Create an Array in Swift from an NSData Object - ios

Question

I'm trying to store an array of integers to disk in swift. I can get them into an NSData object to store, but getting them back out into an array is difficult. I can get a raw COpaquePointer to the data with data.bytes but can't find a way to initialize a new swift array with that pointer. Does anyone know how to do it?
import Foundation
var arr : UInt32[] = [32,4,123,4,5,2];
let data = NSData(bytes: arr, length: arr.count * sizeof(UInt32))
println(data) //data looks good in the inspector
// now get it back into an array?

Answer 1

You can use the getBytes method of NSData:
// the number of elements:
let count = data.length / sizeof(UInt32)
// create array of appropriate length:
var array = [UInt32](count: count, repeatedValue: 0)
// copy bytes into array
data.getBytes(&array, length:count * sizeof(UInt32))
print(array)
// Output: [32, 4, 123, 4, 5, 2]
Update for Swift 3 (Xcode 8): Swift 3 has a new type struct Data
which is a wrapper for NS(Mutable)Data with proper value semantics.
The accessor methods are slightly different.
Array to Data:
var arr: [UInt32] = [32, 4, UInt32.max]
let data = Data(buffer: UnsafeBufferPointer(start: &arr, count: arr.count))
print(data) // <20000000 04000000 ffffffff>
Data to Array:
let arr2 = data.withUnsafeBytes {
Array(UnsafeBufferPointer<UInt32>(start: $0, count: data.count/MemoryLayout<UInt32>.stride))
}
print(arr2) // [32, 4, 4294967295]
Update for Swift 5:
Array to Data:
let arr: [UInt32] = [32, 4, UInt32.max]
let data = Data(buffer: UnsafeBufferPointer(start: arr, count: arr.count))
print(data) // <20000000 04000000 ffffffff>
Data to Array:
var arr2 = Array<UInt32>(repeating: 0, count: data.count/MemoryLayout<UInt32>.stride)
_ = arr2.withUnsafeMutableBytes { data.copyBytes(to: $0) }
print(arr2) // [32, 4, 4294967295]

Answer 2

It's also possible to do this using an UnsafeBufferPointer, which is essentially an "array pointer", as it implements the Sequence protocol:
let data = NSData(/* ... */)
// Have to cast the pointer to the right size
let pointer = UnsafePointer<UInt32>(data.bytes)
let count = data.length / 4
// Get our buffer pointer and make an array out of it
let buffer = UnsafeBufferPointer<UInt32>(start:pointer, count:count)
let array = [UInt32](buffer)
This eliminates the need for initializing an empty array with duplicated elements first, to then overwrite it, although I have no idea if it's any faster. As it uses the Sequence protocol this implies iteration rather than fast memory copy, though I don't know if it's optimized when passed a buffer pointer. Then again, I'm not sure how fast the "create an empty array with X identical elements" initializer is either.

Answer 3

Here is a generic way to do it.
import Foundation
extension Data {
func elements <T> () -> [T] {
return withUnsafeBytes {
Array(UnsafeBufferPointer<T>(start: $0, count: count/MemoryLayout<T>.size))
}
}
}
let array = [1, 2, 3]
let data = Data(buffer: UnsafeBufferPointer(start: array, count: array.count))
let array2: [Int] = data.elements()
array == array2
// IN THE PLAYGROUND, THIS SHOWS AS TRUE
You must specify the type in the array2 line. Otherwise, the compiler cannot guess.

Answer 4

If you are dealing with Data to Array (I know for sure my array is going to be [String]), I am quite happy with this:
NSKeyedUnarchiver.unarchiveObject(with: yourData)
I hope it helps