QObjectComputedProperty 类

template <typename Class, typename T, auto Offset, auto Getter> class QObjectComputedProperty

QObjectComputedProperty 类是帮助将旧特性移植到可绑定特性系统的模板类。 更多...

头：	#include <QObjectComputedProperty>
CMake:	find_package(Qt6 REQUIRED COMPONENTS Core) target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake:	QT += core
Since:	Qt 6.0
继承：	QUntypedPropertyData

宏

(从 6.0 起)

Q_OBJECT_COMPUTED_PROPERTY ( containingClass , type , name , callback )

详细描述

QObjectComputedProperty is a read-only property which is recomputed on each read. It does not store the computed value. It is one of the Qt internal classes implementing Qt Bindable Properties . QObjectComputedProperty is usually not used directly, instead an instance of it is created by using the Q_OBJECT_COMPUTED_PROPERTY 宏。

见以下范例。

class Client{};
class MyClassPrivate : public QObjectPrivate
{
public:
    QList<Client> clients;
    bool hasClientsActualCalculation() const { return clients.size() > 0; }
    Q_OBJECT_COMPUTED_PROPERTY(MyClassPrivate, bool, hasClientsData,
                               &MyClassPrivate::hasClientsActualCalculation)
};
class MyClass : public QObject
{
    Q_OBJECT
    Q_PROPERTY(bool hasClients READ hasClients STORED false BINDABLE bindableHasClients)
public:
    QBindable<bool> bindableHasClients()
    {
        return QBindable<bool>(&d_func()->hasClientsData);
    }
    bool hasClients() const
    {
        return d_func()->hasClientsData.value();
    }
    void addClient(const Client &c)
    {
        Q_D(MyClass);
        d->clients.push_back(c);
        // notify that the value could have changed
        d->hasClientsData.notify();
    }
private:
    Q_DECLARE_PRIVATE(MyClass)
};
					

The rules for getters in Bindable Property Getters and Setters also apply for QObjectComputedProperty. Especially, the getter should be trivial and only return the value of the QObjectComputedProperty object. The callback given to the QObjectComputedProperty should usually be a private method which is only called by the QObjectComputedProperty.

No setter is required or allowed, as QObjectComputedProperty is read-only.

To correctly participate in dependency handling, QObjectComputedProperty has to know when its value, the result of the callback given to it, might have changed. Whenever a bindable property used in the callback changes, this happens automatically. If the result of the callback might change because of a change in a value which is not a bindable property, it is the developer's responsibility to call notify on the QObjectComputedProperty object. This will inform dependent properties about the potential change.

Note that calling notify might trigger change handlers in dependent properties, which might in turn use the object the QObjectComputedProperty is a member of. So notify must not be called when in a transitional or invalid state.

QObjectComputedProperty is not suitable for use with a computation that depends on any input that might change without notice, such as the contents of a file.

另请参阅 Q_OBJECT_COMPUTED_PROPERTY , QProperty , QObjectBindableProperty , Qt 的特性系统 ，和 Qt Bindable Properties .

宏文档编制

[since 6.0] Q_OBJECT_COMPUTED_PROPERTY ( containingClass , type , name , callback )

Declares a QObjectComputedProperty inside containingClass 类型 type with name name 。自变量 callback specifies a GETTER function to be called when the property is evaluated.

该宏在 Qt 6.0 引入。

另请参阅 QObjectBindableProperty , Qt 的特性系统 ，和 Qt Bindable Properties .

内容

• 宏

• 详细描述