Safer KVO by referencing the Mike Ash article
Safer KVO by referencing the Mike Ash article

https://www.mikeash.com/pyblog/key-value-observing-done-right.html
mxcl/PromiseKit,mxcl/PromiseKit,allen-zeng/PromiseKit,allen-zeng/PromiseKit,mxcl/PromiseKit