NSURLConnection sendAsynchronousRequest不能从封闭中获取变量

Question

I'm trying to get a simple text response from a PHP page using POST. I have the following code:

func post(url: String, info: String) -> String {
    var URL: NSURL = NSURL(string: url)!
    var request:NSMutableURLRequest = NSMutableURLRequest(URL:URL)
    var output = "Nothing Returned";
    request.HTTPMethod = "POST";
    var bodyData = info;
    request.HTTPBody = bodyData.dataUsingEncoding(NSUTF8StringEncoding);

    NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue.mainQueue()){

        response, data, error in

        output = (NSString(data: data, encoding: NSUTF8StringEncoding))!


    }

    return output
}

While this code does not throw any errors, when I make a call to it like this:

println(post(url, info: data))

It only prints: "Nothing Returned" even though if I were to change the line:

output = (NSString(data: data, encoding: NSUTF8StringEncoding))!

to this:

println((NSString(data: data, encoding: NSUTF8StringEncoding)))

it does print out the proper response. Am I doing something wrong with my variables here?

Answer 1

This is calling asynchronous function that is using a completion handler block/closure. So, you need to employ the completion handler pattern in your own code. This consists of changing the method return type to Void and adding a new completionHandler closure that will be called when the asynchronous call is done:

func post(url: String, info: String, completionHandler: (NSString?, NSError?) -> ()) {
    let URL = NSURL(string: url)!
    let request = NSMutableURLRequest(URL:URL)
    request.HTTPMethod = "POST"
    let bodyData = info
    request.HTTPBody = bodyData.dataUsingEncoding(NSUTF8StringEncoding);

    NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue.mainQueue()) { response, data, error in
        guard data != nil else {
            completionHandler(nil, error)
            return
        }

        completionHandler(NSString(data: data!, encoding: NSUTF8StringEncoding), nil)
    }
}

Or, since NSURLConnection is now formally deprecated, it might be better to use NSURLSession :

func post(url: String, info: String, completionHandler: (NSString?, NSError?) -> ()) -> NSURLSessionTask {
    let URL = NSURL(string: url)!
    let request = NSMutableURLRequest(URL:URL)
    request.HTTPMethod = "POST"
    let bodyData = info
    request.HTTPBody = bodyData.dataUsingEncoding(NSUTF8StringEncoding);

    let task = NSURLSession.sharedSession().dataTaskWithRequest(request) { data, response, error in
        dispatch_async(dispatch_get_main_queue()) {
            guard data != nil else {
                completionHandler(nil, error)
                return
            }

            completionHandler(NSString(data: data!, encoding: NSUTF8StringEncoding), nil)
        }
    }
    task.resume()

    return task
}

And you call it like so:

post(url, info: info) { responseString, error in
    guard responseString != nil else {
        print(error)
        return
    }

    // use responseString here
}

// but don't try to use response string here ... the above closure will be called
// asynchronously (i.e. later)

Note, to keep this simple, I've employed the trailing closure syntax (see Trailing Closure section of The Swift Programming Language: Closures ), but hopefully it illustrates the idea: You cannot immediately return the result of an asynchronous method, so provide a completion handler closure that will be called when the asynchronous method is done.