Make it significant easier to create custom property matchers

lib/src/spot/selectors.dart

@@ -663,6 +663,83 @@ extension WidgetMatcherExtensions<W extends Widget> on WidgetMatcher<W> {
     }
     return this;
   }
+
+  WidgetMatcher<W> hasWidgetProp<T>({
+    required String name,
+    required T Function(W) prop,
+    required MatchProp<T> match,
+  }) {
+    final ConditionSubject<Element> conditionSubject = it<Element>();
+    final Subject<T> subject = conditionSubject.context.nest<T>(
+      () => ['$W', "with prop '$name'"],
+      (element) {
+        final widget = selector.mapElementToWidget(element);
+        return Extracted.value(prop.call(widget));
+      },
+    );
+
+    match(subject);
+    final failure = softCheck(element, conditionSubject);
+    if (failure != null) {
+      final errorParts =
+          describe(conditionSubject).map((it) => it.trim()).toList();
+      // workaround allowing to use
+      // hasPropertyXWhere((subject)=> subject.equals(X));
+      // instead of
+      // hasPropertyXWhere((subject)=> subject.isNotNull().equals(X));
+      //
+      // when Subject is Subject<T> but the value can actually be null (should be Subject<T?>).
+      final errorMessage = errorParts.join(' ');
+      if (errorParts.last == 'is null' &&
+          failure.rejection.actual.firstOrNull == '<null>') {
+        // property is null and isNull() was called
+        // not error because null == null
+        return this;
+      }
+      throw PropertyCheckFailure(
+        'Failed to match widget: $errorMessage, actual: ${failure.rejection.actual.joinToString()}',
+        matcherDescription: errorParts.skip(1).join(' ').removePrefix('with '),
+      );
+    }
+    return this;
+  }
+
+  WidgetMatcher<W> hasElementProp<T>({
+    required String name,
+    required T Function(Element) prop,
+    required MatchProp<T> match,
+  }) {
+    final ConditionSubject<Element> conditionSubject = it<Element>();
+    final Subject<T> subject = conditionSubject.context.nest<T>(
+      () => ['Element of $W', "with prop '$name'"],
+      (element) => Extracted.value(prop.call(element)),
+    );
+
+    match(subject);
+    final failure = softCheck(element, conditionSubject);
+    if (failure != null) {
+      final errorParts =
+          describe(conditionSubject).map((it) => it.trim()).toList();
+      // workaround allowing to use
+      // hasPropertyXWhere((subject)=> subject.equals(X));
+      // instead of
+      // hasPropertyXWhere((subject)=> subject.isNotNull().equals(X));
+      //
+      // when Subject is Subject<T> but the value can actually be null (should be Subject<T?>).
+      final errorMessage = errorParts.join(' ');
+      if (errorParts.last == 'is null' &&
+          failure.rejection.actual.firstOrNull == '<null>') {
+        // property is null and isNull() was called
+        // not error because null == null
+        return this;
+      }
+      throw PropertyCheckFailure(
+        'Failed to match widget: $errorMessage, actual: ${failure.rejection.actual.joinToString()}',
+        matcherDescription: errorParts.skip(1).join(' ').removePrefix('with '),
+      );
+    }
+    return this;
+  }
 }
 
 class PropertyCheckFailure extends TestFailure {

test/selectors/custom_matcher_test.dart

@@ -0,0 +1,55 @@
+// ignore_for_file: prefer_const_constructors
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spot/spot.dart';
+
+import '../util/assert_error.dart';
+
+void main() {
+  testWidgets('match custom property', (widgetTester) async {
+    await widgetTester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: Checkbox(
+            value: true,
+            onChanged: (_) {},
+          ),
+        ),
+      ),
+    );
+
+    final checkbox = spotSingle<Checkbox>();
+
+    checkbox
+        .existsOnce()
+        .hasProp(
+          elementSelector: (e) => e.context.nest(
+            () => ['Checkbox', 'value'],
+            (e) => Extracted.value((e.widget as Checkbox).value),
+          ),
+          match: (it) => it.equals(true),
+        )
+        .hasProp(
+          widgetSelector: (widget) => widget.context.nest(
+            () => ['value'],
+            (widget) => Extracted.value(widget.value),
+          ),
+          match: (it) => it.equals(true),
+        )
+        .hasElementProp(
+          name: 'value',
+          prop: (it) => (it.widget as Checkbox).value,
+          match: (it) => it.equals(true),
+        )
+        .hasWidgetProp(
+          name: 'value',
+          prop: (it) => it.value,
+          match: (it) => it.equals(false),
+        );
+    //     .hasProp(
+    //   prop: ('value', (it) => it.value),
+    //   match: (it) => it.equals(false),
+    // );
+  });
+}