diff --git a/CHANGELOG.md b/CHANGELOG.md
index 52ddb14..aef42c5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 3.0.6
+
+- Added `UniqueCaller` and `parseIntList`.
+- Null safety migration adjustments. 
+
 ## 3.0.5
 
 - Null safety migration adjustments. 
diff --git a/lib/src/events.dart b/lib/src/events.dart
index 554a95b..1d5330f 100644
--- a/lib/src/events.dart
+++ b/lib/src/events.dart
@@ -2,6 +2,8 @@ import 'dart:async';
 
 import 'package:swiss_knife/src/collections.dart';
 
+import 'math.dart';
+
 class _ListenSignature {
   final Object _identifier;
 
@@ -322,7 +324,7 @@ class EventStream<T> implements Stream<T> {
   ListenerWrapper<T>? listenOneShot(void Function(T event) onData,
       {Function? onError,
       void Function()? onDone,
-      required bool cancelOnError,
+      bool cancelOnError = false,
       Object? singletonIdentifier,
       bool? singletonIdentifyByInstance = true}) {
     var listenerWrapper = ListenerWrapper<T>(this, onData,
@@ -421,20 +423,21 @@ class EventStream<T> implements Stream<T> {
 class EventStreamDelegator<T> implements EventStream<T> {
   EventStream<T>? _eventStream;
 
-  final EventStream<T> Function()? _eventStreamProvider;
+  final EventStream<T>? Function()? _eventStreamProvider;
 
   EventStreamDelegator(EventStream<T> eventStream)
       : _eventStream = eventStream,
         _eventStreamProvider = null;
 
-  EventStreamDelegator.provider(EventStream<T> Function() eventStreamProvider)
+  EventStreamDelegator.provider(EventStream<T>? Function() eventStreamProvider)
       : _eventStream = null,
         _eventStreamProvider = eventStreamProvider;
 
   /// Returns the main [EventStream].
   EventStream<T>? get eventStream {
     if (_eventStream == null) {
-      _eventStream = _eventStreamProvider!();
+      _eventStream =
+          _eventStreamProvider != null ? _eventStreamProvider!() : null;
       if (_eventStream != null) {
         flush();
       }
@@ -1274,3 +1277,139 @@ class ListenerWrapper<T> {
     _subscription = null;
   }
 }
+
+/// Ensures that a call is executed only 1 per time.
+class UniqueCaller<R> {
+  final FutureOr<R?> Function() function;
+
+  final void Function(UniqueCaller<R> caller)? onDuplicatedCall;
+
+  final _IdentifierWrapper _identifier;
+
+  static String stackTraceIdentifier([int stackOffset = 0]) {
+    var stackTracer = StackTrace.current;
+    var stackTraceStr = stackTracer.toString();
+
+    var lines = stackTraceStr.split(RegExp(r'[\r\n]+', multiLine: false));
+
+    var start = Math.min(1 + stackOffset, lines.length - 1);
+    var end = Math.min(3, lines.length);
+
+    lines = lines.sublist(start, Math.max(start, end));
+
+    var s = lines.join('\n');
+    return s;
+  }
+
+  UniqueCaller(this.function,
+      {Object? identifier,
+      StackTrace? stackTraceIdentifier,
+      this.onDuplicatedCall})
+      : _identifier = _IdentifierWrapper(identifier ??
+            stackTraceIdentifier?.toString() ??
+            UniqueCaller.stackTraceIdentifier());
+
+  _IdentifierWrapper get identifier => _identifier;
+
+  static final Set<_IdentifierWrapper> _calling = {};
+
+  static final Map<_IdentifierWrapper, Future> _callsFuture = {};
+
+  static Future<R?> getCallFuture<R>(Object identifier) {
+    var identifierWrapper = _IdentifierWrapper(identifier);
+    return _callsFuture[identifierWrapper] as Future<R?>;
+  }
+
+  static List<Future> get calling {
+    return _callsFuture.values.toList();
+  }
+
+  Future<R?> callAsync() async {
+    if (_calling.contains(_identifier)) {
+      if (onDuplicatedCall != null) onDuplicatedCall!(this);
+      return Future<R?>.value(null);
+    }
+
+    _calling.add(_identifier);
+    try {
+      var ret = function();
+      if (ret is Future) {
+        _callsFuture[_identifier] = ret as Future;
+        return await ret;
+      } else {
+        return ret;
+      }
+    } finally {
+      _finalizeCall();
+    }
+  }
+
+  R? call() {
+    if (_calling.contains(_identifier)) {
+      if (onDuplicatedCall != null) onDuplicatedCall!(this);
+      return null;
+    }
+
+    _calling.add(_identifier);
+    try {
+      var ret = function();
+      if (ret is Future) {
+        var future = ret as Future;
+        _callsFuture[_identifier] = future;
+        future.then((value) {
+          _finalizeCall();
+          return value;
+        });
+        return null;
+      } else {
+        _finalizeCall();
+        return ret;
+      }
+    } catch (e) {
+      _finalizeCall();
+      rethrow;
+    }
+  }
+
+  void _finalizeCall() {
+    // ignore: unawaited_futures
+    _callsFuture.remove(_identifier);
+    _calling.remove(_identifier);
+  }
+}
+
+class _IdentifierWrapper {
+  final Object identifier;
+
+  _IdentifierWrapper(this.identifier);
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is _IdentifierWrapper &&
+          runtimeType == other.runtimeType &&
+          _identical(identifier, other.identifier);
+
+  bool _identical(Object? o1, Object? o2) {
+    if (o1 == null && o2 == null) return true;
+    if (identical(o1, o2)) return true;
+    if (o1 != null && o2 == null) return false;
+    if (o1 == null && o2 != null) return false;
+
+    if (o1.runtimeType != o2.runtimeType) return false;
+
+    if (o1 is Future || o1 is Function) {
+      return false;
+    } else {
+      return o1 == o2;
+    }
+  }
+
+  @override
+  int get hashCode => identifier.hashCode;
+
+  @override
+  String toString() {
+    return '_IdentifierWrapper{hashCode: $hashCode ; identifier: $identifier}';
+  }
+}
diff --git a/lib/src/math.dart b/lib/src/math.dart
index 0789ec5..20cad72 100644
--- a/lib/src/math.dart
+++ b/lib/src/math.dart
@@ -314,6 +314,23 @@ int? parseInt(Object? v, [int? def]) {
   return n as int? ?? def;
 }
 
+/// Parses [l] as [List<int>].
+///
+/// [def] The default value if [l] is invalid.
+List<int>? parseIntList(Object? l, [List<int>? def]) {
+  if (l == null) return def;
+
+  if (l is List) {
+    var l2 = l.map((e) => parseInt(e)).whereType<int>().toList();
+    return l2.isNotEmpty ? l2 : (def ?? l2);
+  } else if (l is String) {
+    var l2 = parseIntsFromInlineList(l, _REGEXP_SPLIT_COMMA);
+    return l2 != null && l2.isNotEmpty ? l2 : (def ?? l2);
+  } else {
+    return def;
+  }
+}
+
 /// Parses [v] to [double].
 ///
 /// [def] The default value if [v] is invalid.
diff --git a/lib/src/resource.dart b/lib/src/resource.dart
index d714197..b8fb0ff 100644
--- a/lib/src/resource.dart
+++ b/lib/src/resource.dart
@@ -329,7 +329,7 @@ class ContextualResource<T, C extends Comparable<C>>
         context = _resolveContext<T, C>(resource, context);
 
   static List<ContextualResource<T, C>> toList<T, C extends Comparable<C>>(
-          Iterable<T> resources, C Function(T resource) context) =>
+          Iterable<T> resources, C? Function(T resource) context) =>
       resources.map((r) => ContextualResource<T, C>(r, context)).toList();
 
   @override
@@ -482,14 +482,14 @@ class ContextualResourceResolver<T, C extends Comparable<C>> {
     var low = 0;
     var high = options.length - 1;
 
-    var comparator = contextComparator as int Function(C?, C)?;
+    var comparator = contextComparator;
 
     while (low <= high) {
       var mid = (low + high) ~/ 2;
       var midVal = options[mid];
 
       var cmp = comparator != null
-          ? comparator(midVal.context, context)
+          ? comparator(midVal.context!, context)
           : midVal.compareContext(context);
 
       if (cmp < 0) {
diff --git a/pubspec.yaml b/pubspec.yaml
index f512fe7..aef6b00 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,6 +1,6 @@
 name: swiss_knife
 description: Dart Useful Tools - collections, math, date, uri, json, events, resources, regexp, etc...
-version: 3.0.5
+version: 3.0.6
 homepage: https://github.com/gmpassos/swiss_knife
 
 environment:
diff --git a/test/swiss_knife_events_test.dart b/test/swiss_knife_events_test.dart
index 2f70ef8..bfc2fbe 100644
--- a/test/swiss_knife_events_test.dart
+++ b/test/swiss_knife_events_test.dart
@@ -6,6 +6,18 @@ Future<void> _sleep(int delayMs) async {
   await Future.delayed(Duration(milliseconds: delayMs), () {});
 }
 
+void _asyncCall(List<String> calls) async {
+  var callID = DateTime.now().microsecondsSinceEpoch;
+  calls.add('$callID> a');
+  await Future.delayed(Duration(seconds: 2));
+  calls.add('$callID> b');
+}
+
+void _doUniqueCall(List<String> calls) async {
+  var uniqueCaller = UniqueCaller(() => _asyncCall(calls));
+  uniqueCaller.call();
+}
+
 void main() {
   group('Events', () {
     setUp(() {});
@@ -68,5 +80,39 @@ void main() {
       expect(interactionCompleter.isTriggerScheduled, isFalse);
       expect(counter.value, equals(1));
     });
+
+    test('NON UniqueCaller', () async {
+      var calls = <String>[];
+
+      expect(UniqueCaller.calling, isEmpty);
+
+      _asyncCall(calls);
+      _asyncCall(calls);
+
+      expect(UniqueCaller.calling, isEmpty);
+
+      print('NON UniqueCaller> $calls');
+
+      expect(calls.where((e) => e.contains(' a')).length, equals(2));
+    });
+
+    test('UniqueCaller', () async {
+      var calls = <String>[];
+
+      expect(UniqueCaller.calling, isEmpty);
+
+      _doUniqueCall(calls);
+      _doUniqueCall(calls);
+
+      expect(UniqueCaller.calling.length, equals(1));
+
+      await Future.wait(UniqueCaller.calling);
+
+      expect(UniqueCaller.calling.length, equals(0));
+
+      print('UniqueCaller> $calls');
+
+      expect(calls.where((e) => e.contains(' a')).length, equals(1));
+    });
   });
 }