Ui::dnd_drop_zone() swallows the inner Response

[sowbug]

The signature is

pub fn dnd_drop_zone<Payload>(
        &mut self,
        frame: Frame,
        add_contents: impl FnOnce(&mut Ui),
    ) -> (Response, Option<Arc<Payload>>)
    where
        Payload: Any + Send + Sync;

Note that add_contents: impl FnOnce(&mut Ui) doesn't return a result. It's thus unclear how we're supposed to know what happened to the contents.

I can do this:

let (inner_response, response, payload) = {
    let mut inner_response = None;
    let (response, payload) =
        ui.dnd_drop_zone::<Payload>(Frame::default(), |ui| {
            inner_response =
                Some(ui.add(Slider::new(&mut value, Normal::range()).text("Label")));
        });
    (inner_response, response, payload)
};
if let Some(inner_response) = inner_response {
    if inner_response.changed() {
      // do something
    }
}

... but that's quite a bit more cumbersome than something like this:

let (response, payload) = {
    ui.dnd_drop_zone::<Payload>(Frame::default(), |ui| {
        ui.add(Slider::new(&mut value, Normal::range()).text("Label"))
    })
};
if let Some(response) = response.inner_response {
    if response.changed() {
        // do_something
    }
}

(That hypothetical code assumes that dnd_drop_zone uses something like an InnerResponse rather than a plain old Response.)

I hope there isn't anything obvious I'm missing here, but I looked at the dnd_drop_zone source, and it looks like there was no provision for add_contents's response to be preserved:

let mut frame = frame.begin(self);
add_contents(&mut frame.content_ui);
let response = frame.allocate_space(self);

[emilk]

It should be easy enough to change the signature of dnd_drop_zone to:

pub fn dnd_drop_zone<Payload, R>(
        &mut self,
        frame: Frame,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> (InnerResponse<R>, Option<Arc<Payload>>)

if someone wants to take a stab at it

[sowbug]

I'll give it a look today.