bugfix/ fixed range of default dates for the datepicker

example/lib/main.dart

@@ -172,8 +172,8 @@ class _MyAppState extends State<MyApp> {
           title: 'When was your last holiday?',
           answerFormat: DateAnswerFormat(
             minDate: DateTime.utc(1970),
-            maxDate: DateTime.now(),
             defaultDate: DateTime.now(),
+            maxDate: DateTime.now(),
           ),
         ),
         CompletionStep(

lib/src/answer_format/date_answer_format.dart

@@ -1,13 +1,26 @@
 import 'package:survey_kit/src/answer_format/answer_format.dart';
 
 class DateAnswerFormat implements AnswerFormat {
+  /// Default date which will be preselected on datepicker opening
   final DateTime? defaultDate;
+
+  /// Lowest date which can be selected via the datepicker
   final DateTime? minDate;
+
+  /// Highest date which can be selected via the datepicker
   final DateTime? maxDate;
 
   DateAnswerFormat({
     this.defaultDate,
     this.minDate,
     this.maxDate,
-  });
+  })  : assert(minDate == null || maxDate == null || minDate.isBefore(maxDate)),
+        assert(defaultDate == null ||
+            minDate == null ||
+            defaultDate.isAtSameMomentAs(minDate) ||
+            defaultDate.isAfter(minDate)),
+        assert(defaultDate == null ||
+            maxDate == null ||
+            defaultDate.isAtSameMomentAs(maxDate) ||
+            defaultDate.isBefore(maxDate));
 }

lib/src/views/date_answer_view.dart

@@ -1,5 +1,3 @@
-import 'dart:io';
-
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
@@ -11,7 +9,10 @@ import 'package:survey_kit/src/steps/predefined_steps/question_step.dart';
 import 'package:survey_kit/src/views/widget/step_view.dart';
 
 class DateAnswerView extends StatefulWidget {
+  /// [QuestionStep] which includes the [DateAnswerFormat]
   final QuestionStep questionStep;
+
+  /// [DateQuestionResult] which boxes the result
   final DateQuestionResult? result;
 
   const DateAnswerView({
@@ -27,14 +28,16 @@ class DateAnswerView extends StatefulWidget {
 class _DateAnswerViewState extends State<DateAnswerView> {
   final DateFormat _dateFormat = DateFormat('E, MMM d');
   final DateTime _startDate = DateTime.now();
-  late final DateAnswerFormat _dateAnswerFormat;
+  late DateAnswerFormat _dateAnswerFormat;
   DateTime? _result;
 
   @override
   void initState() {
     super.initState();
     _dateAnswerFormat = widget.questionStep.answerFormat as DateAnswerFormat;
-    _result = widget.result?.result ?? DateTime.now();
+    _result = widget.result?.result ??
+        _dateAnswerFormat.defaultDate ??
+        DateTime.now();
   }
 
   void _handleDateChanged(DateTime date) {
@@ -43,6 +46,7 @@ class _DateAnswerViewState extends State<DateAnswerView> {
 
   @override
   Widget build(BuildContext context) {
+    final platform = Theme.of(context).platform;
     return StepView(
       step: widget.questionStep,
       controller: SurveyController(
@@ -73,7 +77,9 @@ class _DateAnswerViewState extends State<DateAnswerView> {
               textAlign: TextAlign.center,
             ),
           ),
-          Platform.isAndroid ? _androidDatePicker() : _iosDatePicker(),
+          platform == TargetPlatform.iOS
+              ? _iosDatePicker()
+              : _androidDatePicker(),
         ],
       ),
     );
@@ -96,7 +102,7 @@ class _DateAnswerViewState extends State<DateAnswerView> {
                 left: 8.0,
                 bottom: 8.0,
                 child: Text(
-                  _result != null ? _dateFormat.format(_result!) : '',
+                  _dateFormat.format(_result!),
                   style: TextStyle(
                     fontSize: 28.0,
                     color: Colors.white,
@@ -110,14 +116,14 @@ class _DateAnswerViewState extends State<DateAnswerView> {
           width: double.infinity,
           height: 300.0,
           child: CalendarDatePicker(
-            firstDate: _dateAnswerFormat.minDate ??
-                DateTime.now().add(Duration(days: 365 * DateTime.now().year)),
-            lastDate: _dateAnswerFormat.maxDate ??
-                DateTime.now().add(Duration(days: 365 * 100)),
-            initialDate: _result ??
-                _dateAnswerFormat.defaultDate ??
-                _dateAnswerFormat.maxDate ??
-                DateTime.now(),
+            firstDate: _dateAnswerFormat.minDate ?? DateTime.utc(1900),
+            lastDate: _dateAnswerFormat.maxDate?.add(
+                  Duration(hours: 1),
+                ) ??
+                DateTime.now().add(
+                  Duration(hours: 1),
+                ),
+            initialDate: _result ?? DateTime.now(),
             currentDate: _result,
             onDateChanged: (DateTime value) => _handleDateChanged(value),
           ),
@@ -135,8 +141,11 @@ class _DateAnswerViewState extends State<DateAnswerView> {
         minimumDate: _dateAnswerFormat.minDate,
         //We have to add an hour to to met the assert maxDate > initDate
         maximumDate: _dateAnswerFormat.maxDate?.add(
-          Duration(hours: 1),
-        ),
+              Duration(hours: 1),
+            ) ??
+            DateTime.now().add(
+              Duration(hours: 1),
+            ),
         initialDateTime: _dateAnswerFormat.defaultDate,
         onDateTimeChanged: (DateTime value) {
           setState(() {

test/date_answer_view_test.dart

@@ -0,0 +1,70 @@
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:survey_kit/src/answer_format/date_answer_format.dart';
+import 'package:survey_kit/src/steps/predefined_steps/question_step.dart';
+import 'package:survey_kit/src/views/date_answer_view.dart';
+
+void main() {
+  DateAnswerView _validDateAnswerView() => DateAnswerView(
+        questionStep: QuestionStep(
+          answerFormat: DateAnswerFormat(
+            minDate: DateTime.now().subtract(const Duration(days: 365 * 70)),
+            maxDate: DateTime.now().subtract(const Duration(days: 365 * 15)),
+            defaultDate:
+                DateTime.now().subtract(const Duration(days: 365 * 20)),
+          ),
+        ),
+        result: null,
+      );
+
+  testWidgets('Detects iOS platform and displays correct widget',
+      (WidgetTester tester) async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: _validDateAnswerView(),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(find.byType(CupertinoDatePicker), findsOneWidget);
+    debugDefaultTargetPlatformOverride = null;
+  });
+
+  testWidgets('Detects Android platform and displays correct widget',
+      (WidgetTester tester) async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.android;
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: _validDateAnswerView(),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(find.byType(CalendarDatePicker), findsOneWidget);
+    debugDefaultTargetPlatformOverride = null;
+  });
+
+  testWidgets('Initial date in between first and last date',
+      (WidgetTester tester) async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.android;
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: DateAnswerView(
+            questionStep: QuestionStep(
+              title: 'Your Birthday?',
+              answerFormat: DateAnswerFormat(),
+            ),
+            result: null,
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(find.byType(CalendarDatePicker), findsOneWidget);
+    debugDefaultTargetPlatformOverride = null;
+  });
+}