How to pass const void *bytes to an Objective-C block from Swift - objective-c

Question

I'm writing some Swift (1.2) code that calls an Objective-C library (transcribing some earlier Objective-C code). It's not clear how it's supposed to work. I've tried a bunch of variations, and nothing satisfies the compiler. Here's the Objective-C code I started with:
// Objective-C
NSData *fileData = //
const void *bytes = fileData.bytes;
unsigned int bufferSize = 1024; //Arbitrary
[object writeDataWithBlock:
^BOOL(BOOL(^writeData)(const void *bytes, unsigned int length), NSError**(actionError)) {
for (NSUInteger offset = 0; offset <= fileData.length; offset += bufferSize) {
unsigned int size = (unsigned int)MIN(fileData.length - i, bufferSize);
writeData(&bytes[i], size);
}
return YES;
}];
So far, I've gotten as far as this Swift code:
// Swift
let fileData = //
let ptr = UnsafePointer<UInt8>(fileData.bytes)
let bufferSize = 1024 // Arbitrary
object.writeDataWithBlock() { (writeData, actionError) -> Bool in
for var offset = 0; offset <= fileData.length; offset += bufferSize {
let size = CUnsignedInt(min(fileData.length - offset, bufferSize))
writeData(UnsafePointer<Void>(ptr[offset]), size)
}
return true
}
That's giving me this error:
Cannot find an initializer for type 'UnsafePointer' that accepts an arguments list of type '(UInt8)'
When I remove the UnsafePointer<Void> conversion to do a more direct translation, yielding this line:
writeData(&ptr[offset], size)
I get this error, pointing at the & character:
Type of expression is ambiguous without more context
Without the &, it yields a UInt8, giving me this error:
Cannot invoke 'writeData' with an argument list of type '(Uint8, UInt32)'
What do I need to do to read the bytes out of the NSData sequentially and pass them onto another method?

Answer 1

Pointer arithmetic should (and does) get you the results you're looking for.
let fileData = //
let bufferSize = 1024 // Arbitrary
object.writeDataWithBlock() { (writeData, actionError) -> Bool in
for var offset = 0; offset <= fileData.length; offset += bufferSize {
let size = CUnsignedInt(min(fileData.length - offset, bufferSize))
writeData(fileData.bytes + offset, size)
}
return true
}