[core] Make the mbgl/actor headers public

They will be needed by the DefaultFileSource, something that we
also export as public.

3 files changed, 168 insertions, 0 deletions

diff --git a/include/mbgl/actor/actor.hpp b/include/mbgl/actor/actor.hpp
new file mode 100644
index 0000000000..810114c513
--- /dev/null
+++ b/include/mbgl/actor/actor.hpp
@@ -0,0 +1,77 @@
+#pragma once
+
+#include <mbgl/actor/mailbox.hpp>
+#include <mbgl/actor/message.hpp>
+#include <mbgl/actor/actor_ref.hpp>
+#include <mbgl/util/noncopyable.hpp>
+
+#include <memory>
+
+namespace mbgl {
+
+/*
+    An `Actor<O>` is an owning reference to an asynchronous object of type `O`: an "actor".
+    Communication with an actor happens via message passing: you send a message to the object
+    (using `invoke`), passing a pointer to the member function to call and arguments which
+    are then forwarded to the actor.
+
+    The actor receives messages sent to it asynchronously, in a manner defined its `Scheduler`.
+    To store incoming messages before their receipt, each actor has a `Mailbox`, which acts as
+    a FIFO queue. Messages sent from actor S to actor R are guaranteed to be processed in the
+    order sent. However, relative order of messages sent by two *different* actors S1 and S2
+    to R is *not* guaranteed (and can't be: S1 and S2 may be acting asynchronously with respect
+    to each other).
+
+    An `Actor<O>` can be converted to an `ActorRef<O>`, a non-owning value object representing
+    a (weak) reference to the actor. Messages can be sent via the `Ref` as well.
+
+    It's safe -- and encouraged -- to pass `Ref`s between actors via messages. This is how two-way
+    communication and other forms of collaboration between multiple actors is accomplished.
+
+    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.)
+
+    Construction and destruction of an actor is currently synchronous: the corresponding `O`
+    object is constructed synchronously by the `Actor` constructor, and destructed synchronously
+    by the `~Actor` destructor, after ensuring that the `O` is not currently receiving an
+    asynchronous message. (Construction and destruction may change to be asynchronous in the
+    future.) The constructor of `O` is passed an `ActorRef<O>` referring to itself (which it
+    can use to self-send messages), followed by the forwarded arguments passed to `Actor<O>`.
+
+    Please don't send messages that contain shared pointers or references. That subverts the
+    purpose of the actor model: prohibiting direct concurrent access to shared state.
+*/
+
+template <class Object>
+class Actor : public util::noncopyable {
+public:
+    template <class... Args>
+    Actor(Scheduler& scheduler, Args&&... args_)
+        : mailbox(std::make_shared<Mailbox>(scheduler)),
+          object(self(), std::forward<Args>(args_)...) {
+    }
+
+    ~Actor() {
+        mailbox->close();
+    }
+
+    template <typename Fn, class... Args>
+    void invoke(Fn fn, Args&&... args) {
+        mailbox->push(actor::makeMessage(object, fn, std::forward<Args>(args)...));
+    }
+
+    ActorRef<std::decay_t<Object>> self() {
+        return ActorRef<std::decay_t<Object>>(object, mailbox);
+    }
+
+    operator ActorRef<std::decay_t<Object>>() {
+        return self();
+    }
+
+private:
+    std::shared_ptr<Mailbox> mailbox;
+    Object object;
+};
+
+} // namespace mbgl
diff --git a/include/mbgl/actor/actor_ref.hpp b/include/mbgl/actor/actor_ref.hpp
new file mode 100644
index 0000000000..9d858d823f
--- /dev/null
+++ b/include/mbgl/actor/actor_ref.hpp
@@ -0,0 +1,43 @@
+#pragma once
+
+#include <mbgl/actor/mailbox.hpp>
+#include <mbgl/actor/message.hpp>
+
+#include <memory>
+
+namespace mbgl {
+
+/*
+    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) {
+        if (auto mailbox = weakMailbox.lock()) {
+            mailbox->push(actor::makeMessage(object, fn, std::forward<Args>(args)...));
+        }
+    }
+
+private:
+    Object& object;
+    std::weak_ptr<Mailbox> weakMailbox;
+};
+
+} // namespace mbgl
diff --git a/include/mbgl/actor/message.hpp b/include/mbgl/actor/message.hpp
new file mode 100644
index 0000000000..cf071d4933
--- /dev/null
+++ b/include/mbgl/actor/message.hpp
@@ -0,0 +1,48 @@
+#pragma once
+
+#include <utility>
+
+namespace mbgl {
+
+// A movable type-erasing function wrapper. This allows to store arbitrary invokable
+// things (like std::function<>, or the result of a movable-only std::bind()) in the queue.
+// Source: http://stackoverflow.com/a/29642072/331379
+class Message {
+public:
+    virtual ~Message() = default;
+    virtual void operator()() = 0;
+};
+
+template <class Object, class MemberFn, class ArgsTuple>
+class MessageImpl : public Message {
+public:
+    MessageImpl(Object& object_, MemberFn memberFn_, ArgsTuple argsTuple_)
+      : object(object_),
+        memberFn(memberFn_),
+        argsTuple(std::move(argsTuple_)) {
+    }
+
+    void operator()() override {
+        invoke(std::make_index_sequence<std::tuple_size<ArgsTuple>::value>());
+    }
+
+    template <std::size_t... I>
+    void invoke(std::index_sequence<I...>) {
+        (object.*memberFn)(std::move(std::get<I>(argsTuple))...);
+    }
+
+    Object& object;
+    MemberFn memberFn;
+    ArgsTuple argsTuple;
+};
+
+namespace actor {
+
+template <class Object, class MemberFn, class... Args>
+std::unique_ptr<Message> makeMessage(Object& object, MemberFn memberFn, Args&&... args) {
+    auto tuple = std::make_tuple(std::forward<Args>(args)...);
+    return std::make_unique<MessageImpl<Object, MemberFn, decltype(tuple)>>(object, memberFn, std::move(tuple));
+}
+
+} // namespace actor
+} // namespace mbgl