Qt's Property System

Using Enums and Flags in Properties

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;
 };

Note that if you want to register an enum (using Q_ENUMS()) that is declared in another class, the enum must be fully qualified with the name of the class defining it. In addition, the class defining the enum has to inherit QObject as well as declare the enum using Q_ENUMS().

Another similar macro is Q_FLAGS(). Like Q_ENUMS(), it registers an enumeration type but marks it in addition as a set of "flags", 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_FLAGS(), rather than Q_ENUMS().

The remaining keywords in the Q_PROPERTY() section are RESET, DESIGNABLE, SCRIPTABLE, and STORED.

• RESET names a function that will set the property to its default state (which may have changed since initialization). The function must return void and take no arguments.
• DESIGNABLE declares whether this property is suitable for modification by a GUI design tool. The default is true for writable properties; otherwise false. Instead of true or false, you can specify a boolean member function.
• SCRIPTABLE declares whether this property is suited for access by a scripting engine. The default is true. Instead of true or false, you can specify a boolean member function.
• STORED declares whether the property's value must be remembered when storing an object's state. Stored makes only sense for writable properties. The default value is true. Technically superfluous properties (like QPoint pos if QRect geometry is already a property) define this to be false.

 Q_CLASSINFO("Version", "3.0.0")