable file is created in the current directory. On
120 Windows, this file may be located in a \c debug or \c release
121 subdirectory. You can run this executable to see the example code
126 \example tutorials/widgets/toplevel
127 \title Widgets Tutorial - Creating a Window
129 If a widget is created without a parent, it is treated as a window, or
130 \e{top-level widget}, when it is shown. Since it has no parent object to
131 ensure that it is deleted when no longer needed, it is up to the
132 developer to keep track of the top-level widgets in an application.
134 In the following example, we use QWidget to create and show a window with
137 \div {class="qt-code"}
140 \o \snippet tutorials/widgets/toplevel/main.cpp main program
141 \o \inlineimage widgets-tutorial-toplevel.png
145 To create a real GUI, we need to place widgets inside the window. To do
146 this, we pass a QWidget instance to a widget's constructor, as we will
147 demonstrate in the next part of this tutorial.
152 \example tutorials/widgets/childwidget
153 \title Widgets Tutorial - Child Widgets
155 We can add a child widget to the window created in the previous example by
156 passing \c window as the parent to its constructor. In this case, we add a
157 button to the window and place it in a specific location:
159 \div {class="qt-code"}
162 \o \snippet tutorials/widgets/childwidget/main.cpp main program
163 \o \inlineimage widgets-tutorial-childwidget.png
167 The button is now a child of the window and will be deleted when the
168 window is destroyed. Note that hiding or closing the window does not
169 automatically destroy it. It will be destroyed when the example exits.
173 \example tutorials/widgets/windowlayout
174 \title Widgets Tutorial - Using Layouts
176 Usually, child widgets are arranged inside a window using layout objects
177 rather than by specifying positions and sizes explicitly. Here, we
178 construct a label and line edit widget that we would like to arrange
181 \div {class="qt-code"}
184 \o \snippet tutorials/widgets/windowlayout/main.cpp main program
185 \o \inlineimage widgets-tutorial-windowlayout.png
189 The \c layout object we construct manages the positions and sizes of
190 widgets supplied to it with the \l{QHBoxLayout::}{addWidget()} function.
191 The layout itself is supplied to the window itself in the call to
192 \l{QWidget::}{setLayout()}. Layouts are only visible through the effects
193 they have on the widgets (and other layouts) they are responsible for
196 In the example above, the ownership of each widget is not immediately
197 clear. Since we construct the widgets and the layout without parent
198 objects, we would expect to see an empty window and two separate windows
199 containing a label and a line edit. However, when we tell the layout to
200 manage the label and line edit and set the layout on the window, both the
201 widgets and the layout itself are ''reparented'' to become children of
206 \example tutorials/widgets/nestedlayouts
207 \title Widgets Tutorial - Nested Layouts
209 Just as widgets can contain other widgets, layouts can be used to provide
210 different levels of grouping for widgets. Here, we want to display a
211 label alongside a line edit at the top of a window, above a table view
212 showing the results of a query.
214 We achieve this by creating two layouts: \c{queryLayout} is a QHBoxLayout
215 that contains QLabel and QLineEdit widgets placed side-by-side;
216 \c{mainLayout} is a QVBoxLayout that contains \c{queryLayout} and a
217 QTableView arranged vertically.
219 \div {class="qt-code"}
222 \o \snippet tutorials/widgets/nestedlayouts/main.cpp first part
223 \snippet tutorials/widgets/nestedlayouts/main.cpp last part
224 \o \inlineimage widgets-tutorial-nestedlayouts.png
228 Note that we call the \c{mainLayout}'s \l{QBoxLayout::}{addLayout()}
229 function to insert the \c{queryLayout} above the \c{resultView} table.
231 We have omitted the code that sets up the model containing the data shown
232 by the QTableView widget, \c resultView. For completeness, we show this below.
234 As well as QHBoxLayout and QVBoxLayout, Qt also provides QGridLayout
235 and QFormLayout classes to help with more complex user interfaces.
236 These can be seen if you run \l{Qt Designer}.
238 \section1 Setting up the Model
240 In the code above, we did not show where the table's data came from
241 because we wanted to concentrate on the use of layouts. Here, we see
242 that the model holds a number of items corresponding to rows, each of
243 which is set up to contain data for two columns.
245 \snippet tutorials/widgets/nestedlayouts/main.cpp set up the model
247 The use of models and views is covered in the
248 \l{Item Views Examples} and in the \l{Model/View Programming} overview.