Adding observer for KVO without pointers using Swift

Question

In Objective-C, I would normally use something like this:

static NSString *kViewTransformChanged = @"view transform changed";
// or
static const void *kViewTransformChanged = &kViewTransformChanged;

[clearContentView addObserver:self
                       forKeyPath:@"transform"
                          options:NSKeyValueObservingOptionNew
                          context:&kViewTransformChanged];

I have two overloaded methods to choose from to add an observer for KVO with the only difference being the context argument:

 clearContentView.addObserver(observer: NSObject?, forKeyPath: String?, options: NSKeyValueObservingOptions, context: CMutableVoidPointer)
 clearContentView.addObserver(observer: NSObject?, forKeyPath: String?, options: NSKeyValueObservingOptions, kvoContext: KVOContext)

With Swift not using pointers, I'm not sure how to dereference a pointer to use the first method.

If I create my own KVOContext constant for use with the second method, I wind up with it asking for this:

let test:KVOContext = KVOContext.fromVoidContext(context: CMutableVoidPointer)

EDIT: What is the difference between CMutableVoidPointer and KVOContext? Can someone give me an example how how to use them both and when I would use one over the other?

EDIT #2: A dev at Apple just posted this to the forums: KVOContext is going away; using a global reference as your context is the way to go right now.

Answer 1

There is now a technique officially recommended in the documentation, which is to create a private mutable variable and use its address as the context.

(Updated for Swift 3 on 2017-01-09)

// Set up non-zero-sized storage. We don't intend to mutate this variable,
// but it needs to be `var` so we can pass its address in as UnsafeMutablePointer.
private static var myContext = 0
// NOTE: `static` is not necessary if you want it to be a global variable

observee.addObserver(self, forKeyPath: …, options: [], context: &MyClass.myContext)

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey: Any]?, context: UnsafeMutableRawPointer?) {
    if context == &myContext {
        …
    }
    else {
        super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
    }
}