ios - Getting custom class property from contact.bodyA.node -

i developing ios game , having issue didbegincontact().

i trying .difference property 1 of custom classes, "fullbarclass". here code:

func didbegincontact(contact: skphysicscontact) {     var a: skphysicsbody     var b: skphysicsbody      if contact.bodya.categorybitmask < contact.bodyb.categorybitmask{         = contact.bodya         b = contact.bodyb     } else {         b = contact.bodya         = contact.bodyb     }      let bar : fullbarclass = contact.bodya.node     let dif = int(bar.difference)     println(dif) } 

on "let bar : ..." line, getting error: "sknode? not convertible 'fullbarclass' ".

does know why not working?

since contact.bodya.node optional and may not fullbarclass, can't assign body node object fullbarclass constant. can conditionally assign object bar if it's appropriate type by

if let bar = contact.bodya.node as? fullbarclass {    // execute if body node fullbarclass    let dif = int(bar.difference)    print(dif) }