Migrating from 0.19 to 0.19

Observer overhaul

TransitionListener has been deleted, use observers instead:

> app.add_observer(
>     |trigger: On<LoadingCompleted>| println!("do stuff"),
> );
< active_machines.push_listener::<ReloadingInitialized<P>>(OnLoadingCompleteListener);

This allows you to use more convenient system parameter syntax to access resources and queries, but you may not use exclusive systems anymore.

If you want observers to respect ordering between themselves, you should prefer to issue commands in these instead (see related bevy issue).