Here are 2 common pitfalls to avoid when you’re trying to use Swift 4 Key-Value Observation for the first time:
Keep that Observation object!
Calling YourObject.observe(_:options:changeHandler:) returns a NSKeyValueObservation object. The observation will only continue for as long as the observation object exists! If you fail to store the observation object in a persistent variable or array, then the it will immediately deallocate and no observations will occur.
Always Specify Options!
The 2nd parameter of the observe() method has a default value which is unhelpfully just called default in the quick documentation. It is the equivalent of providing an empty option set, which means your change handler closure will be invoked when the value changes, but you will not get any information about the new or old value! Even if this is the behavior you want, it is better to explicitly specify an empty option set so that someone else looking at your code immediately knows not to expect a value in the change handler.