Handle removal of sprite physics bodies during the physics simulation

examples/game/test_physics.dart

@@ -83,6 +83,7 @@ class TestBed extends NodeWithSize {
 
   void myCallback(PhysicsContactType type, PhysicsContact contact) {
     print("CONTACT type: $type");
+    contact.nodeB.removeFromParent();
   }
 
   bool handleEvent(SpriteBoxEvent event) {

packages/flutter_sprites/lib/physics_body.dart

@@ -120,7 +120,7 @@ class PhysicsBody {
 
   void _detach() {
     if (_attached) {
-      _physicsNode.b2World.destroyBody(_body);
+      _physicsNode._bodiesScheduledForDestruction.add(_body);
       _attached = false;
     }
   }

packages/flutter_sprites/lib/physics_node.dart

@@ -31,6 +31,8 @@ class PhysicsNode extends Node {
 
   _ContactHandler _contactHandler;
 
+  List<box2d.Body> _bodiesScheduledForDestruction = [];
+
   double b2WorldToNodeConversionFactor = 500.0;
 
   Offset get gravity {
@@ -57,6 +59,9 @@ class PhysicsNode extends Node {
   }
 
   void _stepPhysics(double dt) {
+    // Remove bodies that were marked for destruction during the update phase
+    _removeBodiesScheduledForDestruction();
+
     // Calculate a step in the simulation
     b2World.stepDt(dt, 10, 10);
 
@@ -71,6 +76,16 @@ class PhysicsNode extends Node {
 
       body._node._setRotationFromPhysics(degrees(b2Body.getAngle()));
     }
+
+    // Remove bodies that were marked for destruction during the simulation
+    _removeBodiesScheduledForDestruction();
+  }
+
+  void _removeBodiesScheduledForDestruction() {
+    for (box2d.Body b2Body in _bodiesScheduledForDestruction) {
+      b2World.destroyBody(b2Body);
+    }
+    _bodiesScheduledForDestruction.clear();
   }
 
   void _updatePosition(PhysicsBody body, Point position) {
@@ -257,6 +272,11 @@ class _ContactHandler extends box2d.ContactListener {
           b2Contact.isTouching(),
           b2Contact.isEnabled()
         );
+
+        if (type == PhysicsContactType.postSolve) {
+
+        }
+
         // Make callback
         info.callback(type, contact);