core: rename mbgl directories to mln (#4511)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/mln/actor/actor_ref.hpp

@@ -0,0 +1,60 @@
+#pragma once
+
+#include <mln/actor/mailbox.hpp>
+#include <mln/actor/message.hpp>
+
+#include <memory>
+
+namespace mln {
+
+/**
+    An `ActorRef<O>` is a *non*-owning, weak reference to an actor of type `O`.
+   You can send it messages just like an `Actor<O>`. It's a value object: safe
+   to copy and pass between actors via messages.
+
+    An `ActorRef<O>` does not extend the lifetime of the corresponding
+   `Actor<O>`. That's determined entirely by whichever object owns the
+   `Actor<O>` -- the actor's "supervisor".
+
+    It's safe for a `Ref` to outlive its `Actor` -- the reference is "weak", and
+   does not extend the lifetime of the owning Actor, and sending a message to a
+   `Ref` whose `Actor` has died is a no-op. (In the future, a dead-letters queue
+   or log may be implemented.)
+*/
+template <class Object>
+class ActorRef {
+public:
+    ActorRef(Object& object_, std::weak_ptr<Mailbox> weakMailbox_)
+        : object(&object_),
+          weakMailbox(std::move(weakMailbox_)) {}
+
+    template <typename Fn, class... Args>
+    void invoke(Fn fn, Args&&... args) const {
+        if (auto mailbox = weakMailbox.lock()) {
+            mailbox->push(actor::makeMessage(*object, fn, std::forward<Args>(args)...));
+        }
+    }
+
+    template <typename Fn, class... Args>
+    auto ask(Fn fn, Args&&... args) const {
+        // Result type is deduced from the function's return type
+        using ResultType = std::invoke_result_t<decltype(fn), Object, Args...>;
+
+        std::promise<ResultType> promise;
+        auto future = promise.get_future();
+
+        if (auto mailbox = weakMailbox.lock()) {
+            mailbox->push(actor::makeMessage(std::move(promise), *object, fn, std::forward<Args>(args)...));
+        } else {
+            promise.set_exception(std::make_exception_ptr(std::runtime_error("Actor has gone away")));
+        }
+
+        return future;
+    }
+
+private:
+    Object* object;
+    std::weak_ptr<Mailbox> weakMailbox;
+};
+
+} // namespace mln