Improve DropdownButton assert message (#43511)

- Adds an assert message checking for exactly one unique value for `DropdownItem`s

packages/flutter/lib/src/material/dropdown.dart

@@ -719,7 +719,15 @@ class DropdownButton<T> extends StatefulWidget {
     this.focusColor,
     this.focusNode,
     this.autofocus = false,
-  }) : assert(items == null || items.isEmpty || value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1),
+  }) : assert(items == null || items.isEmpty || value == null ||
+              items.where((DropdownMenuItem<T> item) {
+                return item.value == value;
+              }).length == 1,
+                'There should be exactly one item with [DropdownButton]\'s value: '
+                '$value. \n'
+                'Either zero or 2 or more [DropdownMenuItem]s were detected '
+                'with the same value',
+              ),
        assert(elevation != null),
        assert(iconSize != null),
        assert(isDense != null),
@@ -1287,7 +1295,15 @@ class DropdownButtonFormField<T> extends FormField<T> {
     bool isDense = false,
     bool isExpanded = false,
     double itemHeight,
-  }) : assert(items == null || items.isEmpty || value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1),
+  }) : assert(items == null || items.isEmpty || value == null ||
+              items.where((DropdownMenuItem<T> item) {
+                return item.value == value;
+              }).length == 1,
+                'There should be exactly one item with [DropdownButton]\'s value: '
+                '$value. \n'
+                'Either zero or 2 or more [DropdownMenuItem]s were detected '
+                'with the same value',
+              ),
        assert(decoration != null),
        assert(elevation != null),
        assert(iconSize != null),

packages/flutter/test/material/dropdown_test.dart

@@ -342,6 +342,68 @@ void main() {
     expect(value, equals('two'));
   });
 
+  testWidgets('DropdownButton does not allow duplicate item values', (WidgetTester tester) async {
+    final List<DropdownMenuItem<String>> itemsWithDuplicateValues = <String>['a', 'b', 'c', 'c']
+      .map<DropdownMenuItem<String>>((String value) {
+        return DropdownMenuItem<String>(
+          value: value,
+          child: Text(value),
+        );
+      }).toList();
+
+    try {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: DropdownButton<String>(
+              value: 'c',
+              onChanged: (String newValue) {},
+              items: itemsWithDuplicateValues,
+            ),
+          ),
+        ),
+      );
+
+      fail('Should not be possible to have duplicate item value');
+    } on AssertionError catch (error) {
+      expect(
+        error.toString(),
+        contains('There should be exactly one item with [DropdownButton]\'s value'),
+      );
+    }
+  });
+
+  testWidgets('DropdownButton value should only appear in one menu item', (WidgetTester tester) async {
+    final List<DropdownMenuItem<String>> itemsWithDuplicateValues = <String>['a', 'b', 'c', 'd']
+      .map<DropdownMenuItem<String>>((String value) {
+        return DropdownMenuItem<String>(
+          value: value,
+          child: Text(value),
+        );
+      }).toList();
+
+    try {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: DropdownButton<String>(
+              value: 'e',
+              onChanged: (String newValue) {},
+              items: itemsWithDuplicateValues,
+            ),
+          ),
+        ),
+      );
+
+      fail('Should not be possible to have no items with passed in value');
+    } on AssertionError catch (error) {
+      expect(
+        error.toString(),
+        contains('There should be exactly one item with [DropdownButton]\'s value'),
+      );
+    }
+  });
+
   testWidgets('Dropdown in ListView', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/12053
     // Positions a DropdownButton at the left and right edges of the screen,
@@ -2219,6 +2281,68 @@ void main() {
     expect(tester.getCenter(item40.first).dy, tester.getCenter(item40.last).dy);
   });
 
+  testWidgets('DropdownButtonFormField does not allow duplicate item values', (WidgetTester tester) async {
+    final List<DropdownMenuItem<String>> itemsWithDuplicateValues = <String>['a', 'b', 'c', 'c']
+      .map<DropdownMenuItem<String>>((String value) {
+        return DropdownMenuItem<String>(
+          value: value,
+          child: Text(value),
+        );
+      }).toList();
+
+    try {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: DropdownButtonFormField<String>(
+              value: 'c',
+              onChanged: (String newValue) {},
+              items: itemsWithDuplicateValues,
+            ),
+          ),
+        ),
+      );
+
+      fail('Should not be possible to have duplicate item value');
+    } on AssertionError catch (error) {
+      expect(
+        error.toString(),
+        contains('There should be exactly one item with [DropdownButton]\'s value'),
+      );
+    }
+  });
+
+  testWidgets('DropdownButtonFormField value should only appear in one menu item', (WidgetTester tester) async {
+    final List<DropdownMenuItem<String>> itemsWithDuplicateValues = <String>['a', 'b', 'c', 'd']
+      .map<DropdownMenuItem<String>>((String value) {
+        return DropdownMenuItem<String>(
+          value: value,
+          child: Text(value),
+        );
+      }).toList();
+
+    try {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: DropdownButton<String>(
+              value: 'e',
+              onChanged: (String newValue) {},
+              items: itemsWithDuplicateValues,
+            ),
+          ),
+        ),
+      );
+
+      fail('Should not be possible to have no items with passed in value');
+    } on AssertionError catch (error) {
+      expect(
+        error.toString(),
+        contains('There should be exactly one item with [DropdownButton]\'s value'),
+      );
+    }
+  });
+
   testWidgets('DropdownButton hint is selected item', (WidgetTester tester) async {
     const double hintPaddingOffset = 8;
     const List<String> itemValues = <String>['item0', 'item1', 'item2', 'item3'];
@@ -2286,6 +2410,7 @@ void main() {
 
     expect(tester.getTopLeft(find.text('-item0-')).dx, 8);
   });
+
   testWidgets('DropdownButton can be focused, and has focusColor', (WidgetTester tester) async {
     final UniqueKey buttonKey = UniqueKey();
     final FocusNode focusNode = FocusNode(debugLabel: 'DropdownButton');
@@ -2298,6 +2423,7 @@ void main() {
     await tester.pumpWidget(buildFrame(buttonKey: buttonKey, onChanged: onChanged, focusNode: focusNode, focusColor: const Color(0xff00ff00)));
     expect(buttonFinder, paints ..rrect(rrect: const RRect.fromLTRBXY(0.0, 0.0, 104.0, 48.0, 4.0, 4.0), color: const Color(0xff00ff00)));
   });
+
   testWidgets("DropdownButton won't be focused if not enabled", (WidgetTester tester) async {
     final UniqueKey buttonKey = UniqueKey();
     final FocusNode focusNode = FocusNode(debugLabel: 'DropdownButton');
@@ -2306,6 +2432,7 @@ void main() {
     expect(focusNode.hasPrimaryFocus, isFalse);
     expect(find.byKey(buttonKey), isNot(paints ..rrect(rrect: const RRect.fromLTRBXY(0.0, 0.0, 104.0, 48.0, 4.0, 4.0), color: const Color(0xff00ff00))));
   });
+
   testWidgets('DropdownButton is activated with the enter key', (WidgetTester tester) async {
     final FocusNode focusNode = FocusNode(debugLabel: 'DropdownButton');
     String value = 'one';