summaryrefslogtreecommitdiffstats
path: root/src/corelib/doc/src/objectmodel/properties.qdoc
blob: abb0720fe945751ed4f5e98bc7d58ff14cfe3095 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
/****************************************************************************
**
** Copyright (C) 2015 The Qt Company Ltd.
** Contact: http://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see http://www.qt.io/terms-conditions. For further
** information use the contact form at http://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: http://www.gnu.org/copyleft/fdl.html.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \page properties.html
    \title The Property System
    \brief An overview of Qt's property system.

    \ingroup qt-basic-concepts
    \target Qt's Property System

    Qt provides a sophisticated property system similar to the ones
    supplied by some compiler vendors. However, as a compiler- and
    platform-independent library, Qt does not rely on non-standard
    compiler features like \c __property or \c [property]. The Qt
    solution works with \e any standard C++ compiler on every platform
    Qt supports. It is based on the \l {Meta-Object System} that also
    provides inter-object communication via \l{signals and slots}.

    \section1 Requirements for Declaring Properties

    To declare a property, use the \l {Q_PROPERTY()} {Q_PROPERTY()}
    macro in a class that inherits QObject.

    \snippet code/doc_src_properties.cpp 0

    Here are some typical examples of property declarations taken from
    class QWidget.

    \snippet code/doc_src_properties.cpp 1

    Here is an example showing how to export member variables as Qt
    properties using the \c MEMBER keyword.
    Note that a \c NOTIFY signal must be specified to allow QML property bindings.

    \snippet code/doc_src_properties.cpp 8

    A property behaves like a class data member, but it has additional
    features accessible through the \l {Meta-Object System}.

    \list

    \li A \c READ accessor function is required if no \c MEMBER variable was
    specified. It is for reading the property value. Ideally, a const function
    is used for this purpose, and it must return either the property's type or a
    const reference to that type. e.g., QWidget::focus is a read-only
    property with \c READ function, QWidget::hasFocus().

    \li A \c WRITE accessor function is optional. It is for setting the
    property value. It must return void and must take exactly one
    argument, either of the property's type or a pointer or reference
    to that type. e.g., QWidget::enabled has the \c WRITE function
    QWidget::setEnabled().  Read-only properties do not need \c WRITE
    functions. e.g., QWidget::focus has no \c WRITE function.

    \li A \c MEMBER variable association is required if no \c READ accessor
    function is specified. This makes the given member variable
    readable and writable without the need of creating \c READ and \c WRITE accessor
    functions. It's still possible to use \c READ or \c WRITE accessor functions in
    addition to \c MEMBER variable association (but not both), if you need to
    control the variable access.

    \li A \c RESET function is optional. It is for setting the property
    back to its context specific default value. e.g., QWidget::cursor
    has the typical \c READ and \c WRITE functions, QWidget::cursor()
    and QWidget::setCursor(), and it also has a \c RESET function,
    QWidget::unsetCursor(), since no call to QWidget::setCursor() can
    mean \e {reset to the context specific cursor}. The \c RESET
    function must return void and take no parameters.

    \li A \c NOTIFY signal is optional. If defined, it should specify one
    existing signal in that class that is emitted whenever the value
    of the property changes.
    \c NOTIFY signals for \c MEMBER variables must take zero or one parameter,
    which must be of the same type as the property. The parameter will take the
    new value of the property. The \c NOTIFY signal should only be emitted when
    the property has really been changed, to avoid bindings being unnecessarily
    re-evaluated in QML, for example. Qt emits automatically that signal when
    needed for MEMBER properties that do not have an explicit setter.

    \li A \c REVISION number is optional. If included, it defines
    the property and its notifier signal to be used in a particular
    revision of the API (usually for exposure to QML). If not included, it
    defaults to 0.

    \li The \c DESIGNABLE attribute indicates whether the property
    should be visible in the property editor of GUI design tool (e.g.,
    \l {Qt Designer Manual}{Qt Designer}). Most properties are \c DESIGNABLE
    (default true). Instead of true or false, you can specify a boolean
    member function.

    \li The \c SCRIPTABLE attribute indicates whether this property
    should be accessible by a scripting engine (default true).
    Instead of true or false, you can specify a boolean member
    function.

    \li The \c STORED attribute indicates whether the property should
    be thought of as existing on its own or as depending on other
    values. It also indicates whether the property value must be saved
    when storing the object's state. Most properties are \c STORED
    (default true), but e.g., QWidget::minimumWidth() has \c STORED
    false, because its value is just taken from the width component
    of property QWidget::minimumSize(), which is a QSize.

    \li The \c USER attribute indicates whether the property is
    designated as the user-facing or user-editable property for the
    class. Normally, there is only one \c USER property per class
    (default false). e.g., QAbstractButton::checked is the user
    editable property for (checkable) buttons. Note that QItemDelegate
    gets and sets a widget's \c USER property.

    \li The presence of the \c CONSTANT attibute indicates that the property
    value is constant.  For a given object instance, the READ method of a
    constant property must return the same value every time it is called.  This
    constant value may be different for different instances of the object.  A
    constant property cannot have a WRITE method or a NOTIFY signal.

    \li The presence of the \c FINAL attribute indicates that the property
    will not be overridden by a derived class.  This can be used for performance
    optimizations in some cases, but is not enforced by moc.  Care must be taken
    never to override a \c FINAL property.

    \endlist

    The \c READ, \c WRITE, and \c RESET functions can be inherited.
    They can also be virtual. When they are inherited in classes where
    multiple inheritance is used, they must come from the first
    inherited class.

    The property type can be any type supported by QVariant, or it can
    be a user-defined type. In this example, class QDate is considered
    to be a user-defined type.

    \snippet code/doc_src_properties.cpp 2

    Because QDate is user-defined, you must include the \c{<QDate>}
    header file with the property declaration.

    For QMap, QList, and QValueList properties, the property value is
    a QVariant whose value is the entire list or map.  Note that the
    Q_PROPERTY string cannot contain commas, because commas separate
    macro arguments. Therefore, you must use \c QMap as the property
    type instead of \c QMap<QString,QVariant>. For consistency, also
    use \c QList and \c QValueList instead of \c QList<QVariant> and
    \c QValueList<QVariant>.

    \section1 Reading and Writing Properties with the Meta-Object System

    A property can be read and written using the generic functions
    QObject::property() and QObject::setProperty(), without knowing
    anything about the owning class except the property's name.  In
    the code snippet below, the call to QAbstractButton::setDown() and
    the call to QObject::setProperty() both set property "down".

    \snippet code/doc_src_properties.cpp 3

    Accessing a property through its \c WRITE accessor is the better
    of the two, because it is faster and gives better diagnostics at
    compile time, but setting the property this way requires that you
    know about the class at compile time. Accessing properties by name
    lets you access classes you don't know about at compile time. You
    can \e discover a class's properties at run time by querying its
    QObject, QMetaObject, and \l {QMetaProperty} {QMetaProperties}.

    \snippet code/doc_src_properties.cpp 4

    In the above snippet, QMetaObject::property() is used to get \l
    {QMetaProperty} {metadata} about each property defined in some
    unknown class. The property name is fetched from the metadata and
    passed to QObject::property() to get the \l {QVariant} {value} of
    the property in the current \l {QObject}{object}.

    \section1 A Simple Example

    Suppose we have a class MyClass, which is derived from QObject and
    which uses the Q_OBJECT macro in its private section. We want to
    declare a property in MyClass to keep track of a priority
    value. The name of the property will be \e priority, and its type
    will be an enumeration type named \e Priority, which is defined in
    MyClass.

    We declare the property with the Q_PROPERTY() macro in the private
    section of the class. The required \c READ function is named \c
    priority, and we include a \c WRITE function named \c setPriority.
    The enumeration type must be registered with the \l {Meta-Object
    System} using the Q_ENUM() macro. Registering an enumeration type
    makes the enumerator names available for use in calls to
    QObject::setProperty(). We must also provide our own declarations
    for the \c READ and \c WRITE functions. The declaration of MyClass
    then might look like this:

    \snippet code/doc_src_properties.cpp 5

    The \c READ function is const and returns the property type. The
    \c WRITE function returns void and has exactly one parameter of
    the property type. The meta-object compiler enforces these
    requirements.

    Given a pointer to an instance of MyClass or a pointer to a
    QObject that is an instance of MyClass, we have two ways to set
    its priority property:

    \snippet code/doc_src_properties.cpp 6

    In the example, the enumeration type that is the property type is
    declared in MyClass and registered with the \l{Meta-Object System}
    using the Q_ENUM() macro. This makes the enumeration values
    available as strings for use as in the call to \l{QObject::}{setProperty()}. Had
    the enumeration type been declared in another class, its fully
    qualified name (i.e., OtherClass::Priority) would be required, and
    that other class would also have to inherit QObject and register
    the enumeration type there using the Q_ENUM() macro.

    A similar macro, Q_FLAG(), is also available. Like Q_ENUM(), it
    registers an enumeration type, but it marks the type as being a
    set of \e flags, i.e. values that can be OR'd together. An I/O
    class might have enumeration values \c Read and \c Write and then
    QObject::setProperty() could accept \c{Read | Write}. Q_FLAG()
    should be used to register this enumeration type.

    \section1 Dynamic Properties

    QObject::setProperty() can also be used to add \e new properties
    to an instance of a class at runtime. When it is called with a
    name and a value, if a property with the given name exists in the
    QObject, and if the given value is compatible with the property's
    type, the value is stored in the property, and true is returned.
    If the value is \e not compatible with the property's type, the
    property is \e not changed, and false is returned. But if the
    property with the given name doesn't exist in the QObject (i.e.,
    if it wasn't declared with Q_PROPERTY()), a new property with the
    given name and value is automatically added to the QObject, but
    false is still returned. This means that a return of false can't
    be used to determine whether a particular property was actually
    set, unless you know in advance that the property already exists
    in the QObject.

    Note that \e dynamic properties are added on a per instance basis,
    i.e., they are added to QObject, not QMetaObject. A property can
    be removed from an instance by passing the property name and an
    invalid QVariant value to QObject::setProperty(). The default
    constructor for QVariant constructs an invalid QVariant.

    Dynamic properties can be queried with QObject::property(), just
    like properties declared at compile time with Q_PROPERTY().

    \sa {Meta-Object System}, {Signals and Slots}

    \section1 Properties and Custom Types

    Custom types used by properties need to be registered using the
    Q_DECLARE_METATYPE() macro so that their values can be stored in
    QVariant objects. This makes them suitable for use with both
    static properties declared using the Q_PROPERTY() macro in class
    definitions and dynamic properties created at run-time.

    \sa Q_DECLARE_METATYPE(), QMetaType, QVariant

    \section1 Adding Additional Information to a Class

    Connected to the property system is an additional macro,
    Q_CLASSINFO(), that can be used to attach additional
    \e{name}--\e{value} pairs to a class's meta-object, for example:

    \snippet code/doc_src_properties.cpp 7

    Like other meta-data, class information is accessible at run-time
    through the meta-object; see QMetaObject::classInfo() for details.
*/