Open Radar

 
Summary:
I just discovered NSFormattingContextDynamic today and was blown away by this feature. I started digging as I would love to see this working for custom formatters as well. I guess all it needs is a public API for the proxy that the system formatters return. A minimal version could even just make `_dynamicContextEvaluation:patternString:` publicly available. 

If that would be publicly available, this would bring a huge benefit for custom formatters.

Notes:
From playing around a bit with it, I guess the only pitfall with `_dynamicContextEvaluation:patternString:` is that currently `context` contains `pattern` multiple times when there are multiple placeholders in a string so it’s important to check for the first occurrence of `pattern` inside `context` when trying to determine the correct position. This could either be documented or changed by making `pattern` unique, e.g. by just adding a number at the end that increments. (So make it `%@NSCONTEXT1`, `%@NSCONTEXT2`,…)