very, very simple WP with midi CC out

include/libremidi/detail/observer.hpp

@@ -0,0 +1,26 @@
+#pragma once
+#include <libremidi/api.hpp>
+#include <libremidi/config.hpp>
+#include <libremidi/observer_configuration.hpp>
+
+#include <memory>
+#include <vector>
+
+namespace libremidi
+{
+class observer_api
+{
+public:
+  virtual ~observer_api() = default;
+
+  virtual libremidi::API get_current_api() const noexcept = 0;
+  virtual std::vector<libremidi::input_port> get_input_ports() const noexcept = 0;
+  virtual std::vector<libremidi::output_port> get_output_ports() const noexcept = 0;
+};
+
+template <typename T, typename Arg>
+std::unique_ptr<observer_api> make(libremidi::observer_configuration&& conf, Arg&& arg)
+{
+  return std::make_unique<T>(std::move(conf), std::move(arg));
+}
+}