QQuickAttachedPropertyPropagator Class

详细说明

在 QML 中，可以为对象附加属性和信号处理器。提供附加属性》（Providing Attached Properties）详细介绍了如何公开自己的 C++ 附加类型。

QQuickAttachedPropertyPropagator 提供了从父对象向子对象传播附加属性的 API，类似于font 和palette 传播。它支持通过items 、popups 和windows 传播。

如果属性传播并不重要，可以考虑使用C++ 或QML单例，因为它更适合这种用例，而且只需要一个QObject ，效率更高。

实现自定义附加属性

1. 从 QQuickAttachedPropertyPropagator 派生一个公开附加属性的类。

   例如，要实现MyStyle.theme 附加属性，请声明MyStyle 类：

   class MYSTYLE_EXPORT MyStyle : public QQuickAttachedPropertyPropagator
2. 在类的构造函数中调用initialize() ：

   MyStyle::MyStyle(QObject *parent)
       : QQuickAttachedPropertyPropagator(parent)
       , m_theme(globalTheme)
   {
       // A static function could be called here that reads globalTheme from a
       // settings file once at startup. That value would override the global
       // value. This is similar to what the Imagine and Material styles do, for
       // example.
   
       initialize();
   }
3. 根据需要为附加属性定义 set/inherit/propagate/reset 函数。例如，定义一个theme 附加属性：

   MyStyle::Theme MyStyle::theme() const
   {
       return m_theme;
   }
   
   void MyStyle::setTheme(Theme theme)
   {
       // When this function is called, we know that the user has explicitly
       // set a theme on this attached object. We set this to true even if
       // the effective theme didn't change, because it's important that
       // the user's specified value is respected (and not inherited from
       // from the parent).
       m_explicitTheme = true;
       if (m_theme == theme)
           return;
   
       m_theme = theme;
       propagateTheme();
       themeChange();
   
   }
   
   void MyStyle::inheritTheme(Theme theme)
   {
       if (m_explicitTheme || m_theme == theme)
           return;
   
       m_theme = theme;
       propagateTheme();
       themeChange();
   }
   
   void MyStyle::propagateTheme()
   {
       const auto styles = attachedChildren();
       for (QQuickAttachedPropertyPropagator *child : styles) {
           MyStyle *myStyle = qobject_cast<MyStyle *>(child);
           if (myStyle)
               myStyle->inheritTheme(m_theme);
       }
   }
   
   void MyStyle::resetTheme()
   {
       if (!m_explicitTheme)
           return;
   
       m_explicitTheme = false;
       MyStyle *myStyle = qobject_cast<MyStyle *>(attachedParent());
       inheritTheme(myStyle ? myStyle->theme() : globalTheme);
   }
4. 重新实现attachedParentChange() 以处理属性继承：

   void MyStyle::attachedParentChange(QQuickAttachedPropertyPropagator *newParent, QQuickAttachedPropertyPropagator *oldParent)
   {
       Q_UNUSED(oldParent);
       MyStyle *attachedParentStyle = qobject_cast<MyStyle *>(newParent);
       if (attachedParentStyle) {
           inheritTheme(attachedParentStyle->theme());
           // Do any other inheriting here...
       }
   }
5. 实现静态qmlAttachedProperties 函数，并用QML_ELEMENT 和QML_ATTACHED 声明该类型为附加 QML 类型，详见Providing Attached Properties（提供附加属性）：

   MyStyle *MyStyle::qmlAttachedProperties(QObject *object)
   {
       return new MyStyle(object);
   }

完整的实现可参见Qt Quick Controls - 附加样式属性示例。