Add diagnostic prop tests

test/selectors/diagnostic_prop_test.dart

@@ -0,0 +1,123 @@
+// ignore_for_file: prefer_const_constructors, prefer_const_literals_to_create_immutables
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spot/spot.dart';
+
+import '../util/assert_error.dart';
+
+void main() {
+  group('diagnostic prop', () {
+    testWidgets('getDiagnosticProp', (tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Text('a', maxLines: 4),
+        ),
+      );
+
+      final maxLines =
+          spot<Text>().existsOnce().getDiagnosticProp<int>('maxLines');
+      expect(maxLines, 4);
+    });
+
+    testWidgets('generated getDiagnosticProp', (tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Text('a', maxLines: 4),
+        ),
+      );
+
+      final maxLines2 = spot<Text>().existsOnce().getMaxLines();
+      expect(maxLines2, 4);
+    });
+
+    testWidgets('hasDiagnosticProp', (tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Text('a', maxLines: 4),
+        ),
+      );
+
+      spot<Text>()
+          .existsOnce()
+          .hasDiagnosticProp<int>('maxLines', (it) => it.equals(4));
+
+      expect(
+        () => spot<Text>()
+            .existsOnce()
+            .hasDiagnosticProp<int>('maxLines', (it) => it.equals(2)),
+        throwsSpotErrorContaining([
+          'Text with property maxLines',
+          'equals <2>, actual: <4>',
+        ]),
+      );
+    });
+
+    testWidgets('generated hasDiagnosticProp', (tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Text('a', maxLines: 4),
+        ),
+      );
+
+      spot<Text>()
+          .existsOnce()
+          .hasMaxLines(4)
+          .hasMaxLinesWhere((it) => it.equals(4));
+
+      expect(
+        () => spot<Text>().existsOnce().hasMaxLines(2),
+        throwsSpotErrorContaining([
+          'Text with property maxLines',
+          'equals <2>, actual: <4>',
+        ]),
+      );
+    });
+
+    testWidgets('withDiagnosticProp', (tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Text('a', maxLines: 4),
+        ),
+      );
+
+      spot<Text>()
+          .withDiagnosticProp<int>('maxLines', (it) => it.equals(4))
+          .existsOnce();
+
+      expect(
+        () => spot<Text>()
+            .withDiagnosticProp<int>('maxLines', (it) => it.equals(2))
+            .existsOnce(),
+        throwsSpotErrorContaining([
+          'Could not find Text with prop "maxLines" equals <2> in widget tree',
+          'A less specific search (Text) discovered 1 matches!',
+          'Text("a", maxLines: 4,',
+        ]),
+      );
+    });
+
+    testWidgets('generated withDiagnosticProp', (tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Text('a', maxLines: 4),
+        ),
+      );
+
+      spot<Text>().withMaxLines(4).existsOnce();
+      spot<Text>().whereMaxLines((it) => it.equals(4)).existsOnce();
+
+      spot<Text>().withMaxLines(3).doesNotExist();
+      spot<Text>().whereMaxLines((it) => it.equals(3)).doesNotExist();
+
+      expect(
+        () => spot<Text>().withMaxLines(2).existsOnce(),
+        throwsSpotErrorContaining([
+          'Could not find Text with prop "maxLines" equals <2> in widget tree',
+          'A less specific search (Text) discovered 1 matches!',
+          'Text("a", maxLines: 4,',
+        ]),
+      );
+    });
+  });
+}