Why my return is nil but if i press the url in chrome/safari, i can get data?

Question

@IBAction func mainButtonnBeTapped(sender: AnyObject) {
        let session = NSURLSession.sharedSession()
        let request = NSURLRequest(URL: NSURL(string: "http://hq.sinajs.cn/list=sz000609")!)
        let task = session.dataTaskWithRequest(request, completionHandler: {
            (data, response, error) -> Void in            
            let myString = NSString(data: data, encoding: NSUTF8StringEncoding)
            println("this is my string: \(myString)")
        })
        task.resume()
    }

I am using above url to try to get some data, but the return is nil, but i enter the url in chrome/safari, i can get some data. I really don't why, can anyone help to explain?

Monospermous

What exactly is nil? – Parsee

@nhgrif, you see, i have a println() function, in the console, the result is: this is my string: nil – Monospermous

What's the response parameter? (It is non-nil?) – Parsee

Answer 1

The text you try to get is probably not UTF-8, try with another encoding, like this for example:

let myString = NSString(data: data, encoding: NSASCIIStringEncoding)

Update: read Martin R's answer for how to find the right encoding.

Longhair

While it might be possible that the result was not UTF-8 encoding... I don't think this would actually result in the string being nil (versus simply unintelligible). – Parsee

It seems like it is GBK and according to the list of available encodings it is probably kCFStringEncodingGB_18030_2000 – Spiegelman

@Parsee I just tested in a Playground, it is nil with NSUTF8StringEncoding but works with NSASCIIStringEncoding. :) Although I'm not sure this ASCII encoding is the right one for OP, it doesn't return nil anymore with it. – Longhair

@nhgrif: NSString(data:encoding:) does return nil if the data is not valid in the specified encoding, and data in GBK encoding is unlikely to be valid UTF-8. – Ever

Answer 2

This HTTP server sends a

Content-Type = application/x-javascript; charset=GBK

header field in the response, therefore you get the correct encoding from the textEncodingName property of the NSURLResponse. This can be converted to a NSStringEncoding.

This is just a translation of the solution presented in https://mcmap.net/q/1330688/-function-to-detect-the-nsstringencoding-from-nsurlresponse to Swift, plus some simple error checking:

let session = NSURLSession.sharedSession()
let request = NSURLRequest(URL: NSURL(string: "http://hq.sinajs.cn/list=sz000609")!)
let task = session.dataTaskWithRequest(request, completionHandler: {
    (data, response, error) -> Void in   

    var usedEncoding = NSUTF8StringEncoding // Some fallback value
    if let encodingName = response.textEncodingName {
        let encoding = CFStringConvertEncodingToNSStringEncoding(CFStringConvertIANACharSetNameToEncoding(encodingName))
        if encoding != UInt(kCFStringEncodingInvalidId) {
            usedEncoding = encoding
        }
    }
    if let myString = NSString(data: data, encoding: usedEncoding) {
        println("this is my string: \(myString)")
    } else {
        println("failed to decode data")
    }
})
task.resume()

Output:

this is my string: var hq_str_sz000609="绵世股份, ....

Minor changes are necessary for Swift 2:

let session = NSURLSession.sharedSession()
let request = NSURLRequest(URL: NSURL(string: "http://hq.sinajs.cn/list=sz000609")!)
let task = session.dataTaskWithRequest(request, completionHandler: {
    (data, response, error) -> Void in   

    var usedEncoding = NSUTF8StringEncoding // Some fallback value
    if let encodingName = response?.textEncodingName {
        let encoding = CFStringConvertEncodingToNSStringEncoding(CFStringConvertIANACharSetNameToEncoding(encodingName))
        if encoding != UInt(kCFStringEncodingInvalidId) {
            usedEncoding = encoding
        }
    }
    if let myString = String(data: data!, encoding: usedEncoding) {
        print("this is my string: \(myString)")
    } else {
        print("failed to decode data")
    }
})
task.resume()

Update for Swift 3:

let session = URLSession.shared
let request = URLRequest(url: URL(string: "http://hq.sinajs.cn/list=sz000609")!)
let task = session.dataTask(with: request, completionHandler: {
    (data, response, error) -> Void in

    guard let data = data else { return }

    var usedEncoding = String.Encoding.utf8 // Some fallback value
    if let encodingName = response?.textEncodingName {
        let encoding = CFStringConvertEncodingToNSStringEncoding(CFStringConvertIANACharSetNameToEncoding(encodingName as CFString))
        if encoding != UInt(kCFStringEncodingInvalidId) {
            usedEncoding = String.Encoding(rawValue: encoding)
        }
    }
    if let myString = String(data: data, encoding: usedEncoding) {
        print("this is my string: \(myString)")
    } else {
        print("failed to decode data")
    }
})
task.resume()

Ever

i understand the reason, great thanks @Martin R, thanks for the answer and the good soultion – Monospermous