Prevent multiple instances and center settings dialog

main.cpp

@@ -1,7 +1,12 @@
 #include <QApplication>
+#include <QByteArray>
 #include <QCoreApplication>
 #include <QIcon>
+#include <QIODevice>
+#include <QLocalServer>
+#include <QLocalSocket>
 #include <QObject>
+#include <QString>
 
 #include "src/config/ConfigManager.h"
 #include "src/tray/TrayController.h"
@@ -9,12 +14,41 @@
 #include "src/util/Logger.h"
 #include "src/util/ResourcePaths.h"
 
+namespace
+{
+const QString SingleInstanceServerName = QStringLiteral("Make.QtDesktopPet.SingleInstance");
+const QByteArray ActivateMessage("activate\n");
+constexpr int SingleInstanceConnectTimeoutMs = 500;
+constexpr int SingleInstanceWriteTimeoutMs = 500;
+
+bool notifyRunningInstance()
+{
+    QLocalSocket socket;
+    socket.connectToServer(SingleInstanceServerName, QIODevice::WriteOnly);
+    if (!socket.waitForConnected(SingleInstanceConnectTimeoutMs))
+    {
+        return false;
+    }
+
+    socket.write(ActivateMessage);
+    socket.flush();
+    socket.waitForBytesWritten(SingleInstanceWriteTimeoutMs);
+    socket.disconnectFromServer();
+    return true;
+}
+}
+
 int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);
     QApplication::setApplicationName("QtDesktopPet");
     QApplication::setOrganizationName("QtDesktopPet");
 
+    if (notifyRunningInstance())
+    {
+        return 0;
+    }
+
     const QIcon appIcon(ResourcePaths::appIconPath());
     if (!appIcon.isNull())
     {
@@ -23,6 +57,19 @@ int main(int argc, char *argv[])
 
     Logger::info(QStringLiteral("Application started."));
 
+    QLocalServer singleInstanceServer;
+    QLocalServer::removeServer(SingleInstanceServerName);
+    if (!singleInstanceServer.listen(SingleInstanceServerName))
+    {
+        if (notifyRunningInstance())
+        {
+            return 0;
+        }
+
+        Logger::warning(QStringLiteral("Unable to start single instance server: ")
+            + singleInstanceServer.errorString());
+    }
+
     ConfigManager configManager;
     PetWindow window;
     window.applyAppConfig(configManager.loadAppConfig());
@@ -31,6 +78,37 @@ int main(int argc, char *argv[])
     window.setSettingsFallbackInContextMenuEnabled(!trayController.isAvailable());
     trayController.show();
 
+    QObject::connect(&singleInstanceServer, &QLocalServer::newConnection, [&singleInstanceServer, &window]() {
+        while (singleInstanceServer.hasPendingConnections())
+        {
+            QLocalSocket *socket = singleInstanceServer.nextPendingConnection();
+            if (socket == nullptr)
+            {
+                continue;
+            }
+
+            const auto handleActivationMessage = [socket, &window]() {
+                const QByteArray message = socket->readAll().trimmed();
+                if (message == ActivateMessage.trimmed())
+                {
+                    window.activateFromExternalInstance();
+                }
+            };
+            QObject::connect(socket, &QLocalSocket::readyRead, handleActivationMessage);
+            QObject::connect(socket, &QLocalSocket::disconnected, [socket, handleActivationMessage]() {
+                if (socket->bytesAvailable() > 0)
+                {
+                    handleActivationMessage();
+                }
+                socket->deleteLater();
+            });
+            if (socket->bytesAvailable() > 0)
+            {
+                handleActivationMessage();
+            }
+        }
+    });
+
     QObject::connect(&app, &QCoreApplication::aboutToQuit, [&configManager, &window]() {
         if (!configManager.saveAppConfig(window.currentAppConfig()))
         {