[feat] activate existing window on duplicate launches, refs #119

apps/desktop/src/platform/instance_activation.h

@@ -0,0 +1,59 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2026-10-17
+ * Copyright (c) 2026 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _INSTANCE_ACTIVATION_H_
+#define _INSTANCE_ACTIVATION_H_
+
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <cstdint>
+#endif
+
+#include <string>
+
+namespace crossdesk::platform {
+
+// Created only by the interactive GUI, on its event-loop thread. Requests can
+// arrive during startup and remain pending until the UI is ready to show
+// itself.
+class InstanceActivationReceiver {
+ public:
+  explicit InstanceActivationReceiver(std::string application = "CrossDesk");
+  ~InstanceActivationReceiver();
+  InstanceActivationReceiver(const InstanceActivationReceiver&) = delete;
+  InstanceActivationReceiver& operator=(const InstanceActivationReceiver&) =
+      delete;
+
+  // The caller must hold this application's client single-instance lease.
+  bool Start(std::string& error);
+  bool TakeRequest(std::string* activation_token = nullptr);
+
+ private:
+  std::string application_;
+#ifdef _WIN32
+  static LRESULT CALLBACK WindowProc(HWND window, UINT message, WPARAM wparam,
+                                     LPARAM lparam);
+
+  HWND window_ = nullptr;
+  UINT activation_message_ = 0;
+  bool requested_ = false;
+#else
+  int socket_ = -1;
+  std::string socket_path_;
+  std::uint64_t socket_device_ = 0;
+  std::uint64_t socket_inode_ = 0;
+#endif
+};
+
+// Targets the current user's GUI on the current desktop, including a daemon
+// child. Waits briefly for startup/restart, without waiting on its UI thread.
+bool RequestInstanceActivation(std::string& error,
+                               const std::string& application = "CrossDesk");
+
+}  // namespace crossdesk::platform
+
+#endif