How to byte reverse NSData output in Swift the littleEndian way?

I have this output from NSData: <00000100 84000c00 071490fe 4dfbd7e9>

So how could I byte reverse it in Swift and have this output: <00000001 0084000c 1407fe90 fb4de9d7>?

This should work to swap each pair of adjacent bytes in the data. The idea is to interpret the bytes as an array of UInt16 integers and use the built-in byteSwapped property.

func swapUInt16Data(data : NSData) -> NSData {
    
    // Copy data into UInt16 array:
    let count = data.length / sizeof(UInt16)
    var array = [UInt16](count: count, repeatedValue: 0)
    data.getBytes(&array, length: count * sizeof(UInt16))
    
    // Swap each integer:
    for i in 0 ..< count {
        array[i] = array[i].byteSwapped // *** (see below)
    }

    // Create NSData from array:
    return NSData(bytes: &array, length: count * sizeof(UInt16))
}

If your actual intention is to convert data from an (external) big-endian representation to the host (native) byte order (which happens to be little-endian on all current iOS and OS X devices) then you should replace *** by

array[i] = UInt16(bigEndian: array[i])

Example:

var bytes : [UInt8] = [1, 2, 3, 4, 5, 6, 7, 8]
let data = NSData(bytes: &bytes, length: bytes.count)
print(data)
// <01020304 05060708>
print(swapUInt16Data(data))
// <02010403 06050807>

Update for Swift 3: The generic withUnsafeMutableBytes() methods allows to obtain a UnsafeMutablePointer<UInt16> to the bytes and modify them directly:

func swapUInt16Data(data : Data) -> Data {
    var mdata = data // make a mutable copy
    let count = data.count / MemoryLayout<UInt16>.size
    mdata.withUnsafeMutableBytes { (i16ptr: UnsafeMutablePointer<UInt16>) in
        for i in 0..<count {
            i16ptr[i] =  i16ptr[i].byteSwapped
        }
    }
    return mdata
}

Example:

let data = Data(bytes: [1, 2, 3, 4, 5, 6, 7, 8])
print(data as NSData) // <01020304 05060708>

let swapped = swapUInt16Data(data: data)
print(swapped as NSData) // <02010403 06050807>

Update for Swift 5, (fixing a deprecated warning):

func swapUInt16Data(data : Data) -> Data {
    var mdata = data // make a mutable copy
    let count = data.count / MemoryLayout<UInt16>.size
    mdata.withUnsafeMutableBytes { ptr in
        let i16ptr = ptr.assumingMemoryBound(to: UInt16.self)
        for i in 0..<count {
            i16ptr[i] =  i16ptr[i].byteSwapped
        }
    }
    return mdata
}