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
    }
}