Joints and Motors - Box2D for Flash Games - page 120

5. As we are using a custom contact listener, we'll disable collisions between the

projectile and the siege machine using the customContact class.

6. Finally, the updateWorld must also be changed to include the lines to remove

destroyed bodies.
private function updateWorld(e:Event):void {
if (left) {
motorSpeed-=0.1;
}
if (right) {
motorSpeed+=0.1;
}
motorSpeed*0.99;
if (motorSpeed>5) {
motorSpeed=5;
}
if (motorSpeed<-5) {
motorSpeed=-5;
}
frj.SetMotorSpeed(motorSpeed);
rrj.SetMotorSpeed(motorSpeed);
world.Step(1/30,10,10);
world.ClearForces();
for (var b:b2Body=world.GetBodyList(); b; b=b.GetNext())
{
if (b.GetUserData()=="remove") {
world.DestroyBody(b);
}
}
world.DrawDebugData();
}

7. And last but not the least, something new to learn about collisions. The

following is how I am using the PreSolve callback function to determine
if the cart collided with the projectile and disable the collision before the
contact is solved, adding this function to the customContact class:
override public function PreSolve(contact:b2Contact,
oldManifold:b2Manifold):void {
var fixtureA:b2Fixture=contact.GetFixtureA();
var fixtureB:b2Fixture=contact.GetFixtureB();
var dataA:String=fixtureA.GetBody().GetUserData();
var dataB:String=fixtureB.GetBody().GetUserData();
if (dataA=="cart" && dataB=="projectile") {
contact.SetEnabled(false);