very, very simple WP with midi CC out

include/libremidi/observer.cpp

@@ -0,0 +1,87 @@
+#if !defined(LIBREMIDI_HEADER_ONLY)
+  #include <libremidi/libremidi.hpp>
+#endif
+
+#include <libremidi/backends.hpp>
+#include <libremidi/detail/midi_api.hpp>
+
+namespace libremidi
+{
+LIBREMIDI_INLINE auto make_observer(auto base_conf, std::any api_conf)
+{
+  std::unique_ptr<observer_api> ptr;
+  auto from_api = [&]<typename T>(T& /*backend*/) mutable {
+    if (auto conf = std::any_cast<typename T::midi_observer_configuration>(&api_conf))
+    {
+      ptr = libremidi::make<typename T::midi_observer>(std::move(base_conf), std::move(*conf));
+      return true;
+    }
+    return false;
+  };
+  std::apply([&](auto&&... b) { (from_api(b) || ...); }, midi1::available_backends);
+  std::apply([&](auto&&... b) { (from_api(b) || ...); }, midi2::available_backends);
+  return ptr;
+}
+
+LIBREMIDI_INLINE observer::observer(const observer_configuration& base_conf) noexcept
+{
+  for (const auto& api : available_apis())
+  {
+    try
+    {
+      impl_ = make_observer(base_conf, observer_configuration_for(api));
+    }
+    catch (const std::exception& e)
+    {
+    }
+
+    if (impl_)
+      return;
+  }
+  if (!impl_)
+    impl_ = std::make_unique<observer_dummy>(observer_configuration{}, dummy_configuration{});
+}
+
+LIBREMIDI_INLINE observer::observer(observer_configuration base_conf, std::any api_conf)
+    : impl_{make_observer(base_conf, api_conf)}
+{
+  if (!impl_)
+    throw midi_exception("Could not open observer for the given API");
+}
+
+LIBREMIDI_INLINE observer::observer(observer&& other) noexcept
+    : impl_{std::move(other.impl_)}
+{
+  other.impl_ = std::make_unique<libremidi::observer_dummy>(
+      observer_configuration{}, dummy_configuration{});
+}
+
+LIBREMIDI_INLINE observer& observer::operator=(observer&& other) noexcept
+{
+  this->impl_ = std::move(other.impl_);
+  other.impl_ = std::make_unique<libremidi::observer_dummy>(
+      observer_configuration{}, dummy_configuration{});
+  return *this;
+}
+
+LIBREMIDI_INLINE
+observer::~observer() = default;
+
+LIBREMIDI_INLINE
+libremidi::API observer::get_current_api() const noexcept
+{
+  return impl_->get_current_api();
+}
+
+LIBREMIDI_INLINE
+std::vector<libremidi::input_port> observer::get_input_ports() const noexcept
+{
+  return impl_->get_input_ports();
+}
+
+LIBREMIDI_INLINE
+std::vector<libremidi::output_port> observer::get_output_ports() const noexcept
+{
+  return impl_->get_output_ports();
+}
+}