Redesign druid::widget::ListIter trait?

[changhe3]

Currently this trait is not very efficient when you want to implement ListIter<(K, V)> on OrdMap<K, V> or HashMap<K, V>, as you have to clone the keys and values each time you call the callback cb.

[changhe3]

I did find this implementation which only implements ListIter<V>, thus not requiring you to clone the value, as a workaround though.

[arthmis]

What was the implementation you found?

[changhe3]

@arthmis Here: https://github.com/linebender/druid/pull/1641/files#diff-0bb03eb637d659cef6bf00945fc260762de7365f068a308ee92a633fc2cc4f0bR122

[cmyr]

Anything that is part of your Data should be cheap to clone.

I do agree that these types should be extensively reworked or replaced, but I don't think that the cloning, on its own, would be sufficient rationale. :)

[ratmice]

Currently this trait is not very efficient when you want to implement ListIter<(K, V)> on OrdMap<K, V> or HashMap<K, V>,

I'm not sure how to link directly to PR comments, but it's worth considering why it doesn't already implement ListIter<(K, V)>, It seems there was an implementation in #1641 which was removed before merge.
The comments there are enlightening, particularly in regards to ListIter mutating keys, which then causes reordering

In the comments on druid/src/widget/list.rs
#1641 (comment)

[rbtcollins]

A closely related thing is this choice: https://github.com/linebender/druid/blob/c02452ddeebc527992e8f112f434f23ce24c934d/druid/src/widget/list.rs#LL129C3-L129C33 - the projection through OrdMap<K,V> only passes V to the interior widget.

e.g.

for some OrdMap<String,String>

 LensWrap::new(
                List::new(|| {
                    Label::dynamic(|data), _| {
                        format!("List item: {:?}", data)
                    })
                }),
                lens_pointing_at_the_collection,
            ),

Now the reason this is a nuisance is that things like toml files that deserialise collections won't include the collection K inside the V, forcing postprocessing after serde to put the data into a form accessible to druid.