class MyClass {
     var completion : ((MyObject) -> Void)? // or ...! for an implicitly unwrapped optional
}

您可以设置属性

completion = {
    (obj : MyObject) -> Void in
    // do something with obj ...
}

可以缩短(由于自动类型推断)

completion = {
    obj in
    // do something with obj ...
}