add tests, improve logic

Author: solid-daniyilyevtyushkin

lib/flutter_switch.dart

@@ -173,11 +173,12 @@ class _FlutterSwitchState extends State<FlutterSwitch>
     super.initState();
     _animationController = AnimationController(
       vsync: this,
+      value: widget.value ? 1.0 : 0.0,
       duration: Duration(milliseconds: 60),
     );
     _toggleAnimation = AlignmentTween(
-      begin: widget.value ? Alignment.centerRight : Alignment.centerLeft,
-      end: widget.value ? Alignment.centerLeft : Alignment.centerRight,
+      begin: Alignment.centerLeft,
+      end: Alignment.centerRight,
     ).animate(
       CurvedAnimation(parent: _animationController, curve: Curves.linear),
     );

test/flutter_switch_test.dart

@@ -0,0 +1,67 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../lib/flutter_switch.dart';
+
+void main() {
+  testWidgets(
+    "displays the toggle indicator on the right if the given value is true",
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        _FlutterSwitchTestbed(
+          value: true,
+        ),
+      );
+
+      final align = tester.widget<Align>(find.byType(Align));
+
+      expect(align.alignment, equals(Alignment.centerRight));
+    },
+  );
+
+  testWidgets(
+    "displays the toggle indicator on the left if the given value is false",
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        _FlutterSwitchTestbed(
+          value: false,
+        ),
+      );
+
+      final align = tester.widget<Align>(find.byType(Align));
+
+      expect(align.alignment, equals(Alignment.centerLeft));
+    },
+  );
+}
+
+/// A testbed needed to test the switch widget.
+class _FlutterSwitchTestbed extends StatelessWidget {
+  /// A callback that is called when the switch is toggled.
+  final ValueChanged<bool> onToggle;
+
+  /// An initial value of the switch.
+  final bool value;
+
+  /// A default on toggle callback of the switch.
+  static void _defaultOnToggle(bool value) {}
+
+  /// Creates a new instance of this testbed.
+  const _FlutterSwitchTestbed({
+    Key key,
+    this.onToggle = _defaultOnToggle,
+    this.value,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        body: FlutterSwitch(
+          value: value,
+          onToggle: (value) => {},
+        ),
+      ),
+    );
+  }
+}