C++: Fixed sixtyfps::blocking_invoke_from_main_loop when the callable returns void

Fixes #623

CHANGELOG.md

@@ -6,6 +6,7 @@ All notable changes to this project will be documented in this file.
 ### Fixed
 
  - The Slider's changed callback not being called with the fluent style (#621)
+ - C++: Fixed compilation error in `sixtyfps::blocking_invoke_from_main_loop` when the callable returns `void` (#623)
 
 ## [0.1.4] - 2021-10-22
 

api/sixtyfps-cpp/include/sixtyfps.h

@@ -855,7 +855,7 @@ void invoke_from_event_loop(Functor f)
 ///     ...
 /// }
 /// ```
-template<typename Functor>
+template<typename Functor, typename = std::enable_if_t<!std::is_void_v<std::invoke_result_t<Functor>>>>
 auto blocking_invoke_from_event_loop(Functor f) -> std::invoke_result_t<Functor> {
     std::optional<std::invoke_result_t<Functor>> result;
     std::mutex mtx;
@@ -871,6 +871,21 @@ auto blocking_invoke_from_event_loop(Functor f) -> std::invoke_result_t<Functor>
     return std::move(*result);
 }
 
+template<typename Functor, typename = std::enable_if_t<std::is_void_v<std::invoke_result_t<Functor>>>>
+auto blocking_invoke_from_event_loop(Functor f) -> void {
+    std::mutex mtx;
+    std::condition_variable cv;
+    bool ok = false;
+    invoke_from_event_loop([&] {
+        f();
+        std::unique_lock lock(mtx);
+        ok = true;
+        cv.notify_one();
+    });
+    std::unique_lock lock(mtx);
+    cv.wait(lock, [&] { return ok; });
+}
+
 namespace private_api {
 
 /// Registers a font by the specified path. The path must refer to an existing

api/sixtyfps-cpp/tests/eventloop.cpp

@@ -67,9 +67,12 @@ TEST_CASE("Blocking Event from thread")
             return std::make_unique<int>(42);
         });
         called = *foo;
-        sixtyfps::invoke_from_event_loop([&] {
+        int xxx = 123;
+        sixtyfps::blocking_invoke_from_event_loop([&] {
             sixtyfps::quit_event_loop();
+            xxx = 888999;
         });
+        REQUIRE(xxx == 888999);
     });
 
     sixtyfps::run_event_loop();