System Inputs, Outputs, Chaining, and Registration Ergo

[cart]

Time for another round of ECS improvements!

• System Registration Ergonomics: add_system() now takes a generic IntoSystem type instead of a boxed system

  // old
  App::build()
    .add_system(my_system.system())
    .add_system(my_thread_local_system.thread_local_system())
    .run()
  
  // new
  App::build()
    .add_system(my_system)
    .add_system(my_thread_local_system)
     // you can do this now, which is pretty cool
    .add_system(|| println!("hello world"))
    .run()

• System Inputs and Outputs: The System trait now has Input and Output associated types. System<Input = X, Output = Y>

  fn count_materials_system(
    In(count_materials): In<bool>,
    materials: Res<Assets<ColorMaterial>>) -> Option<usize> {
    if count_materials {
      Some(materials.len())
    } else {
       None
    }
  }
  
  fn system(world: &mut World, resources: &mut Resources) {
    let system = count_materials_system.system();
    if let Some(material_count) = system.run(true, world, resources) {
      println!("there are {} materials", material_count);
    } 
  }

• System Chaining. This allows us to make "result systems" with error handlers (among other things)

  fn main() {
    App::build()
      .add_plugins(DefaultPlugins)
      .add_system(result_system.chain(error_handler))
      .run();
  }
  
  fn result_system(query: Query<&Transform>) -> Result<()> {
    let transform = query.get(SOME_ENTITY)?;
    println!("found entity transform: {:?}", transform);
    Ok(())
  }
  
  fn error_handler(In(result): In<Result<()>>) {
    if let Err(err) = result {
        println!("encountered an error {:?}", err);
    }
  }

Note: only System<Input = (), Output =()> systems can be added to a schedule. Schedules don't know how to populate inputs, nor do they know what to do with outputs.

[Plecra]

A test for the inputs and outputs should be added to the examples for the normal reasons. They're unique enough that illustrating the feature is important.

[bjorn3]

The message is already not an integer. It is "hello".

[cart]

Oops! I changed it to hello to test the error case, then forgot to change it back. Thanks!

[cart]

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