bevy/crates/bevy_utils/src/label.rs

//! Traits used by label implementations

use std::{
    any::Any,
    hash::{Hash, Hasher},
};

/// An object safe version of [`Eq`]. This trait is automatically implemented
/// for any `'static` type that implements `Eq`.
pub trait DynEq: Any {
    /// Casts the type to `dyn Any`.
    fn as_any(&self) -> &dyn Any;

    /// This method tests for `self` and `other` values to be equal.
    ///
    /// Implementers should avoid returning `true` when the underlying types are
    /// not the same.
    fn dyn_eq(&self, other: &dyn DynEq) -> bool;
}

impl<T> DynEq for T
where
    T: Any + Eq,
{
    fn as_any(&self) -> &dyn Any {
        self
    }

    fn dyn_eq(&self, other: &dyn DynEq) -> bool {
        if let Some(other) = other.as_any().downcast_ref::<T>() {
            return self == other;
        }
        false
    }
}

/// An object safe version of [`Hash`]. This trait is automatically implemented
/// for any `'static` type that implements `Hash`.
pub trait DynHash: DynEq {
    /// Casts the type to `dyn Any`.
    fn as_dyn_eq(&self) -> &dyn DynEq;

    /// Feeds this value into the given [`Hasher`].
    ///
    /// [`Hash`]: std::hash::Hasher
    fn dyn_hash(&self, state: &mut dyn Hasher);
}

impl<T> DynHash for T
where
    T: DynEq + Hash,
{
    fn as_dyn_eq(&self) -> &dyn DynEq {
        self
    }

    fn dyn_hash(&self, mut state: &mut dyn Hasher) {
        T::hash(self, &mut state);
        self.type_id().hash(&mut state);
    }
}

/// Macro to define a new label trait
///
/// # Example
///
/// ```
/// # use bevy_utils::define_label;
/// define_label!(
///     /// A class of labels.
///     MyNewLabelTrait,
///     /// Identifies a value that implements `MyNewLabelTrait`.
///     MyNewLabelId,
/// );
/// ```
#[macro_export]
macro_rules! define_label {
    (
        $(#[$label_attr:meta])*
        $label_name:ident,

        $(#[$id_attr:meta])*
        $id_name:ident $(,)?
    ) => {
        $(#[$id_attr])*
        #[derive(Clone, Copy, PartialEq, Eq, Hash)]
        pub struct $id_name(::core::any::TypeId, &'static str);

        impl ::core::fmt::Debug for $id_name {
            fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
                write!(f, "{}", self.1)
            }
        }

        $(#[$label_attr])*
        pub trait $label_name: 'static {
            /// Converts this type into an opaque, strongly-typed label.
            fn as_label(&self) -> $id_name {
                let id = self.type_id();
                let label = self.as_str();
                $id_name(id, label)
            }
            /// Returns the [`TypeId`] used to differentiate labels.
            fn type_id(&self) -> ::core::any::TypeId {
                ::core::any::TypeId::of::<Self>()
            }
            /// Returns the representation of this label as a string literal.
            ///
            /// In cases where you absolutely need a label to be determined at runtime,
            /// you can use [`Box::leak`] to get a `'static` reference.
            fn as_str(&self) -> &'static str;
        }

        impl $label_name for $id_name {
            fn as_label(&self) -> Self {
                *self
            }
            fn type_id(&self) -> ::core::any::TypeId {
                self.0
            }
            fn as_str(&self) -> &'static str {
                self.1
            }
        }

        impl $label_name for &'static str {
            fn as_str(&self) -> Self {
                self
            }
        }
    };
}