Combining plugins with chain (pipe) systems is very complex

[alice-i-cecile]

Bevy version

0.8

What you did

I wanted to make a plugin that abstracts over different ways to handle errors from a shared system (for building a wheel menu widget).

We can get a skeleton of the plugin using the following definition.

enum MyError {
   Rainy,
   Sunny,
}

fn error_producing_system() -> Result<(), MyErr> {
   Ok(())
}

pub struct MyPlugin<H, Out, Params>
where
    H: IntoSystem<Result<(), MyError>, Out, Params> + Send + Sync + 'static,
    Out: Send + Sync + 'static,
    Params: Send + Sync + 'static,
{
    handler: H,
    _phantom: PhantomData<(Out, Params)>,
}

impl<
        H: IntoSystem<Result<C, NoWheelSelection>, Out, Params> + Send + Sync + 'static,
        Out: Send + Sync + 'static,
        Params: Send + Sync + 'static,
    > Plugin for WheelMenuPlugin<C, H, Out, Params>
{
    fn build(&self, app: &mut bevy::prelude::App) {
        app.add_system(error_producing_system::<C>.chain(self.handler))
    }
}

What went wrong

The code above satisfies almost all of the trait bounds, but the ChainSystem trait is not satisfied.

The error message received is incredibly noisy, and it's unclear what trait bounds to add to fix this problem.

Full error message

the trait bound `ChainSystem<FunctionSystem<(), Result<C, NoWheelSelection>, (bevy::prelude::Query<'_, '_, &leafwing_input_manager::action_state::ActionState<PlayerAction>, bevy::prelude::With<PlayerCharacter>>, bevy::prelude::Query<'_, '_, (&WheelMenuCenter<C>, &geometry::coordinates::WindowPos, &bevy::prelude::Children)>, bevy::prelude::Query<'_, '_, &geometry::coordinates::WindowPos, bevy::prelude::With<ui::cursor::Cursor>>, bevy::prelude::Query<'_, '_, &mut WheelMenuWedge<C>>), (), for<'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k, 'l, 'm, 'n> fn(bevy::prelude::Query<'a, 'b, &'c leafwing_input_manager::action_state::ActionState<PlayerAction>, bevy::prelude::With<PlayerCharacter>>, bevy::prelude::Query<'d, 'e, (&'f WheelMenuCenter<C>, &'g geometry::coordinates::WindowPos, &'h bevy::prelude::Children)>, bevy::prelude::Query<'i, 'j, &'k geometry::coordinates::WindowPos, bevy::prelude::With<ui::cursor::Cursor>>, bevy::prelude::Query<'l, 'm, &'n mut WheelMenuWedge<C>>) -> Result<C, NoWheelSelection> {select_from_wheel_menu::<C>}>, <H as bevy::prelude::IntoSystem<Result<C, NoWheelSelection>, Out, Params>>::System>: bevy::prelude::IntoSystem<(), (), _>` is not satisfied
the following other types implement trait `IntoSystemDescriptor<Params>`:
  ConditionalSystemDescriptor
  ExclusiveSystemCoerced
  ExclusiveSystemDescriptor
  ExclusiveSystemFn<F>
  ParallelSystemDescriptor
  SystemDescriptor
  std::boxed::Box<(dyn bevy::prelude::System<Out = (), In = ()> + 'static)>
required for `ChainSystem<FunctionSystem<(), Result<C, NoWheelSelection>, (bevy::prelude::Query<'_, '_, &leafwing_input_manager::action_state::ActionState<PlayerAction>, bevy::prelude::With<PlayerCharacter>>, bevy::prelude::Query<'_, '_, (&WheelMenuCenter<C>, &geometry::coordinates::WindowPos, &bevy::prelude::Children)>, bevy::prelude::Query<'_, '_, &geometry::coordinates::WindowPos, bevy::prelude::With<ui::cursor::Cursor>>, bevy::prelude::Query<'_, '_, &mut WheelMenuWedge<C>>), (), for<'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k, 'l, 'm, 'n> fn(bevy::prelude::Query<'a, 'b, &'c leafwing_input_manager::action_state::ActionState<PlayerAction>, bevy::prelude::With<PlayerCharacter>>, bevy::prelude::Query<'d, 'e, (&'f WheelMenuCenter<C>, &'g geometry::coordinates::WindowPos, &'h bevy::prelude::Children)>, bevy::prelude::Query<'i, 'j, &'k geometry::coordinates::WindowPos, bevy::prelude::With<ui::cursor::Cursor>>, bevy::prelude::Query<'l, 'm, &'n mut WheelMenuWedge<C>>) -> Result<C, NoWheelSelection> {select_from_wheel_menu::<C>}>, <H as bevy::prelude::IntoSystem<Result<C, NoWheelSelection>, Out, Params>>::System>` to implement `IntoSystemDescriptor<_>`

[mockersf]

you have two issues:

• your handler type has an Out type parameter, it should be () to be able to finish a chain
• your handler is owned by the plugin and won't be able to be given to the chain

making H Clone and removing Out works for me 👍

[alice-i-cecile]

Right! Let me see if I can get this working in practice...

[mockersf]

making H Clone works to make it compile, in practice I would probably use something that allows interior mutability on the handler to be able to take ownership of it when building the plugin

[alice-i-cecile]

Alright, so this is compiling in isolation, but I'm still at a loss for how to actually use it in practice. We can remove both In and Out, as they're known, and pass in the handler by value to avoid the need to clone.

    pub struct WheelPlugin<C, H, Params>
    where
        C: Choice + LoadAssets,
        H: IntoSystem<Result<C, NoWheelSelection>, (), Params> + Send + Sync + 'static,
        Params: Send + Sync + 'static,
    {
        handler: H,
        _phantom: PhantomData<(C, H, Params)>,
    }

    impl<C, H, Params> WheelPlugin<C, H, Params>
    where
        C: Choice + LoadAssets,
        H: IntoSystem<Result<C, NoWheelSelection>, (), Params> + Send + Sync + 'static,
        Params: Send + Sync + 'static,
    {
        pub const fn new(handler: H) -> Self {
            WheelPlugin {
                handler,
                _phantom: PhantomData,
            }
        }
    }

This then leads us with a question of how to actually call this.

        app.add_plugin(WheelPlugin::<Instruction, H, Params>::new(
            handle_selected_instruction,
        ))

This requires us to fill in three generics: the first is obvious, but the last two are quite mysterious. I'm going to give up here (as this won't be reasonable to ask anyone to use, even if I do get it working), but I'll leave my notes up here in case some poor soul in the future finds them helpful.