来自 UInt8 的 NSData

Question

I have recently found a source code in swift and I am trying to get it to objective-C. The one thing I was unable to understand is this:

var theData:UInt8!

theData = 3;
NSData(bytes: [theData] as [UInt8], length: 1)

Can anybody help me with the Obj-C equivalent?

Just to give you some context, I need to send UInt8 to a CoreBluetooth peripheral (CBPeripheral) as UInt8. Float or integer won't work because the data type would be too big.

Answer 1

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? ).

Answer 2

In Swift 3

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

Answer 3

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)