Improve testability and add unit tests

lib/src/unix_single_instance_base.dart

@@ -41,12 +41,13 @@ enum ErrorMode {
 // kDebugMode makes the application noisy.
 Future<bool> unixSingleInstance(
     List<String> arguments, void Function(List<dynamic> args) cmdProcessor,
-    {bool kDebugMode = false, ErrorMode errorMode = ErrorMode.exit}) async {
-  // TODO make a named arg
+    {bool kDebugMode = false,
+    ErrorMode errorMode = ErrorMode.exit,
+    String? customConfigPath,
+    String socketFilename = 'socket'}) async {
   // Kept short because of mac os x sandboxing makes the name too long for unix sockets.
-  var socketFilename = 'socket';
   // TODO make configurable so it can be per X, per User, or for the whole machine based on optional named args
-  var configPath = await _applicationConfigDirectory();
+  var configPath = customConfigPath ?? await _applicationConfigDirectory();
   await Directory(configPath).create(recursive: true);
   var socketFilepath = p.join(configPath, socketFilename);
   final InternetAddress host =
@@ -63,7 +64,11 @@ Future<bool> unixSingleInstance(
         print("Message sent");
         print("Quiting");
       }
-      exit(0);
+      if (errorMode == ErrorMode.returnFalse) {
+        return false;
+      } else {
+        exit(0);
+      }
     } else {
       if (kDebugMode) {
         print("Deleting dead socket");

pubspec.yaml

@@ -15,6 +15,7 @@ dependencies:
   path_provider: '>=2.1.5 <4.0.0'
 
 dev_dependencies:
+  test: ^1.30.0
   lints: '>=6.0.0 <8.0.0'
 
 # This package supports only Linux and macOS.

test/unix_single_instance_test.dart

@@ -0,0 +1,69 @@
+import 'dart:io';
+
+import 'package:test/test.dart';
+import 'package:unix_single_instance/unix_single_instance.dart';
+import 'package:path/path.dart' as p;
+
+void main() {
+  group('unixSingleInstance', () {
+    late Directory tempDir;
+
+    setUp(() async {
+      tempDir =
+          await Directory.systemTemp.createTemp('unix_single_instance_test_');
+    });
+
+    tearDown(() async {
+      if (await tempDir.exists()) {
+        await tempDir.delete(recursive: true);
+      }
+    });
+
+    test('First instance returns true', () async {
+      var isFirst = await unixSingleInstance(
+        ['arg1', 'arg2'],
+        (args) {},
+        customConfigPath: tempDir.path,
+        errorMode: ErrorMode.returnFalse,
+      );
+
+      expect(isFirst, isTrue);
+
+      // Verify socket file was created
+      var socketFile = File(p.join(tempDir.path, 'socket'));
+      expect(await socketFile.exists(), isTrue);
+    });
+
+    test('Second instance returns false and sends args to first instance',
+        () async {
+      var receivedArgs = <dynamic>[];
+
+      // Start the "first instance"
+      var isFirst = await unixSingleInstance(
+        ['first'],
+        (args) {
+          receivedArgs.addAll(args);
+        },
+        customConfigPath: tempDir.path,
+        errorMode: ErrorMode.returnFalse,
+      );
+
+      expect(isFirst, isTrue);
+
+      // Start the "second instance"
+      var isSecondFirst = await unixSingleInstance(
+        ['second1', 'second2'],
+        (args) {},
+        customConfigPath: tempDir.path,
+        errorMode: ErrorMode.returnFalse,
+      );
+
+      expect(isSecondFirst, isFalse);
+
+      // Give it a moment for the socket to process
+      await Future.delayed(Duration(milliseconds: 100));
+
+      expect(receivedArgs, equals(['second1', 'second2']));
+    });
+  });
+}