1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
|
#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>
#include <future>
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)...));
}
template <typename Fn, class... Args>
auto ask(Fn fn, Args&&... args) {
// Result type is deduced from the function's return type
using ResultType = typename std::result_of<decltype(fn)(Object, Args...)>::type;
std::promise<ResultType> promise;
auto future = promise.get_future();
mailbox->push(actor::makeMessage(std::move(promise), object, fn, std::forward<Args>(args)...));
return future;
}
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
|
