[core] Rework invokeWithCallback so that the callback is last

3 files changed, 25 insertions, 11 deletions

diff --git a/include/mbgl/util/run_loop.hpp b/include/mbgl/util/run_loop.hpp
index af1053dfdc..6559b72ef8 100644
--- a/include/mbgl/util/run_loop.hpp
+++ b/include/mbgl/util/run_loop.hpp
@@ -61,10 +61,10 @@ public:
     }
 
     // Invoke fn(args...) on this RunLoop, then invoke callback(results...) on the current RunLoop.
-    template <class Fn, class Cb, class... Args>
+    template <class Fn, class... Args>
     std::unique_ptr<AsyncRequest>
-    invokeWithCallback(Fn&& fn, Cb&& callback, Args&&... args) {
-        std::shared_ptr<WorkTask> task = WorkTask::makeWithCallback(std::forward<Fn>(fn), std::forward<Cb>(callback), std::forward<Args>(args)...);
+    invokeWithCallback(Fn&& fn, Args&&... args) {
+        std::shared_ptr<WorkTask> task = WorkTask::makeWithCallback(std::forward<Fn>(fn), std::forward<Args>(args)...);
         push(task);
         return std::make_unique<WorkRequest>(task);
     }
diff --git a/include/mbgl/util/work_task.hpp b/include/mbgl/util/work_task.hpp
index e9c2062d9c..dda8e5d00f 100644
--- a/include/mbgl/util/work_task.hpp
+++ b/include/mbgl/util/work_task.hpp
@@ -17,10 +17,10 @@ public:
     virtual void cancel() = 0;
 
     template <class Fn, class... Args>
-    static std::shared_ptr<WorkTask> make(Fn&& fn, Args&&... args);
+    static std::shared_ptr<WorkTask> make(Fn&&, Args&&...);
 
-    template <class Fn, class Cb, class... Args>
-    static std::shared_ptr<WorkTask> makeWithCallback(Fn&& fn, Cb&& callback, Args&&... args);
+    template <class Fn, class... Args>
+    static std::shared_ptr<WorkTask> makeWithCallback(Fn&&, Args&&...);
 };
 
 } // namespace mbgl
diff --git a/include/mbgl/util/work_task_impl.hpp b/include/mbgl/util/work_task_impl.hpp
index 2c016601bb..a2f55e00c5 100644
--- a/include/mbgl/util/work_task_impl.hpp
+++ b/include/mbgl/util/work_task_impl.hpp
@@ -62,10 +62,12 @@ std::shared_ptr<WorkTask> WorkTask::make(Fn&& fn, Args&&... args) {
         flag);
 }
 
-template <class Fn, class Cb, class... Args>
-std::shared_ptr<WorkTask> WorkTask::makeWithCallback(Fn&& fn, Cb&& callback, Args&&... args) {
-    auto flag = std::make_shared<std::atomic<bool>>();
-    *flag = false;
+namespace detail {
+template <class Tuple, size_t... Indexes>
+auto packageArgumentsAndCallback(std::shared_ptr<std::atomic<bool>> flag,
+                                 Tuple&& args,
+                                 std::index_sequence<Indexes...>) {
+    auto callback = std::get<sizeof...(Indexes)>(args);
 
     // Create a lambda L1 that invokes another lambda L2 on the current RunLoop R, that calls
     // the callback C. Both lambdas check the flag before proceeding. L1 needs to check the flag
@@ -82,7 +84,19 @@ std::shared_ptr<WorkTask> WorkTask::makeWithCallback(Fn&& fn, Cb&& callback, Arg
         }
     };
 
-    auto tuple = std::make_tuple(std::move(args)..., after);
+    return std::make_tuple(std::move(std::get<Indexes>(args))..., after);
+}
+} // namespace detail
+
+template <class Fn, class... Args>
+std::shared_ptr<WorkTask> WorkTask::makeWithCallback(Fn&& fn, Args&&... args) {
+    auto flag = std::make_shared<std::atomic<bool>>();
+    *flag = false;
+
+    auto tuple = detail::packageArgumentsAndCallback(flag,
+        std::forward_as_tuple(std::forward<Args>(args)...),
+        std::make_index_sequence<sizeof...(Args) - 1>());
+
     return std::make_shared<WorkTaskImpl<Fn, decltype(tuple)>>(
         std::move(fn),
         std::move(tuple),