First cut of pulling out the relevant parts of my original game to make a little framework.

tests/event_router.cpp

@@ -0,0 +1,57 @@
+#include <catch2/catch_test_macros.hpp>
+#include <fmt/core.h>
+#include <string>
+#include "gui/event_router.hpp"
+
+using namespace fmt;
+using namespace gui;
+using enum gui::routing::Event;
+using enum gui::routing::State;
+
+using EventScript = std::vector<routing::Event>;
+
+void run_script(routing::Router& router, routing::State expected, EventScript script) {
+  for(auto ev : script) {
+    router.event(ev);
+  }
+
+  REQUIRE(router.in_state(expected));
+}
+
+TEST_CASE("basic router operations test", "[event_router]") {
+  routing::Router router;
+
+  // start goes to idle
+  run_script(router, IDLE, {
+    STARTED
+  });
+
+  // simulate drag and drop
+  run_script(router, IDLE, {
+    MOUSE_DOWN,
+    MOUSE_MOVE,
+    MOUSE_UP,
+    KEY_PRESS
+  });
+
+  // moving the mouse outside dnd
+  run_script(router, IDLE, {
+    MOUSE_MOVE,
+    KEY_PRESS,
+    MOUSE_MOVE
+  });
+
+  // regular mouse click
+  run_script(router, IDLE, {
+    MOUSE_DOWN,
+    MOUSE_UP
+  });
+
+  // possible bad key press in a move?
+  run_script(router, IDLE, {
+    MOUSE_DOWN,
+    MOUSE_MOVE,
+    KEY_PRESS,
+    MOUSE_UP,
+  });
+}