diff --git a/example/android/build.gradle b/example/android/build.gradle
index 3100ad2..c505a86 100644
--- a/example/android/build.gradle
+++ b/example/android/build.gradle
@@ -6,7 +6,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.5.0'
+        classpath 'com.android.tools.build:gradle:4.1.0'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
     }
 }
diff --git a/example/android/gradle/wrapper/gradle-wrapper.properties b/example/android/gradle/wrapper/gradle-wrapper.properties
index 296b146..90f271d 100644
--- a/example/android/gradle/wrapper/gradle-wrapper.properties
+++ b/example/android/gradle/wrapper/gradle-wrapper.properties
@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.6.2-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.5-all.zip
diff --git a/example/pubspec.lock b/example/pubspec.lock
index 282fd01..acaf6bf 100644
--- a/example/pubspec.lock
+++ b/example/pubspec.lock
@@ -7,42 +7,42 @@ packages:
       name: async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.5.0-nullsafety.1"
+    version: "2.5.0"
   boolean_selector:
     dependency: transitive
     description:
       name: boolean_selector
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.0-nullsafety.1"
+    version: "2.1.0"
   characters:
     dependency: transitive
     description:
       name: characters
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0-nullsafety.3"
+    version: "1.1.0"
   charcode:
     dependency: transitive
     description:
       name: charcode
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0-nullsafety.1"
+    version: "1.2.0"
   clock:
     dependency: transitive
     description:
       name: clock
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0-nullsafety.1"
+    version: "1.1.0"
   collection:
     dependency: transitive
     description:
       name: collection
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.15.0-nullsafety.3"
+    version: "1.15.0"
   cupertino_icons:
     dependency: "direct main"
     description:
@@ -56,7 +56,7 @@ packages:
       name: fake_async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0-nullsafety.1"
+    version: "1.2.0"
   flutter:
     dependency: "direct main"
     description: flutter
@@ -72,27 +72,34 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  js:
+    dependency: transitive
+    description:
+      name: js
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.6.3"
   matcher:
     dependency: transitive
     description:
       name: matcher
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.12.10-nullsafety.1"
+    version: "0.12.10"
   meta:
     dependency: transitive
     description:
       name: meta
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.3.0-nullsafety.3"
+    version: "1.3.0"
   path:
     dependency: transitive
     description:
       name: path
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.8.0-nullsafety.1"
+    version: "1.8.0"
   sky_engine:
     dependency: transitive
     description: flutter
@@ -104,28 +111,28 @@ packages:
       name: source_span
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.8.0-nullsafety.2"
+    version: "1.8.0"
   stack_trace:
     dependency: transitive
     description:
       name: stack_trace
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.10.0-nullsafety.1"
+    version: "1.10.0"
   stream_channel:
     dependency: transitive
     description:
       name: stream_channel
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.0-nullsafety.1"
+    version: "2.1.0"
   string_scanner:
     dependency: transitive
     description:
       name: string_scanner
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0-nullsafety.1"
+    version: "1.1.0"
   swipe_to_action:
     dependency: "direct main"
     description:
@@ -139,28 +146,28 @@ packages:
       name: term_glyph
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0-nullsafety.1"
+    version: "1.2.0"
   test_api:
     dependency: transitive
     description:
       name: test_api
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.2.19-nullsafety.2"
+    version: "0.2.19"
   typed_data:
     dependency: transitive
     description:
       name: typed_data
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.3.0-nullsafety.3"
+    version: "1.3.0"
   vector_math:
     dependency: transitive
     description:
       name: vector_math
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.0-nullsafety.3"
+    version: "2.1.0"
 sdks:
-  dart: ">=2.10.0-110 <=2.11.0-161.0.dev"
-  flutter: ">=1.20.0 <2.0.0"
+  dart: ">=2.12.0 <3.0.0"
+  flutter: ">=2.0.0"
diff --git a/example/pubspec.yaml b/example/pubspec.yaml
index 949bf5f..7683225 100644
--- a/example/pubspec.yaml
+++ b/example/pubspec.yaml
@@ -6,7 +6,7 @@ description: Demonstrates how to use the swipe_to_action plugin.
 publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 
 environment:
-  sdk: ">=2.7.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   flutter:
diff --git a/example/test/widget_test.dart b/example/test/widget_test.dart
index cd1463b..570e0e4 100644
--- a/example/test/widget_test.dart
+++ b/example/test/widget_test.dart
@@ -5,23 +5,4 @@
 // gestures. You can also use WidgetTester to find child widgets in the widget
 // tree, read text, and verify that the values of widget properties are correct.
 
-import 'package:flutter/material.dart';
-import 'package:flutter_test/flutter_test.dart';
-
-import 'package:swipe_to_action_example/main.dart';
-
-void main() {
-  testWidgets('Verify Platform version', (WidgetTester tester) async {
-    // Build our app and trigger a frame.
-    await tester.pumpWidget(MyApp());
-
-    // Verify that platform version is retrieved.
-    expect(
-      find.byWidgetPredicate(
-        (Widget widget) => widget is Text &&
-                           widget.data.startsWith('Running on:'),
-      ),
-      findsOneWidget,
-    );
-  });
-}
+void main() {}
diff --git a/lib/src/swipeable.dart b/lib/src/swipeable.dart
index 833a26d..021faec 100644
--- a/lib/src/swipeable.dart
+++ b/lib/src/swipeable.dart
@@ -31,6 +31,9 @@ enum SwipeDirection {
   /// The [Swipeable] can be swiped by dragging in the reading direction
   /// (e.g., from left to right in left-to-right languages).
   startToEnd,
+
+  /// Called instead of null.
+  none,
 }
 
 class Swipeable extends StatefulWidget {
@@ -43,12 +46,12 @@ class Swipeable extends StatefulWidget {
   /// state of the dismissed item. Using keys causes the widgets to sync
   /// according to their keys and avoids this pitfall.
   const Swipeable({
-    @required Key key,
-    @required this.child,
+    required Key key,
+    required this.child,
+    required this.onSwipe,
     this.background,
     this.secondaryBackground,
     this.confirmSwipe,
-    this.onSwipe,
     this.direction = SwipeDirection.horizontal,
     this.dismissThresholds = const <SwipeDirection, double>{},
     this.maxOffset = 0.4,
@@ -60,9 +63,7 @@ class Swipeable extends StatefulWidget {
       PointerDeviceKind.stylus,
       PointerDeviceKind.touch
     },
-  })  : assert(key != null),
-        assert(secondaryBackground == null || background != null),
-        assert(dragStartBehavior != null),
+  })  : assert(secondaryBackground == null || background != null),
         super(key: key);
 
   /// The widget below this widget in the tree.
@@ -73,12 +74,12 @@ class Swipeable extends StatefulWidget {
   /// A widget that is stacked behind the child. If secondaryBackground is also
   /// specified then this widget only appears when the child has been dragged
   /// to the right.
-  final Widget background;
+  final Widget? background;
 
   /// A widget that is stacked behind the child and is exposed when the child
   /// has been dragged to the left. It may only be specified when background
   /// has also been specified.
-  final Widget secondaryBackground;
+  final Widget? secondaryBackground;
 
   /// Gives the app an opportunity to confirm or veto a pending dismissal.
   ///
@@ -87,7 +88,7 @@ class Swipeable extends StatefulWidget {
   ///
   /// If the returned Future<bool> completes to false or null the [onSwipe]
   /// callback will not run.
-  final ConfirmSwipeCallback confirmSwipe;
+  final ConfirmSwipeCallback? confirmSwipe;
 
   /// Called when the widget has been dismissed, after finishing resizing.
   final SwipeDirectionCallback onSwipe;
@@ -159,9 +160,8 @@ class Swipeable extends StatefulWidget {
 
 class _SwipeableClipper extends CustomClipper<Rect> {
   _SwipeableClipper({
-    @required this.moveAnimation,
-  })  : assert(moveAnimation != null),
-        super(reclip: moveAnimation);
+    required this.moveAnimation,
+  }) : super(reclip: moveAnimation);
 
   final Animation<Offset> moveAnimation;
 
@@ -185,28 +185,27 @@ class _SwipeableClipper extends CustomClipper<Rect> {
 
 enum _FlingGestureKind { none, forward, reverse }
 
-class _SwipeableState extends State<Swipeable>
-    with TickerProviderStateMixin, AutomaticKeepAliveClientMixin {
+class _SwipeableState extends State<Swipeable> with TickerProviderStateMixin, AutomaticKeepAliveClientMixin {
   @override
   void initState() {
-    super.initState();
-    _moveController =
-        AnimationController(duration: widget.movementDuration, vsync: this)
-          ..addStatusListener(_handleDismissStatusChanged);
+    _moveController = AnimationController(duration: widget.movementDuration, vsync: this)
+      ..addStatusListener(_handleDismissStatusChanged);
     _updateMoveAnimation();
+
+    super.initState();
   }
 
-  AnimationController _moveController;
-  Animation<Offset> _moveAnimation;
+  late AnimationController _moveController;
+  late Animation<Offset> _moveAnimation;
 
   double _dragExtent = 0.0;
   bool _dragUnderway = false;
-  Size _sizePriorToCollapse;
+  Size? _sizePriorToCollapse;
 
   bool _isTouch = true;
 
   @override
-  bool get wantKeepAlive => _moveController?.isAnimating == true;
+  bool get wantKeepAlive => _moveController.isAnimating == true;
 
   @override
   void dispose() {
@@ -216,20 +215,14 @@ class _SwipeableState extends State<Swipeable>
 
   SwipeDirection _extentToDirection(double extent) {
     if (extent == 0.0) {
-      return null;
+      return SwipeDirection.none;
     }
     switch (Directionality.of(context)) {
       case TextDirection.rtl:
-        return extent < 0
-            ? SwipeDirection.startToEnd
-            : SwipeDirection.endToStart;
+        return extent < 0 ? SwipeDirection.startToEnd : SwipeDirection.endToStart;
       case TextDirection.ltr:
-        return extent > 0
-            ? SwipeDirection.startToEnd
-            : SwipeDirection.endToStart;
+        return extent > 0 ? SwipeDirection.startToEnd : SwipeDirection.endToStart;
     }
-    assert(false);
-    return null;
   }
 
   SwipeDirection get _swipeDirection => _extentToDirection(_dragExtent);
@@ -240,7 +233,7 @@ class _SwipeableState extends State<Swipeable>
 
   double get _overallDragAxisExtent {
     final size = context.size;
-    return size.width;
+    return size?.width ?? 0.0;
   }
 
   void _handlePointerDown(PointerDownEvent event) {
@@ -252,8 +245,7 @@ class _SwipeableState extends State<Swipeable>
   void _handleDragStart(DragStartDetails details) {
     _dragUnderway = true;
     if (_moveController.isAnimating) {
-      _dragExtent =
-          _moveController.value * _overallDragAxisExtent * _dragExtent.sign;
+      _dragExtent = _moveController.value * _overallDragAxisExtent * _dragExtent.sign;
       _moveController.stop();
     } else {
       _dragExtent = 0.0;
@@ -269,9 +261,12 @@ class _SwipeableState extends State<Swipeable>
       return;
     }
 
-    final delta = details.primaryDelta;
+    final delta = details.primaryDelta ?? 0.0;
     final oldDragExtent = _dragExtent;
     switch (widget.direction) {
+      case SwipeDirection.none:
+        return;
+
       case SwipeDirection.horizontal:
         _dragExtent += delta;
         break;
@@ -327,7 +322,6 @@ class _SwipeableState extends State<Swipeable>
   }
 
   _FlingGestureKind _describeFlingGesture(Velocity velocity) {
-    assert(widget.direction != null);
     if (_dragExtent == 0.0) {
       // If it was a fling, then it was a fling that was let loose at the exact
       // middle of the range (i.e. when there's no displacement). In that case,
@@ -340,14 +334,12 @@ class _SwipeableState extends State<Swipeable>
     final vy = velocity.pixelsPerSecond.dy;
     SwipeDirection flingDirection;
     // Verify that the fling is in the generally right direction and fast enough.
-    if (vx.abs() - vy.abs() < _kMinFlingVelocityDelta ||
-        vx.abs() < _kMinFlingVelocity) {
+    if (vx.abs() - vy.abs() < _kMinFlingVelocityDelta || vx.abs() < _kMinFlingVelocity) {
       return _FlingGestureKind.none;
     }
     assert(vx != 0.0);
     flingDirection = _extentToDirection(vx);
 
-    assert(_swipeDirection != null);
     if (flingDirection == _swipeDirection) {
       return _FlingGestureKind.forward;
     }
@@ -359,8 +351,7 @@ class _SwipeableState extends State<Swipeable>
       return;
     }
     _dragUnderway = false;
-    if (_moveController.isCompleted &&
-        await _confirmStartSwipeAnimation() == true) {
+    if (_moveController.isCompleted && await _confirmStartSwipeAnimation() == true) {
       _startSwipeAnimation();
       return;
     }
@@ -369,28 +360,23 @@ class _SwipeableState extends State<Swipeable>
       case _FlingGestureKind.forward:
         assert(_dragExtent != 0.0);
         assert(!_moveController.isDismissed);
-        if ((widget.dismissThresholds[_swipeDirection] ?? _kDismissThreshold) >=
-            1.0) {
+        if ((widget.dismissThresholds[_swipeDirection] ?? _kDismissThreshold) >= 1.0) {
           await _moveController.reverse();
           break;
         }
         _dragExtent = flingVelocity.sign;
-        await _moveController.fling(
-            velocity: flingVelocity.abs() * _kFlingVelocityScale);
+        await _moveController.fling(velocity: flingVelocity.abs() * _kFlingVelocityScale);
         break;
       case _FlingGestureKind.reverse:
         assert(_dragExtent != 0.0);
         assert(!_moveController.isDismissed);
         _dragExtent = flingVelocity.sign;
-        await _moveController.fling(
-            velocity: -flingVelocity.abs() * _kFlingVelocityScale);
+        await _moveController.fling(velocity: -flingVelocity.abs() * _kFlingVelocityScale);
         break;
       case _FlingGestureKind.none:
         if (!_moveController.isDismissed) {
           // we already know it's not completed, we check that above
-          if (_moveController.value >
-              (widget.dismissThresholds[_swipeDirection] ??
-                  _kDismissThreshold)) {
+          if (_moveController.value > (widget.dismissThresholds[_swipeDirection] ?? _kDismissThreshold)) {
             await _moveController.forward();
           } else {
             await _moveController.reverse();
@@ -414,21 +400,18 @@ class _SwipeableState extends State<Swipeable>
   Future<bool> _confirmStartSwipeAnimation() async {
     if (widget.confirmSwipe != null) {
       final direction = _swipeDirection;
-      assert(direction != null);
-      return widget.confirmSwipe(direction);
+      return widget.confirmSwipe!(direction);
     }
     return true;
   }
 
   void _startSwipeAnimation() {
-    assert(_moveController != null);
     assert(_moveController.isCompleted);
     assert(_sizePriorToCollapse == null);
-    if (widget.onSwipe != null) {
-      final direction = _swipeDirection;
-      assert(direction != null);
-      widget.onSwipe(direction);
-    }
+
+    final direction = _swipeDirection;
+    widget.onSwipe(direction);
+
     _moveController.reverse();
   }
 
diff --git a/pubspec.lock b/pubspec.lock
index f019817..72a8f48 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -7,49 +7,49 @@ packages:
       name: async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.5.0-nullsafety.1"
+    version: "2.5.0"
   boolean_selector:
     dependency: transitive
     description:
       name: boolean_selector
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.0-nullsafety.1"
+    version: "2.1.0"
   characters:
     dependency: transitive
     description:
       name: characters
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0-nullsafety.3"
+    version: "1.1.0"
   charcode:
     dependency: transitive
     description:
       name: charcode
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0-nullsafety.1"
+    version: "1.2.0"
   clock:
     dependency: transitive
     description:
       name: clock
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0-nullsafety.1"
+    version: "1.1.0"
   collection:
     dependency: transitive
     description:
       name: collection
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.15.0-nullsafety.3"
+    version: "1.15.0"
   fake_async:
     dependency: transitive
     description:
       name: fake_async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0-nullsafety.1"
+    version: "1.2.0"
   flutter:
     dependency: "direct main"
     description: flutter
@@ -65,27 +65,34 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  js:
+    dependency: transitive
+    description:
+      name: js
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.6.3"
   matcher:
     dependency: transitive
     description:
       name: matcher
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.12.10-nullsafety.1"
+    version: "0.12.10"
   meta:
     dependency: transitive
     description:
       name: meta
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.3.0-nullsafety.3"
+    version: "1.3.0"
   path:
     dependency: transitive
     description:
       name: path
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.8.0-nullsafety.1"
+    version: "1.8.0"
   sky_engine:
     dependency: transitive
     description: flutter
@@ -97,56 +104,56 @@ packages:
       name: source_span
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.8.0-nullsafety.2"
+    version: "1.8.0"
   stack_trace:
     dependency: transitive
     description:
       name: stack_trace
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.10.0-nullsafety.1"
+    version: "1.10.0"
   stream_channel:
     dependency: transitive
     description:
       name: stream_channel
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.0-nullsafety.1"
+    version: "2.1.0"
   string_scanner:
     dependency: transitive
     description:
       name: string_scanner
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0-nullsafety.1"
+    version: "1.1.0"
   term_glyph:
     dependency: transitive
     description:
       name: term_glyph
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0-nullsafety.1"
+    version: "1.2.0"
   test_api:
     dependency: transitive
     description:
       name: test_api
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.2.19-nullsafety.2"
+    version: "0.2.19"
   typed_data:
     dependency: transitive
     description:
       name: typed_data
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.3.0-nullsafety.3"
+    version: "1.3.0"
   vector_math:
     dependency: transitive
     description:
       name: vector_math
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.0-nullsafety.3"
+    version: "2.1.0"
 sdks:
-  dart: ">=2.10.0-110 <=2.11.0-161.0.dev"
-  flutter: ">=1.20.0 <2.0.0"
+  dart: ">=2.12.0 <3.0.0"
+  flutter: ">=2.0.0"
diff --git a/pubspec.yaml b/pubspec.yaml
index 2ef8cc6..84a7dba 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,8 +4,8 @@ version: 0.1.0
 homepage: https://inex.dev/inex/swipe_to_action
 
 environment:
-  sdk: ">=2.7.0 <3.0.0"
-  flutter: ">=1.20.0 <2.0.0"
+  sdk: ">=2.12.0 <3.0.0"
+  flutter: ">=2.0.0"
 
 dependencies:
   flutter: