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Qt provides a sophisticated property system similar to those supplied by some compiler vendors. However, as a compiler- and platform-independent library, Qt cannot rely on non-standard compiler features like __property or [property]. Our solution works with any standard C++ compiler on every platform we support. It's based on the meta-object system that also provides object communication through signals and slots.
The Q_PROPERTY macro in a class declaration declares a property. Properties can only be declared in classes that inherit QObject. A second macro, Q_OVERRIDE, can be used to override some aspects of an inherited property in a subclass.
To the outer world, a property appears to be similar to a data member. But properties have several features that distinguish them from ordinary data members:
The read, write and reset functions can be just about any member functions, inherited or not, virtual or not. The only exception is that member functions must be inherited from the first inherited class in the case of multiple inheritance.
Properties can be read and written through generic functions in QObject without knowing anything about the class in use. In the code snippet below, the QObject::setProperty() call is equivalent QButton::setDown() call is equivalent to the QObject::setProperty() call:
QPushButton *button = new QPushButton; QObject *object = button; // button and object point to the same object button->setDown(true); object->setProperty("down", true);
Equivalent, that is, except that the first is faster, and provides much better diagnostics at compile time. When practical, the first is better. However, since you can get a list of all available properties for any QObject through its QMetaObject, QObject::setProperty() can give you control over classes that weren't available at compile time.
As well as QObject::setProperty(), there is a corresponding QObject::property() function. QMetaObject::propertyCount() returns the number of all available properties. QMetaObject::property() returns the property data for a given property index: a QMetaProperty object.
Here's a simple example that shows the most important property functions in use:
class MyClass : public QObject { Q_OBJECT public: MyClass(QObject *parent = 0); ~MyClass(); enum Priority { High, Low, VeryHigh, VeryLow }; void setPriority(Priority priority); Priority priority() const; };
The class has a property priority that is not yet known to the meta object system. In order to make the property known, you must declare it with the Q_PROPERTY macro. The syntax is as follows:
Q_PROPERTY(type name READ getFunction [WRITE setFunction] [RESET resetFunction] [DESIGNABLE bool] [SCRIPTABLE bool] [STORED bool])
For the declaration to be valid, the get function must be const and to return either the type itself, a pointer to it, or a reference to it. The optional write function must return void and must take exactly one argument, either the type itself, a pointer or a const reference to it. The meta object compiler enforces this.
The type of a property can be any QVariant supported type or an enumeration type declared in the class itself. Since MyClass uses the enumeration type Priority for the property, this type must be registered with the property system as well.
There are two exceptions to the above: The type of a property can also be either QList<QVariant> or QMap<QString,QVariant>. In these cases the type must be specified as QList or as QMap (i.e. without their template parameters).
It is possible to set a value by name, like this:
obj->setProperty("priority", "VeryHigh");
In the case of QList and QMap properties the value passes is a QVariant whose value is the entire list or map.
Enumeration types are registered with the Q_ENUMS macro. Here's the final class declaration including the property related declarations:
class MyClass : public QObject { Q_OBJECT Q_PROPERTY(Priority priority READ priority WRITE setPriority) Q_ENUMS(Priority) public: MyClass(QObject *parent = 0); ~MyClass(); enum Priority { High, Low, VeryHigh, VeryLow }; void setPriority(Priority priority); Priority priority() const; };
Another similar macro is Q_SETS. Like Q_ENUMS, it registers an enumeration type but marks it in addition as a "set", i.e. the enumeration values can be OR'd together. An I/O class might have enumeration values Read and Write and accept Read | Write: such an enum is best handled with Q_SETS, rather than Q_ENUMS.
The remaining keywords in the Q_PROPERTY section are RESET, DESIGNABLE, SCRIPTABLE, and STORED.
Connected to the property system is an additional macro, Q_CLASSINFO, that can be used to attach additional name/value-pairs to a class' meta object, for example:
Q_CLASSINFO("Version", "3.0.0")
Like other meta data, class information is accessible at runtime through the meta object, see QMetaObject::classInfo() for details.
When you inherit a QObject subclass you may wish to override some aspects of some of the class's properties.
For example, in QWidget we have the autoMask property defined like this:
Q_PROPERTY(bool autoMask READ autoMask WRITE setAutoMask DESIGNABLE false SCRIPTABLE false)
But we need to make the auto mask property designable in some QWidget subclasses. Similarly some classes will need this property to be scriptable (e.g., for QSA). This is achieved by overriding these features of the property in a subclass. In QCheckBox, for example, we achieve this using the following code:
Q_OVERRIDE(bool autoMask DESIGNABLE true SCRIPTABLE true)
Another example is QToolButton. By default QToolButton has a read-only toggleButton property, because that's what it inherits from QButton:
Q_PROPERTY(bool toggleButton READ isToggleButton)
But we want to make our tool buttons able to be toggled, so we write a WRITE function in QToolButton, and use the following property override to make it acessible:
Q_OVERRIDE(bool toggleButton WRITE setToggleButton)
The result is read-write (and scriptable and designable, since we now have a "write" function) boolean property toggleButton for tool buttons.
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