add observer_handler class

src/util/observer_handler.hpp

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <functional>
+
+class observer_handler {
+public:
+    observer_handler() = default;
+
+    observer_handler(std::function<void()> destructor)
+        : destructor(std::move(destructor)) {
+    }
+
+    observer_handler(const observer_handler&) = delete;
+
+    observer_handler(observer_handler&& handler) noexcept
+        : destructor(std::move(handler.destructor)) {
+    }
+
+    ~observer_handler() {
+        if (destructor) {
+            destructor();
+        }
+    }
+
+    bool operator==(std::nullptr_t) const {
+        return destructor == nullptr;
+    }
+
+    observer_handler& operator=(const observer_handler& handler) = delete;
+
+    observer_handler& operator=(observer_handler&& handler) noexcept {
+        if (destructor) {
+            destructor();
+        }
+        destructor = std::move(handler.destructor);
+        return *this;
+    }
+private:
+    std::function<void()> destructor;
+};