Singleton Custom Field (Meta Box)

Question

How do I create a custom meta box for posts which have a boolean data type and can be true only for one post. I want it to be falsified for all other posts when it's set on one post.

1. Is there a hook that runs right after a custom field is saved in the create post screen?
2. Is there a way to do this with the metabox plugin?
3. For example if you want to create a featured post feature, only one post can be a featured post. Is there a way to create a single featured post without using custom meta boxes?

Answer 1

Storing of your own meta fields is typically a custom process, more so if you are using a third party framework.

Technically it's possible, but clunky:

1. Hooking into save_post would allow you to check if meta is added to it
2. If so you can immediately query for previous (if any) post that had it and unset the meta for it

However I concur with the comment you got that this does not essentially seem to fit a purpose of post meta well. There are precedents in core itself (for example front/posts page handling) to store such things as post's ID in an option.