Game Logic Integration

Loading LDtk levels into Bevy doesn't get you very far if you cannot play them.

Aside from rendering tilemaps, LDtk has features for placing gameplay objects on Entity layers. Even within tilemaps, IntGrid layers imply a categorization of tiles, and perhaps a game designerly meaning. It is fundamental to associate the LDtk entities and IntGrid tiles with Bevy entities/components. bevy_ecs_ldtk is designed around a couple core strategies for doing so, which will be discussed here.

LdtkEntity and LdtkIntCell registration

The LdtkEntity/LdtkIntCell registration API allows you to hook custom bevy Bundles into the level spawning process. You define what components you want on the entity with a bundle, define how they should be constructed with the LdtkEntity or LdtkIntCell derive, and register the bundle to the App for a given LDtk entity identifier, or IntGrid value.

use bevy::prelude::*;
use bevy_ecs_ldtk::prelude::*;

fn main() {
    App::new()
        // other App builders
        .register_ldtk_entity::<PlayerBundle>("Player")
        .run();
}

#[derive(Default, Component)]
struct Player;

#[derive(Default, Bundle, LdtkEntity)]
struct PlayerBundle {
    player: Player,
    #[sprite_bundle]
    sprite_bundle: SpriteBundle,
}

How does LdtkEntity/LdtkIntCell construct the bundle when derived? Without any intervention, the bundle's fields are constructed using the bundle's Default implementation. However, various attributes are available to override this behavior, like #[sprite_bundle] in the above example. This attribute gives the entity a sprite using the tileset in its LDtk editor visual. For documentation about all the available attributes, check out the API reference for these traits:

• LdtkEntity
• LdtkIntCell

This approach is suitable for many common, simple use cases. There's also room for more granular, component-level customization within some of the attributes, like #[with(...)] or #[from_entity_instance]. Of course, the traits can also be manually implemented for the even-more-custom cases.

Post-processing plugin-spawned entities

There are still many cases where LdtkEntity/LdtkIntCell registration is insufficient. Perhaps you need to spawn children of the entity, or need access to more resources in the World. For these more demanding cases, post-processing plugin-spawned entities in a custom system is always an option.

If an LDtk entity does not have a matching LdtkEntity registration, it will be spawned with an EntityInstance component by default. This component contains the raw LDtk data for that entity. Querying for newly-spawned EntityInstance entities can be a good starting point for implementing your own custom spawning logic. Intgrid tiles have similar behavior, except their default component is IntGridCell, which simply contains the IntGrid value for that tile.

#![allow(unused)]
fn main() {
use bevy::prelude::*;
use bevy_ecs_ldtk::prelude::*;
#[derive(Default, Component)]
struct PlayerChild;

#[derive(Default, Component)]
struct Player;

fn process_player(
    mut commands: Commands,
    new_entity_instances: Query<(Entity, &EntityInstance, &Transform), Added<EntityInstance>>,
    assets: Res<AssetServer>,
)
{
    for (entity, entity_instance, transform) in new_entity_instances.iter() {
        if entity_instance.identifier == "Player".to_string() {
            commands
                .entity(entity)
                .insert(Player)
                .insert(SpriteBundle {
                    texture: assets.load("player.png"),
                    transform: *transform,
                    ..default()
                })
                .with_children(|commands| {
                    commands.spawn(PlayerChild);
                });
        }
    }
}
}

This approach makes spawning entities from LDtk just as powerful and customizable as a Bevy system, because that's all it is. LdtkEntity and LdtkIntCell ultimately make some assumptions about what data from the LDtk asset and the Bevy world you will need to spawn your entity, which post-processing avoids. However, there are some pretty obvious ergonomics issues to this strategy compared to using registration:

• You need to manually filter EntityInstances for the desired LDtk entity identifier.
• You need to manually perform the iteration of the query.
• You may need to manually find the associated layer data, or tileset image, or tileset definition (if necessary).
• You need to be careful not to overwrite the plugin-provided Transform component.

A combined approach - the blueprint pattern

At least one of these ergonomics issues can be alleviated with a combined approach. If you register an LdtkEntity/LdtkIntCell with a marker component, querying for it later won't require filtering for a particular entity instance identifier. The plugin does that for you when giving the entity your bundle, then you can write queries that filter for the marker component instead of EntityInstance or IntGridCell. Furthermore, if you can add the transform-overwriting bundles within the LdtkEntity bundle, you won't need to tiptoe around the Transform in your post-processing system.

use bevy::prelude::*;
use bevy_ecs_ldtk::prelude::*;
fn main() {
    App::new()
        // other App builders
        .register_ldtk_entity::<PlayerBundle>("Player")
        .add_systems(Update, process_player)
        .run();
}

#[derive(Default, Component)]
struct PlayerChild;

#[derive(Default, Component)]
struct Player;

#[derive(Default, Bundle, LdtkEntity)]
struct PlayerBundle {
    player: Player,
    #[sprite_bundle]
    sprite_bundle: SpriteBundle,
}

fn process_player(
    mut commands: Commands,
    new_players: Query<Entity, Added<Player>>,
)
{
    for player_entity in new_players.iter() {
        commands
            .spawn(PlayerChild)
            .set_parent(player_entity);
    }
}

Using a simple component or a marker component for the initial spawn of an entity and processing it further in another system is called the "blueprint pattern". You may find it desirable to use the LdtkEntity/LdtkIntCell derives to construct most of the components, but need post-processing for the more demanding ones. This approach is recommended over filtering for Added<EntityInstance> or Added<IntGridCell>.