PMKVObserver

PMKVObserver provides a safe block-based wrapper around Key-Value Observing, with APIs for both Obj-C and Swift. 

Features include:

• Thread-safety. Observers can be registered on a different thread than KVO notifications are sent on, and can be cancelled on yet another thread. An observer can even be cancelled from two threads simultaneously.
• Automatic unregistering when the observed object deallocates.
• Note: nested key paths are not supported at this time and manual cancellation should be used; more info here
• Support for providing an observing object that is given to the block, and automatic unregistering when this observing object deallocates. This lets you call methods on self without retaining it or dealing with a weak reference.
• Thread-safety for the automatic deallocation. This protects against receiving messages on another thread while the object is deallocating.
• First-class support for both Obj-C and Swift, including strong typing in the Swift API.