[Feature Request] Get last dirty state of unregistered field (when destroyOnUnregister is false)

[jedwards1211]

Are you submitting a bug report or a feature request?

Feature request

What is the current behavior?

If I register a field foo, it becomes dirty, and then I unregister it, I can't get the last dirty state. form.getFieldState('foo') returns null.

What is the expected behavior?

I didn't set destroyOnUnregister and the default value is false so I kind of expected the field state to be preserved since the field value is. It would be nice to at least have an option to preserve the last field state, since I'll have to hack together some way to do it manually.

What's your environment?

Final Form 4.20.4

Other information

I'm using Final Form for cell values in an infinite scrolling table, and I want to only submit rows that were changed, so I need to be able to know what unmounted rows were changed.

[gertdreyer]

Related to #474

[gertdreyer]

I suspect all meta values become undefined when an input field no longer exists thus the context is lost. destroyOnUnregister only refers to the field value if you read the docs. Therefore the actual value for the field in the values object is destroyed.