set associated objects for literal value in Swift - ios

Question

Well, this possibly a duplicated question. I've found some questions like this one:
Is there a way to set associated objects in Swift?
However, I want to add an Int property into swift's extension and these answers in the link above doesn't work.
Here's my code:
import ObjectiveC
var xoAssociationKey: UInt8 = 0
extension NSData {
var position: Int {
get {
return objc_getAssociatedObject(self, &xoAssociationKey) as Int
}
set {
objc_setAssociatedObject(self, &xoAssociationKey, newValue, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN_NONATOMIC))
}
}
override convenience init() {
self.init()
position = 0
}
}
And I get fatal error: unexpectedly found nil while unwrapping an Optional value everytime I access position
FYI, I did find a solution for this error in Objective C and I'm looking for a swift solution. Here's my code in objective C if you interested:
static char PROPERTY_KEY;
#implementation NSData (Extension)
#dynamic position;
- (NSInteger)position {
return [objc_getAssociatedObject(self, &PROPERTY_KEY) integerValue];
}
- (void)setPosition:(NSInteger)position {
// Must convert to an object for this trick to work
objc_setAssociatedObject(self, &PROPERTY_KEY, #(position), OBJC_ASSOCIATION_COPY);
}
- (instancetype)init {
self = [super init];
if (self) {
self.position = 0;
}
return self;
}

Answer 1

NSData is part of a class cluster, so your custom init method is not necessarily called,
e.g.
let d = NSMutableData()
does not use your init method. The next problem is that your init method calls
itself recursively, therefore
let d = NSData()
crashes with a stack overflow. Note also that the Objective-C code relies on
undefined behaviour, because it replaces a method in a class extension.
So better remove your custom initialization, and change the getter to
return a default value if the associated object has not been set.
This can easily be achieved with an optional cast (as? Int) and the
nil-coalescing operator (??):
extension NSData {
var position: Int {
get {
return objc_getAssociatedObject(self, &xoAssociationKey) as? Int ?? 0
}
set {
objc_setAssociatedObject(self, &xoAssociationKey, newValue, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN_NONATOMIC))
}
}
}