I'm trying to create an NSData var from an array of bytes.
In Obj-C I might have done this:
NSData *endMarker = [[NSData alloc] initWithBytes:{ 0xFF, 0xD9 }, length: 2]
I can't figure out a working equivalent in Swift.
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6 Answers
NSData has an initializer that takes a bytes pointer: init(bytes: UnsafeMutablePointer <Void>, length: Int). An UnsafePointer parameter can accept a variety of different things, including a simple Swift array, so you can use pretty much the same syntax as in Objective-C. When you pass the array, you need to make sure you identify it as a UInt8 array or Swift's type inference will assume you mean to create an Int array.
var endMarker = NSData(bytes: [0xFF, 0xD9] as [UInt8], length: 2)
You can read more about unsafe pointer parameters in Apple's Interacting with C APIs documentation.
4 Comments
Jeff
So that's what I would have thought too. But when I print that out in the debugger I get: <ff00>; and if I do more bytes I get the same thing, all 00 after the first byte.
Nate Cook
Added type information to the array - it was sending an array of
Int instead of an array of Byte.
mash
Just to add,
Byte is removed in swift1.2, use UInt8 instead.
RenniePet
I'm just learning Swift, so this is probably a dumb question. If I write the following: "_fileHandle!.write(NSData(bytes: UnsafePointer(byteData), length: byteLength) as Data)" then it works OK. But I'm worried that that this maybe involves creating a new copy of byteData, which is a 100 KB byte array. Surely not?
var foo : Byte[] = [0xff, 0xD9]
var data = NSData(bytes: foo, length: foo.count)
println("\(data)")
outputs: ff d9
var data = NSData(bytes: [0xFF, 0xD9] as Byte[], length: 2)
println("\(data)")
outputs: ff d9
Edit: Ah, you have to write 'as Byte[]', so then the results are the same
UPDATED for Swift 2.2
var foo:[UInt8] = [0xff, 0xD9]
var data = NSData(bytes: foo, length: foo.count)
print("\(data)")
outputs: ff d9
var data = NSData(bytes: [0xFF, 0xD9] as [UInt8], length: 2)
print("\(data)")
outputs: ff d9
2 Comments
Albert Vila Calvo
Looks like you can't use
Byte anymore in Swift 1.2. Instead of Byte[], write [Uint8]
Dan Rosenstark
No that's
UInt8 (spelling)You don't need to extend Data, in Swift 3 you can do this:
let bytes:[UInt8] = [0x00, 0x01, 0x02, 0x03]
let data = Data(bytes: bytes)
print(data as NSData)
Prints "<00010203>"
To get the byte array again:
let byteArray:[UInt8] = [UInt8](data)
1 Comment
meaning-matters
Thanks. Anything changed in Swift 4?
For Swift 5 I have created another Data extension that works well.
extension Data {
init<T>(fromArray values: [T]) {
var values = values
self.init(buffer: UnsafeBufferPointer(start: &values, count: values.count))
}
func toArray<T>(type: T.Type) -> [T] {
let value = self.withUnsafeBytes {
$0.baseAddress?.assumingMemoryBound(to: T.self)
}
return [T](UnsafeBufferPointer(start: value, count: self.count / MemoryLayout<T>.stride))
}
}
Sample Usage
let data = Data(fromArray: [1, 2, 3, 4, 5])
let array = data.toArray(type: Int.self)
print(array)
// [1, 2, 3, 4, 5]
Comments
Swift 3 extension
extension Data {
init<T>(fromArray values: [T]) {
var values = values
self.init(buffer: UnsafeBufferPointer(start: &values, count: values.count))
}
func toArray<T>(type: T.Type) -> [T] {
return self.withUnsafeBytes {
[T](UnsafeBufferPointer(start: $0, count: self.count/MemoryLayout<T>.stride))
}
}
}
Usage
let bytes:[UInt8] = [0x00, 0xf4, 0x7c]
let data = Data(fromArray: someBytes)
print(data as NSData)
let bytes = data.toArray(type: UInt8.self)
print(bytes)
Comments
Answer in Swift 5 is simple and not found here. Ran into this issue and found answer:
var bytes = [Int8](repeating: 0, count: 32)
let data = Data(bytes: bytes, count: bytes.count)
