Can't convert NSData to NSString in swift

问题:

This is my function. First println print correct hash to console but in the next row program crashes. Can you help me?

func sha256(string: NSString) -> NSString {
    var data : NSData! = string.dataUsingEncoding(NSUTF8StringEncoding)
    var hash = [UInt8](count: Int(CC_SHA256_DIGEST_LENGTH), repeatedValue: 0)
    CC_SHA256(data.bytes, CC_LONG(data.length), &hash)
    let res = NSData(bytes: hash, length: Int(CC_SHA256_DIGEST_LENGTH))
    println(res)
    let resstr = NSString(data: res, encoding: NSUTF8StringEncoding)
    println(resstr)
    return resstr
}

回答1:

let resstr = NSString(data: res, encoding: NSUTF8StringEncoding)

returns nil if the data does not represent a valid UTF-8 sequence (which is very likely). Then the following println() crashes.

A possible string representation for arbitrary binary data would be a hex string or a Base-64 encoded string.

A Base-64 encoded string can simply be obtained with

let resstr = res.base64EncodedStringWithOptions(nil)

There is (as far as I know) no built-in method to convert binary data to a hex string. A possible implementation in Swift (inspired by the lots of available Objective-C solutions) is

extension NSData {
    func hexString() -> NSString {
        var str = NSMutableString()
        let bytes = UnsafeBufferPointer<UInt8>(start: UnsafePointer(self.bytes), count:self.length)
        for byte in bytes {
            str.appendFormat("%02hhx", byte)
        }
        return str
    }
}

But you could integrate that into your hash method directly, without using an intermediate NSData object:

func sha256(string: NSString) -> NSString {
    let data = string.dataUsingEncoding(NSUTF8StringEncoding)!
    var hash = [UInt8](count: Int(CC_SHA256_DIGEST_LENGTH), repeatedValue: 0)
    CC_SHA256(data.bytes, CC_LONG(data.length), &hash)
    let resstr = NSMutableString()
    for byte in hash {
        resstr.appendFormat("%02hhx", byte)
    }
    return resstr
}