Using an FnMut in a button closure?

[ezntek]

I need to have a button that mutates some data, under the context of a state manager. I wrote a simple state manager that works kind of like a state machine with changes in state happening through navigating the UI, and some functions related to mutating the data requires a mutable reference to self, which requires a mutable variable and a mutable reference, which cannot be obtained inside a plain Fn. Is there a way around this:

Note: I did try wrapping it in an Rc of Cell, it did work until I encountered closure may outlive the current function, but it borrows \state manager variable name`, which is owned by the current function.Usingmove would work, but one would need to make a different clone of theRc` per closure, which is kind of clunky.

[gyscos]

Hi, and thanks for the question!

Indeed, a button currently stores a Fn, because in some cases the callback could be called recursively (for example if the callback trigger an event that results in the same callback), and FnMut cannot support that.

If you don't care about the recursive situation, you can wrap a FnMut into a Fn through a Rc and Cell as you mentioned (or more precisely, a Mutex now that cursive moved to be Sync). I added a macro to help do it for you: cursive::immut1!. If you're curious you can see how it's implemented, but it's really just using a Mutex to get mutable access to the inner closure, skipping if it's already locked (recursive call).