Shows error message to user if discord is running

src/main.cpp

@@ -8,9 +8,50 @@
 #include <QApplication>
 #include <QCommandLineParser>
 #include <QLoggingCategory>
+#include <QMessageBox>
+#include <QLocalSocket>
+#include <QLocalServer>
+
+void showErrorMessage(const char* text) {
+  QMessageBox msgBox;
+
+  msgBox.setIcon(QMessageBox::Critical);
+  msgBox.setText(text);
+  msgBox.setStandardButtons(QMessageBox::Ok);
+  msgBox.setDefaultButton(QMessageBox::Ok);
+  msgBox.setWindowIcon(
+      QIcon(":assets/de.shorsh.discord-screenaudio.png"));
+
+  msgBox.exec();
+}
+
+
+bool isProgramRunning(const QString& program_name)
+{
+    QLocalSocket socket;
+    socket.connectToServer(program_name);
+    if (socket.waitForConnected()) {
+        return true; // program is already running
+    }
+    return false;
+}
 
 int main(int argc, char *argv[]) {
   QApplication app(argc, argv);
+
+  // Check if discord is already running
+  QString program_name = "discord-screenaudio";
+  if (isProgramRunning(program_name)) {
+      // if running show error message
+      showErrorMessage("Discord is already running");
+      return 1;
+  }
+
+  // open server so we can check if discord is running
+  QLocalServer server;
+  server.listen(program_name);
+  QObject::connect(&server, &QLocalServer::newConnection, [](){});
+
   QApplication::setApplicationName("discord-screenaudio");
   QApplication::setWindowIcon(
       QIcon(":assets/de.shorsh.discord-screenaudio.png"));