swift 中使用weak self

request.responseString(encoding: NSUTF8StringEncoding) {[weak self] (res) -> Void in
if let strongSelf = self {
//do something...
}
}

weak var weakSelf = self
request.responseString(encoding: NSUTF8StringEncoding) {(res) -> Void in
if let strongSelf = weakSelf {
//do something
}
}