NSData from UInt8

If you write the Swift code slightly simpler as

var theData : UInt8 = 3let data = NSData(bytes: &theData, length: 1)

then it is relatively straight-forward to translate that to Objective-C:

uint8_t theData = 3;NSData *data = [NSData dataWithBytes:&theData length:1];

For multiple bytes you would use an array

var theData : [UInt8] = [ 3, 4, 5 ]let data = NSData(bytes: &theData, length: theData.count)

which translates to Objective-C as

uint8_t theData[] = { 3, 4, 5 };NSData *data = [NSData dataWithBytes:&theData length:sizeof(theData)];

(and you can omit the address-of operator in the last statement,see for example How come an array's address is equal to its value in C?).

In Swift 3

var myValue: UInt8 = 3 // This can't be let propertieslet value = Data(bytes: &myValue, count: MemoryLayout<UInt8>.size)

In Swift,

Data has a native init method.

// Foundation -> Data  /// Creates a new instance of a collection containing the elements of a/// sequence.////// - Parameter elements: The sequence of elements for the new collection.///   `elements` must be finite.@inlinable public init<S>(_ elements: S) where S : Sequence, S.Element == UInt8@available(swift 4.2)@available(swift, deprecated: 5, message: "use `init(_:)` instead")public init<S>(bytes elements: S) where S : Sequence, S.Element == UInt8

So, the following will work.

let values: [UInt8] = [1, 2, 3, 4]let data = Data(values)