Add bt::lazy

libs/bt/include/psemek/bt/lazy.hpp

@@ -0,0 +1,54 @@
+#pragma once
+
+#include <psemek/bt/node.hpp>
+
+namespace psemek::bt
+{
+
+	template <typename Tree, typename NodeFactoryFn>
+	struct lazy_node;
+
+	template <typename Time, typename Event, typename ... Args, typename NodeFactoryFn>
+	struct lazy_node<tree<Time, Event, Args...>, NodeFactoryFn>
+		: node<tree<Time, Event, Args...>>
+	{
+		using tree_type = tree<Time, Event, Args...>;
+		using node_type = node<tree_type>;
+		using typename node_type::finished;
+		using typename node_type::status;
+
+		lazy_node(NodeFactoryFn && node_factory)
+			: node_factory_(node_factory)
+		{}
+
+		void start(Args ... args) override
+		{
+			child_ = node_factory_(args...);
+			child_->start(args...);
+		}
+
+		status update(Time dt, Args ... args) override
+		{
+			auto result = child_->update(dt, args...);
+			if (std::holds_alternative<finished>(result))
+				child_ = nullptr;
+			return result;
+		}
+
+		bool event(Event const & event, Args ... args) override
+		{
+			return child_->event(event, args...);
+		}
+
+	private:
+		NodeFactoryFn node_factory_;
+		node_ptr<tree_type> child_;
+	};
+
+	template <typename Tree, typename NodeFactoryFn>
+	node_ptr<Tree> lazy(NodeFactoryFn && node_factory)
+	{
+		return std::make_unique<lazy_node<Tree, NodeFactoryFn>>(std::move(node_factory));
+	}
+
+}