First class save / load support

[alice-i-cecile]

What problem does this solve or what need does it fill?

Saving and loading game state is a core functionality that will be reused across almost every game made in Bevy, but implementing this functionality requires advanced Bevy knowledge and there isn't consensus on how you might effectively accomplish it.

What solution would you like?

A pair of Commands to save and load a game state, probably in combination with some Resources to set global config of this functionality.

@TheRawMeatball whipped up the following prototype:

struct SaveCommand;
struct LastSaveData(Option<SomeSerializedDataType>)
trait SaveCommandExt {
  fn save(&mut self);
}

impl Command for SaveCommand {
  fn write(self: Box<Self>, world: &mut World, resources: &mut Resources){
    let data = /*serialize stuff*/;
    resources.get_mut::<LastSaveData>().unwrap().0 = Some(data);
  }  
}

impl SaveCommandExt for Commands {
  fn save(&mut self) {
    self.add_command(SaveCommand)
  } 
}

Usage:

fn regular_system(commands: &mut Commands) {
  commands.save();
}

What alternative(s) have you considered?

Clearly document an idiomatic way to do this.

Additional context

Here's an example of a new user trying to implement this on their own and stumbling.

This issue relates directly to #255.

[bjorn3]

A big thing to be taken into consideration is that not everything should be saved or can even be saved. In addition how should changes to the game be handled that for example add or remove ui elements. Loading a game should use the new set of ui elements. Not the saved set. This issue has overlap with how to handle scene saving in the editor.

[ypoonse]

Chiming in on what @bjorn3 mentions here (and in the hope it can be useful for someone else). I'm implementing load/save, also taking inspiration from #166 .

I'm using the TypeRegistry to register which components I want to be saved, and before saving, I filter out the empty entities:

pub fn drop_empty(scene: DynamicScene) -> DynamicScene {
    let new_entities = scene.entities
        .into_iter()
        .filter(|entity| {
            entity.components.len() > 0
        });
    
    DynamicScene {
        entities: new_entities.collect()
    }
}

And here's the system saving:

fn save_scene(world: &mut World) {
    // let type_registry = world.get_resource::<TypeRegistryArc>().unwrap();

    let type_registry = TypeRegistry::default();
    type_registry.write().register::<Health>();
    type_registry.write().register::<Player>();
    type_registry.write().register::<Enemy>();


    let scene = DynamicScene::from_world(&world, &type_registry);
    let scene = helpers::drop_empty(scene);

    let mut file = File::create("assets/scenes/start_scene.scn.ron").unwrap();
    file.write_all(
        scene
            .serialize_ron(&type_registry)
            .unwrap()
            .as_bytes(),
    )
    .unwrap();
}

[chamons]

I ran into this, and instead of using bevy_reflect I write "hard coded" save logic to walk all entities/resources and serialize them.

As someone who had to ask around on discord for help on getting started, I figured an example might be useful for other people who came across this issue - https://gist.github.com/chamons/37e8c6f8753e63eaef08bef36686c2e2

In my use case, I use bevy_ecs but not bevy proper, so dynamic scenes were not an option.

[alice-i-cecile]

A nice third party library experimenting in this space: https://github.com/hankjordan/bevy_save

[pinkponk]

Have to do the same in my game and also found this lib: https://github.com/Zeenobit/bevy_atomic_save

Haven't decided yet how I will go about myself. I guess the options today are:

1. Doing it like @chamons and looping over and saving everything yourself
2. use Dynamic scenes yourself (Havent fully understood this yet)
3. use third party bevy_save or bevy_atomic_save