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  
    }  
}