Add desktop runners (#892)

testing_app/windows/runner/run_loop.h

@@ -0,0 +1,40 @@
+#ifndef RUNNER_RUN_LOOP_H_
+#define RUNNER_RUN_LOOP_H_
+
+#include <flutter/flutter_engine.h>
+
+#include <chrono>
+#include <set>
+
+// A runloop that will service events for Flutter instances as well
+// as native messages.
+class RunLoop {
+ public:
+  RunLoop();
+  ~RunLoop();
+
+  // Prevent copying
+  RunLoop(RunLoop const&) = delete;
+  RunLoop& operator=(RunLoop const&) = delete;
+
+  // Runs the run loop until the application quits.
+  void Run();
+
+  // Registers the given Flutter instance for event servicing.
+  void RegisterFlutterInstance(
+      flutter::FlutterEngine* flutter_instance);
+
+  // Unregisters the given Flutter instance from event servicing.
+  void UnregisterFlutterInstance(
+      flutter::FlutterEngine* flutter_instance);
+
+ private:
+  using TimePoint = std::chrono::steady_clock::time_point;
+
+  // Processes all currently pending messages for registered Flutter instances.
+  TimePoint ProcessFlutterMessages();
+
+  std::set<flutter::FlutterEngine*> flutter_instances_;
+};
+
+#endif  // RUNNER_RUN_LOOP_H_