Set userdata on the body instead of on fixture

doc/box2d.md

@@ -18,6 +18,14 @@ A simple `Box2DGame` implementation example can be seen in the [examples folder]
 
 If you are using `Box2DGame` you can take advantage of its way of handling contacts between two `BodyComponent`s.
 
+When creating the body definition for your `BodyComponent` make sure that you set the userdata to the current object, otherwise it will not be possible to detect collisions.
+Like this:
+```dart
+final bodyDef = BodyDef()
+  // To be able to determine object in collision
+  ..setUserData(this);
+```
+
 To do this you have to make an implementation of `ContactCallback` where you set which two types that it should react when they come in contact.
 If you have two `BodyComponent`s named Ball and Wall and you want to do something when they come in contact you would do like this:
 

doc/examples/box2d/contact_callbacks/lib/main.dart

@@ -38,14 +38,14 @@ class Ball extends BodyComponent {
     shape.radius = radius;
 
     final fixtureDef = FixtureDef()
-      // To be able to determine object in collision
-      ..setUserData(this)
       ..shape = shape
       ..restitution = 1.0
       ..density = 1.0
       ..friction = 0.1;
 
     final bodyDef = BodyDef()
+      // To be able to determine object in collision
+      ..setUserData(this)
       ..position = position
       ..type = BodyType.DYNAMIC;
 

lib/box2d/contact_callbacks.dart

@@ -40,8 +40,8 @@ class ContactCallbacks extends ContactListener {
   }
 
   void _maybeCallback(Contact contact, ContactCallback callback, Function f) {
-    final Object a = contact.fixtureA.userData;
-    final Object b = contact.fixtureB.userData;
+    final Object a = contact.fixtureA.getBody().userData;
+    final Object b = contact.fixtureB.getBody().userData;
     final ContactTypes wanted = callback.types;
 
     if (wanted.match(a, b) || (wanted.has(BodyComponent) && wanted.hasOne(a, b))) {