ios - Error ("'()' is not identical to 'UInt8'") writing NSData bytes to NSOutputStream using the write function in Swift

Question

I'm trying to build an asynchronous file download in Swift based on the Erica Sadun's method. But I need it to handle bigger files, so I found this answer about using a NSOutputStream instead of NSData, makes sense.

However, I can't get it to work. I get this error when I try adding the NSData bytes (in my NSURLConnection didReceiveData function) to the NSOutputStream write function: '()' is not identical to 'UInt8' on this row: bytesWritten = self.downloadStream.write(data.bytes, maxLength: bytesLeftToWrite).

data.bytes is of the type ConstUnsafePointer<()> and the .write() function expects the type to be ConstUnsafePointer<UInt8>, so in that sense, the error make perfect sense. But since I'm new to iOS and of course Swift programming, I can't get my head around how to fix this.

So, how do I convert the data.bytes: ConstUnsafePointer<()> to ConstUnsafePointer<UInt8> alt. make this work some other way?

My didReceiveData function:

func connection(connection: NSURLConnection!, didReceiveData data: NSData!) {

    var bytesLeftToWrite: NSInteger = data.length
    var bytesWritten: NSInteger = 0

    while bytesLeftToWrite > 0 {

        bytesWritten = self.downloadStream.write(data.bytes, maxLength: bytesLeftToWrite)

        if bytesWritten == -1 {

            break

        }

        bytesLeftToWrite -= bytesWritten

        let responseExpectedlenght: NSNumber = NSNumber(longLong: self.downloadResponse!.expectedContentLength)
        let dataLength: NSNumber = NSNumber(long: data.length)

        self.downloadProgressPercentage = ((dataLength / responseExpectedlenght) * 100)

        println("Downloaded: (self.downloadProgressPercentage)%")

    }

}

Answer 1

You can cast the pointer with UnsafePointer():

bytesWritten = self.downloadStream.write(UnsafePointer(data.bytes), maxLength: bytesLeftToWrite)

There is also a problem in your write loop, because you always write the initial bytes of the data object to the output stream.

It should probably look similar to this (untested):

var bytes = UnsafePointer<UInt8>(data.bytes)
var bytesLeftToWrite: NSInteger = data.length

while bytesLeftToWrite > 0 {
    let bytesWritten = self.downloadStream.write(bytes, maxLength: bytesLeftToWrite)
    if bytesWritten == -1 {
        break // Some error occurred ...
    }

    bytesLeftToWrite -= bytesWritten
    bytes += bytesWritten // advance pointer

    // ...
}