Add information about old theme to 'theme_switched' action

[evansolomon]

Currently the 'theme_switch' action passes data about the theme you're switching to, but not the theme you're switching from. Adding it there would make it easier to juggle functionality that needs to know about new and old themes. Right now some of this happens by ​setting an option ('theme_switched') and ​checking for it on ​init, then deleting the option. That could be simplified by just passing the old theme's data in theme_switched.

There's an added benefit that we avoid checking for an option on every single request that usually won't be there.

[rmccue]

Looking at the patch, it looks like check_theme_switched() needs to be reindented?

[SergeyBiryukov]

Related: #15648, #19682

[slobodanmanic]

Fixes check_theme_switched() function indentation.

[obenland]

I anticipate back compat issue if we moved after_switch_theme to the same page load as switch_theme since we're changing the context in which the action is fired. Default themes checking for the required WP version couldn't self deactivate anymore for example.

No objections to adding old theme info to switch_theme though.

[lancewillett]

+1 Adding the old theme info would be helpful. An example use case is logging when people switch themes on a multisite environment — seeing patterns from old to new.

[DrewAPicture]

22401.diff​ needs a changelog entry for the new parameter.

[DrewAPicture]

In 36502:

Themes: Pass information about the old theme in the form of a WP_Theme object when the switch_theme action is fired.

Props MikeHansenMe.
See #22401.

[jorbin]

If there is remaining work that wasn't noted, please reopen (or create new tickets)