[cpp] Add Qt Desginer widget plugin examples

cpp/qt/designer-plugin-collection/widgetplugin.h

@@ -0,0 +1,52 @@
+/*##############################################################################
+## Author: Shaun Reed                                                         ##
+## Legal: All Content (c) 2022 Shaun Reed, all rights reserved                ##
+## About: Example Qt Designer widget plugin                                   ##
+##                                                                            ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0   ##
+################################################################################
+*/
+
+#ifndef KLIPS_WIDGETPLUGIN_H
+#define KLIPS_WIDGETPLUGIN_H
+
+#include <QDesignerCustomWidgetInterface>
+
+class WidgetPlugin : public QObject, public QDesignerCustomWidgetInterface {
+  Q_OBJECT
+  Q_PLUGIN_METADATA(IID "com.Klips.WidgetPlugin")
+  Q_INTERFACES(QDesignerCustomWidgetInterface)
+
+  using Factory = std::function<QWidget *(QWidget *)>;
+
+public:
+  WidgetPlugin(QString group, QString name, Factory factory);
+  WidgetPlugin(QString group, QString name, QString include, Factory factory);
+
+  explicit WidgetPlugin(QObject *parent = nullptr) : QObject(parent) {}
+
+  ~WidgetPlugin() = default;
+
+public:
+  [[nodiscard]] QString group() const override;
+  [[nodiscard]] QString name() const override;
+  [[nodiscard]] QString includeFile() const override;
+  QWidget *createWidget(QWidget *parent) override;
+
+  [[nodiscard]] QString toolTip() const override;
+  [[nodiscard]] QString whatsThis() const override;
+  [[nodiscard]] QIcon icon() const override;
+  [[nodiscard]] bool isContainer() const override;
+  [[nodiscard]] bool isInitialized() const override;
+  void initialize(QDesignerFormEditorInterface *core) override;
+
+private:
+  bool m_initialized = false;
+
+  QString m_group;
+  QString m_name;
+  QString m_includeFile;
+  Factory m_factory;
+};
+
+#endif // KLIPS_WIDGETPLUGIN_H