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World

book/src/concepts/world.md

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World

What is a World?

A World is a container for entities, with some helper functions that make your life easier. This chapter will showcase those functions and their usage. Along with the World there is a concept of Resources which are singletons available across all systems. See Resources for more about them.

Creating entities

rust
# extern crate amethyst;
# use amethyst::ecs::{World};

#[derive(Default)]
struct MyComponent {
    value: u8,
};

# fn main() {
   let mut world = World::default();
    world.push((MyComponent::default(),));
# }

Accessing a Component

rust
# extern crate amethyst;
# use amethyst::ecs::{World};
# #[derive(Default)]
# struct MyComponent {
#     value: u8,
# };
# fn main() {
   let mut world = World::default();
    // Create an `Entity` with `MyComponent`.
    // `World` will implicitly write to the component's storage in `Resources`.
    let my_entity =     world.push((MyComponent::default(),));

    if let Some(entry) = world.entry(my_entity) {
        let my_component = entry.get_component::<MyComponent>().unwrap();
    }
# }

Modifying a Component

This is almost the same as accessing a component:

rust
# extern crate amethyst;
# use amethyst::ecs::{ World};
# #[derive(Default)]
# struct MyComponent {
#     value: u8,
# };
# fn main() {
  let mut world = World::default();
    let my_entity =     world.push((MyComponent::default(),));
    if let Some(mut entry) = world.entry(my_entity) {
        let mut my_component = entry.get_component_mut::<MyComponent>().unwrap();
        my_component.value = 5;
    }
# }

Delete an entity

Single:

rust
# extern crate amethyst;
# use amethyst::ecs::World;
# fn main() {
    let mut world = World::default();
    let my_entity = world.push((MyComponent,));
    assert!(world.remove(my_entity));
# }

All:

rust
# extern crate amethyst;
# use amethyst::ecs::World;
# fn main() {
#   let mut world = World::default();
    world.clear();
# }

Note: Entities are lazily deleted, which means that deletion only happens at the end of the frame and not immediately when calling the delete method.

Check if the entity was deleted

rust
# extern crate amethyst;
# use amethyst::ecs::World;
# fn main() {
#   let mut world = World::default();
    let my_entity = world.push((0usize,));
    assert!(world.contains(my_entity));
    assert!(!world.contains(Entity(100)))
# }