How to hook into unregistering a widget instance?

Question

I would like to hook into a widget instance after it is removed from a sidebar panel on the Appearances -> Widgets admin page.

So if the instance was an active widget and then removed, I would like to hook into that instance before it is removed.

Can it be done?

Answer 1

Yes, this can be done. Ultimately, adding and removing a widget to/from a sidebar means updating an option in the database with a call like this:

update_option( 'sidebars_widgets', array( ... ) )

If you look at the code of update_option you see that there is a filter called just before the actual update. So if you want to do something before a widget is removed you can hook into:

add_filter ('pre_update_option_sidebars_widgets','wpse71075_pre_remove_widget',10,3);

At this point you only know that the list of active widgets is going to change, not whether it is addition or a removal. However, you do have $value and $old_value of the option, which you can compare inside your filter function (an array_search for a widget's label might be enough).

Read the answer to this question to learn about the structure of the sidebars_widgets option. It depends on your goal what you actually want to detect and do.