fix: flame-engine#2017

 -  Unit and Widget tests for Event Mixin
    - `DragCallbacks`
    - `TapCallbacks`
    - `MultiTouchDragDetector`
    - `MultiTouchTapDetector`

packages/flame/test/events/component_mixins/drag_callbacks_test.dart

@@ -0,0 +1,183 @@
+import 'package:flame/components.dart';
+import 'package:flame/experimental.dart';
+import 'package:flame/game.dart';
+import 'package:flame_test/flame_test.dart';
+import 'package:flutter/gestures.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('DragCallbacks', () {
+    testWithGame<_GameWithHasDraggableComponents>(
+      'make sure they can be added to game with HasDraggableComponents',
+      _GameWithHasDraggableComponents.new,
+      (game) async {
+        await game.add(_DragCallbacksComponent());
+        await game.ready();
+      },
+    );
+
+    testWithFlameGame(
+      'make sure DragCallbacks cannot be added to invalid games',
+      (game) async {
+        expect(
+          () => game.ensureAdd(_DragCallbacksComponent()),
+          failsAssert(
+            'The components with DragCallbacks can only be added to a '
+            'FlameGame with '
+            'the HasDraggableComponents mixin',
+          ),
+        );
+      },
+    );
+
+    testWithGame<_GameWithHasDraggableComponents>(
+      'drag event start',
+      _GameWithHasDraggableComponents.new,
+      (game) async {
+        final component = _DragCallbacksComponent()
+          ..x = 10
+          ..y = 10
+          ..width = 10
+          ..height = 10;
+
+        await game.ensureAdd(component);
+        game.onDragStart(
+          DragStartEvent(
+            1,
+            DragStartDetails(
+              localPosition: const Offset(12, 12),
+              globalPosition: const Offset(12, 12),
+            ),
+          ),
+        );
+        expect(component.containsLocalPoint(Vector2(10, 10)), false);
+        expect(game.dragStartEvent, 1);
+      },
+    );
+
+    testWithGame<_GameWithHasDraggableComponents>(
+      'drag event start, update and cancel',
+      _GameWithHasDraggableComponents.new,
+      (game) async {
+        final component = _DragCallbacksComponent()
+          ..x = 10
+          ..y = 10
+          ..width = 10
+          ..height = 10;
+
+        await game.ensureAdd(component);
+        expect(game.dragStartEvent, 0);
+        game.onDragStart(
+          DragStartEvent(
+            1,
+            DragStartDetails(
+              localPosition: const Offset(12, 12),
+              globalPosition: const Offset(12, 12),
+            ),
+          ),
+        );
+        expect(game.dragStartEvent, 1);
+        expect(game.dragUpdateEvent, 0);
+        expect(game.dragEndEvent, 0);
+
+        game.onDragUpdate(
+          DragUpdateEvent(
+            1,
+            DragUpdateDetails(
+              localPosition: const Offset(15, 15),
+              globalPosition: const Offset(15, 15),
+            ),
+          ),
+        );
+
+        expect(game.containsLocalPoint(Vector2(9, 9)), true);
+        expect(game.dragUpdateEvent, 1);
+
+        game.onDragEnd(
+          DragEndEvent(
+            1,
+            DragEndDetails(),
+          ),
+        );
+
+        expect(game.dragEndEvent, 1);
+      },
+    );
+
+    testWithGame<_GameWithHasDraggableComponents>(
+      'drag event update not called without onDragStart',
+      _GameWithHasDraggableComponents.new,
+      (game) async {
+        final component = _DragCallbacksComponent()
+          ..x = 10
+          ..y = 10
+          ..width = 10
+          ..height = 10;
+
+        await game.ensureAdd(component);
+        expect(game.dragStartEvent, 0);
+        expect(game.dragUpdateEvent, 0);
+
+        game.onDragUpdate(
+          DragUpdateEvent(
+            1,
+            DragUpdateDetails(
+              localPosition: const Offset(15, 15),
+              globalPosition: const Offset(15, 15),
+            ),
+          ),
+        );
+
+        expect(game.dragUpdateEvent, 0);
+      },
+    );
+  });
+}
+
+class _GameWithHasDraggableComponents extends FlameGame
+    with HasDraggableComponents {
+  int dragStartEvent = 0;
+  int dragUpdateEvent = 0;
+  int dragEndEvent = 0;
+
+  @override
+  void onDragStart(DragStartEvent event) {
+    super.onDragStart(event);
+    if (event.handled) {
+      dragStartEvent++;
+    }
+  }
+
+  @override
+  void onDragUpdate(DragUpdateEvent event) {
+    super.onDragUpdate(event);
+    if (event.handled) {
+      dragUpdateEvent++;
+    }
+  }
+
+  @override
+  void onDragEnd(DragEndEvent event) {
+    super.onDragEnd(event);
+    if (event.handled) {
+      dragEndEvent++;
+    }
+  }
+}
+
+class _DragCallbacksComponent extends PositionComponent with DragCallbacks {
+  @override
+  void onDragStart(DragStartEvent event) {
+    event.handled = true;
+  }
+
+  @override
+  void onDragUpdate(DragUpdateEvent event) {
+    event.handled = true;
+  }
+
+  @override
+  void onDragEnd(DragEndEvent event) {
+    event.handled = true;
+  }
+}