NSData from UInt8

In Swift 3

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

If you write the Swift code slightly simpler as

var theData : UInt8 = 3
let 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?).

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