[core] Actor/ActorRef - ask calls to void methods

- allows to wait for execution of void methods as well as non-void methods

include/mbgl/actor/message.hpp

@@ -64,6 +64,32 @@ public:
     std::promise<ResultType> promise;
 };
 
+template <class Object, class MemberFn, class ArgsTuple>
+class AskMessageImpl<void, Object, MemberFn, ArgsTuple> : public Message {
+public:
+    AskMessageImpl(std::promise<void> promise_, Object& object_, MemberFn memberFn_, ArgsTuple argsTuple_)
+            : object(object_),
+            memberFn(memberFn_),
+            argsTuple(std::move(argsTuple_)),
+            promise(std::move(promise_)) {
+    }
+
+    void operator()() override {
+        ask(std::make_index_sequence<std::tuple_size<ArgsTuple>::value>());
+        promise.set_value();
+    }
+
+    template <std::size_t... I>
+    void ask(std::index_sequence<I...>) {
+        (object.*memberFn)(std::move(std::get<I>(argsTuple))...);
+    }
+
+    Object& object;
+    MemberFn memberFn;
+    ArgsTuple argsTuple;
+    std::promise<void> promise;
+};
+
 namespace actor {
 
 template <class Object, class MemberFn, class... Args>

test/actor/actor.test.cpp

@@ -306,6 +306,28 @@ TEST(Actor, Ask) {
     ASSERT_EQ(2, result.get());
 }
 
+TEST(Actor, AskVoid) {
+    // Ask waits for void methods
+
+    struct Test {
+        bool& executed;
+
+        Test(bool& executed_) : executed(executed_) {
+        }
+
+        void doIt() {
+            executed = true;
+        }
+    };
+
+    ThreadPool pool { 1 };
+    bool executed = false;
+    Actor<Test> actor(pool, executed);
+
+    actor.ask(&Test::doIt).get();
+    EXPECT_TRUE(executed);
+}
+
 TEST(Actor, NoSelfActorRef) {
     // Not all actors need a reference to self
     

test/actor/actor_ref.test.cpp

@@ -62,6 +62,28 @@ TEST(ActorRef, Ask) {
     EXPECT_EQ(30, ref.ask(&Test::echo, 30).get());
 }
 
+TEST(ActorRef, AskVoid) {
+    // Ask waits for void methods
+    
+    struct Test {
+        bool& executed;
+        
+        Test(bool& executed_) : executed(executed_) {
+        }
+        
+        void doIt() {
+            executed = true;
+        }
+    };
+    
+    ThreadPool pool { 1 };
+    bool executed = false;
+    Actor<Test> actor(pool, executed);
+    ActorRef<Test> ref = actor.self();
+    
+    ref.ask(&Test::doIt).get();
+    EXPECT_TRUE(executed);
+}
 
 TEST(ActorRef, AskOnDestroyedActor) {
     // Tests behavior when calling ask() after the