[core] implement ask pattern in actor

include/mbgl/actor/actor.hpp

@@ -6,6 +6,7 @@
 #include <mbgl/util/noncopyable.hpp>
 
 #include <memory>
+#include <future>
 
 namespace mbgl {
 
@@ -61,6 +62,17 @@ public:
         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);
     }

include/mbgl/actor/message.hpp

@@ -1,5 +1,8 @@
 #pragma once
 
+#include <mbgl/util/optional.hpp>
+
+#include <future>
 #include <utility>
 
 namespace mbgl {
@@ -36,6 +39,31 @@ public:
     ArgsTuple argsTuple;
 };
 
+template <class ResultType, class Object, class MemberFn, class ArgsTuple>
+class AskMessageImpl : public Message {
+public:
+    AskMessageImpl(std::promise<ResultType> promise_, Object& object_, MemberFn memberFn_, ArgsTuple argsTuple_)
+        : object(object_),
+          memberFn(memberFn_),
+          argsTuple(std::move(argsTuple_)),
+          promise(std::move(promise_)) {
+    }
+
+    void operator()() override {
+        promise.set_value(ask(std::make_index_sequence<std::tuple_size<ArgsTuple>::value>()));
+    }
+
+    template <std::size_t... I>
+    ResultType ask(std::index_sequence<I...>) {
+        return (object.*memberFn)(std::move(std::get<I>(argsTuple))...);
+    }
+
+    Object& object;
+    MemberFn memberFn;
+    ArgsTuple argsTuple;
+    std::promise<ResultType> promise;
+};
+
 namespace actor {
 
 template <class Object, class MemberFn, class... Args>
@@ -44,5 +72,11 @@ std::unique_ptr<Message> makeMessage(Object& object, MemberFn memberFn, Args&&..
     return std::make_unique<MessageImpl<Object, MemberFn, decltype(tuple)>>(object, memberFn, std::move(tuple));
 }
 
+template <class ResultType, class Object, class MemberFn, class... Args>
+std::unique_ptr<Message> makeMessage(std::promise<ResultType>&& promise, Object& object, MemberFn memberFn, Args&&... args) {
+    auto tuple = std::make_tuple(std::forward<Args>(args)...);
+    return std::make_unique<AskMessageImpl<ResultType, Object, MemberFn, decltype(tuple)>>(std::move(promise), object, memberFn, std::move(tuple));
+}
+
 } // namespace actor
 } // namespace mbgl

test/actor/actor.test.cpp

@@ -281,3 +281,27 @@ TEST(Actor, NonConcurrentMailbox) {
     test.invoke(&Test::end);
     endedFuture.wait();
 }
+
+TEST(Actor, Ask) {
+    // Asking for a result
+
+    struct Test {
+
+        Test(ActorRef<Test>) {}
+
+        int doubleIt(int i) {
+            return i * 2;
+        }
+    };
+
+    ThreadPool pool { 2 };
+    Actor<Test> test(pool);
+
+    auto result = test.ask(&Test::doubleIt, 1);
+
+    ASSERT_TRUE(result.valid());
+    
+    auto status = result.wait_for(std::chrono::seconds(1));
+    ASSERT_EQ(std::future_status::ready, status);
+    ASSERT_EQ(2, result.get());
+}