//! Resources are unique, singleton-like data types that can be accessed from systems and stored in the [`World`](crate::world::World).

// The derive macro for the `Resource` trait
pub use bevy_ecs_macros::Resource;

/// A type that can be inserted into a [`World`] as a singleton.
///
/// You can access resource data in systems using the [`Res`] and [`ResMut`] system parameters
///
/// Only one resource of each type can be stored in a [`World`] at any given time.
///
/// # Examples
///
/// ```
/// # let mut world = World::default();
/// # let mut schedule = Schedule::default();
/// # use bevy_ecs::prelude::*;
/// #[derive(Resource)]
/// struct MyResource { value: u32 }
///
/// world.insert_resource(MyResource { value: 42 });
///
/// fn read_resource_system(resource: Res<MyResource>) {
///     assert_eq!(resource.value, 42);
/// }
///
/// fn write_resource_system(mut resource: ResMut<MyResource>) {
///     assert_eq!(resource.value, 42);
///     resource.value = 0;
///     assert_eq!(resource.value, 0);
/// }
/// # schedule.add_systems((read_resource_system, write_resource_system).chain());
/// # schedule.run(&mut world);
/// ```
///
/// # `!Sync` Resources
/// A `!Sync` type cannot implement `Resource`. However, it is possible to wrap a `Send` but not `Sync`
/// type in [`SyncCell`] or the currently unstable [`Exclusive`] to make it `Sync`. This forces only
/// having mutable access (`&mut T` only, never `&T`), but makes it safe to reference across multiple
/// threads.
///
/// This will fail to compile since `RefCell` is `!Sync`.
/// ```compile_fail
/// # use std::cell::RefCell;
/// # use bevy_ecs::resource::Resource;
///
/// #[derive(Resource)]
/// struct NotSync {
///    counter: RefCell<usize>,
/// }
/// ```
///
/// This will compile since the `RefCell` is wrapped with `SyncCell`.
/// ```
/// # use std::cell::RefCell;
/// # use bevy_ecs::resource::Resource;
/// use bevy_platform::cell::SyncCell;
///
/// #[derive(Resource)]
/// struct ActuallySync {
///    counter: SyncCell<RefCell<usize>>,
/// }
/// ```
///
/// [`Exclusive`]: https://doc.rust-lang.org/nightly/std/sync/struct.Exclusive.html
/// [`World`]: crate::world::World
/// [`Res`]: crate::system::Res
/// [`ResMut`]: crate::system::ResMut
/// [`SyncCell`]: bevy_platform::cell::SyncCell
#[diagnostic::on_unimplemented(
    message = "`{Self}` is not a `Resource`",
    label = "invalid `Resource`",
    note = "consider annotating `{Self}` with `#[derive(Resource)]`"
)]
pub trait Resource: Send + Sync + 'static {}