Improve Documentation about borrow types

[patmuk]

I am not totally sure, but I assume that a struct with borrowed fields is auto opaque?

If so, can this be stated better in the documentation, for others?

My use case is having a viewModel, but defined in Rust. Thus it needs to be read by flutter, so I can't use an opaque type. But being a viewModel, I want to use borrowed values from the Model ... so the only option left is cloning the model's fields on each access ... or implementing the view logic in Flutter. So either the performance is suboptimal, or the logic might be repeated for different UIs (Flutter, cli, ...).

For example, a model like

struct MyModel {
    items : Vec<Field>
}

could have the viewModel

struct MyViewModel {
    my_items : Vec<&Field>
    not_my_items : Vec<&Field>
    total_item_count : u64
}

This is all very theoretical - as the performance hit might be not noticeable and I might never have a second UI, as Flutter covers all platforms.
But back to my request: It would be good to document this more clear if I understood it correctly.

[fzyzcjy]

a struct with borrowed fields is auto opaque?

By default IMHO; try to add #[frb(non_opaque)] and see whether it works.

IIRC this was originally non-opaque, but later it gives problems on some scenarios and thus the default is flipped and it looks like this...

[patmuk]

Thanks! But it doesn't seem to work:
The lifetime parameter is missing in the generated code:

// Codec=Dco (DartCObject based), see doc to use other codecs
            impl flutter_rust_bridge::IntoDart for crate::domain::share_cycle::ShareCycleViewModel {
                fn into_dart(self) -> flutter_rust_bridge::for_generated::DartAbi {
                    [
                    self.wanted_items.into_into_dart().into_dart(),
self.to_share_tems.into_into_dart().into_dart(),
self.completely_shared_items.into_into_dart().into_dart()
                ].into_dart()
                }
            }
            impl flutter_rust_bridge::for_generated::IntoDartExceptPrimitive for crate::domain::share_cycle::ShareCycleViewModel {}
impl flutter_rust_bridge::IntoIntoDart<crate::domain::share_cycle::ShareCycleViewModel> for crate::domain::share_cycle::ShareCycleViewModel {
            fn into_into_dart(self) -> crate::domain::share_cycle::ShareCycleViewModel {
                self
            }
        }

my struct is

#[flutter_rust_bridge::frb(non_opaque)]
#[derive(Debug, PartialEq, Eq)]
pub struct ShareCycleViewModel<'a> {
    pub wanted_items: Vec<&'a MediaItem>,
    pub to_share_tems: Vec<&'a MediaItem>,
    pub completely_shared_items: Vec<&'a MediaItem>,
}

Probably this was where the problem with non-opaque borrowed fields started?

[fzyzcjy]

Hmm, it looks like borrowed opaque types are not supported (at least yet). To work on it, maybe we can first discuss - what code should it generate? Unsafe code can definitely handle it, but I would like to have as little unsafe code as possible, surely ;)

[patmuk]

Yes, I agree - I would not like unsafe code as well.
Honestly, I think it might not be worth the effort. Probably modifying a Model to be shown in a view is part of the view, not the core ... even if different views have to do the same.
Or more general: If something in rust should be used by flutter (and not just passed through), it should be owned. Technically it might work - but I have the feeling that along the way one gets into trouble sooner or later.

[fzyzcjy]

Rethinking this, I guess another way may be:

struct MyModel {
    items : Vec<Arc<Field>>
}

struct MyViewModel {
    my_items : Vec<Arc<Field>>,
    not_my_items : Vec<Arc<Field>>,
    total_item_count : u64
}

i.e. use Arc<..> (or possibly Arc<Mutex<..>> etc) to make Field shared by MyModel and MyViewModel, instead of borrowed.

[patmuk]

@fzyzcjy thanks, that is a great idea! I currently implemented it with each (complex) item having a distinct ID, and sharing a lookup table of these between dart and rust. But an Arc sounds like a better, more direct approach. Will try that :)