singleton: add reloadable Singleton type

src/core/singleton.hpp

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <qobject.h>
+#include <qqmlintegration.h>
+#include <qtclasshelpermacros.h>
+#include <qtmetamacros.h>
+#include <qurl.h>
+
+#include "reload.hpp"
+
+///! The root component for reloadable singletons.
+/// All singletons should inherit from this type.
+class Singleton: public ReloadPropagator {
+	Q_OBJECT;
+	QML_ELEMENT;
+
+public:
+	void componentComplete() override;
+};
+
+class SingletonRegistry {
+public:
+	SingletonRegistry() = default;
+	~SingletonRegistry();
+	Q_DISABLE_COPY_MOVE(SingletonRegistry);
+
+	void install(const QUrl& url, Singleton* singleton);
+	void flip();
+
+	static SingletonRegistry* instance();
+
+private:
+	QMap<QUrl, QObject*>* previousRegistry = nullptr;
+	QMap<QUrl, QObject*>* currentRegistry = nullptr;
+};