fix world previews generation

src/util/CallbacksSet.hpp

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <memory>
+#include <vector>
+#include <functional>
+
+template<typename... Args>
+class CallbacksSet {
+public:
+    using Func = std::function<void(Args...)>;
+private:
+    std::unique_ptr<std::vector<Func>> callbacks;
+public:
+    void listen(const Func& callback) {
+        if (callbacks == nullptr) {
+            callbacks = std::make_unique<std::vector<Func>>();
+        }
+        callbacks->push_back(callback);
+    }
+
+    void notify(Args&&... args) {
+        if (callbacks) {
+            for (auto& callback : *callbacks) {
+                callback(std::forward<Args>(args)...);
+            }
+        }
+    }
+};