Implement react::join

libs/react/include/psemek/react/join.hpp

@@ -0,0 +1,59 @@
+#pragma once
+
+#include <psemek/react/value.hpp>
+
+namespace psemek::react
+{
+
+	template <typename T>
+	value<T> join(value<value<T>> x)
+	{
+		auto node = std::make_shared<detail::node<T>>();
+		auto weak_node = std::weak_ptr{node};
+
+		auto internal_subscriber = [weak_node]{
+			if (auto node = weak_node.lock())
+			{
+				node->cached_value = std::nullopt;
+				node->internal_signal();
+			}
+		};
+
+		auto inner_external_subscriber = [weak_node](T const &){
+			if (auto node = weak_node.lock())
+				if (!node->cached_value)
+					node->external_signal(node->value());
+		};
+
+		auto inner_internal_token = std::make_shared<util::signal<void>::subscription_token>();
+		auto inner_external_token = std::make_shared<typename util::signal<T>::subscription_token>();
+
+		auto external_subscriber = [weak_node, inner_internal_token, inner_external_token, internal_subscriber, inner_external_subscriber](value<T> const & x){
+			if (auto node = weak_node.lock())
+			{
+				if (!node->cached_value)
+				{
+					*inner_internal_token = x.subscribe(detail::internal_tag{}, internal_subscriber);
+					*inner_external_token = x.subscribe(inner_external_subscriber);
+					node->external_signal(node->value());
+				}
+			}
+		};
+
+		auto internal_token = x.subscribe(detail::internal_tag{}, internal_subscriber);
+		auto external_token = x.subscribe(external_subscriber);
+
+		node->getter = [
+			inner_internal_token,
+			inner_external_token,
+			internal_token,
+			external_token,
+			x
+		]{
+			return **x;
+		};
+
+		return value<T>(detail::internal_tag{}, std::move(node));
+	}
+
+}