Trait bevy_reflect::Enum

source ·

pub trait Enum: Reflect {
Show 14 methods    // Required methods
    fn field(&self, name: &str) -> Option<&dyn Reflect>;
    fn field_at(&self, index: usize) -> Option<&dyn Reflect>;
    fn field_mut(&mut self, name: &str) -> Option<&mut dyn Reflect>;
    fn field_at_mut(&mut self, index: usize) -> Option<&mut dyn Reflect>;
    fn index_of(&self, name: &str) -> Option<usize>;
    fn name_at(&self, index: usize) -> Option<&str>;
    fn iter_fields(&self) -> VariantFieldIter<'_> ⓘ;
    fn field_len(&self) -> usize;
    fn variant_name(&self) -> &str;
    fn variant_index(&self) -> usize;
    fn variant_type(&self) -> VariantType;
    fn clone_dynamic(&self) -> DynamicEnum;

    // Provided methods
    fn is_variant(&self, variant_type: VariantType) -> bool { ... }
    fn variant_path(&self) -> String { ... }
}

Expand description

A trait used to power enum-like operations via reflection.

This allows enums to be processed and modified dynamically at runtime without necessarily knowing the actual type. Enums are much more complex than their struct counterparts. As a result, users will need to be mindful of conventions, considerations, and complications when working with this trait.

Variants

An enum is a set of choices called variants. An instance of an enum can only exist as one of these choices at any given time. Consider Rust’s Option<T>. It’s an enum with two variants: None and Some. If you’re None, you can’t be Some and vice versa.

⚠️ This is very important: The Enum trait represents an enum as one of its variants. It does not represent the entire enum since that’s not true to how enums work.

Variants come in a few flavors:

Variant Type	Syntax
Unit	`MyEnum::Foo`
Tuple	`MyEnum::Foo( i32, i32 )`
Struct	`MyEnum::Foo{ value: String }`

As you can see, a unit variant contains no fields, while tuple and struct variants can contain one or more fields. The fields in a tuple variant is defined by their order within the variant. Index 0 represents the first field in the variant and so on. Fields in struct variants (excluding tuple structs), on the other hand, are represented by a name.

Implementation

💡 This trait can be automatically implemented using #[derive(Reflect)] on an enum definition.

Despite the fact that enums can represent multiple states, traits only exist in one state and must be applied to the entire enum rather than a particular variant. Because of this limitation, the Enum trait must not only represent any of the three variant types, but also define the methods for all three as well.

What does this mean? It means that even though a unit variant contains no fields, a representation of that variant using the Enum trait will still contain methods for accessing fields! Again, this is to account for all three variant types.

We recommend using the built-in #[derive(Reflect)] macro to automatically handle all the implementation details for you. However, if you must implement this trait manually, there are a few things to keep in mind…

Field Order

While tuple variants identify their fields by the order in which they are defined, struct variants identify fields by their name. However, both should allow access to fields by their defined order.

The reason all fields, regardless of variant type, need to be accessible by their order is due to field iteration. We need a way to iterate through each field in a variant, and the easiest way of achieving that is through the use of field order.

The derive macro adds proper struct variant handling for Enum::index_of, Enum::name_at and Enum::field_at[_mut] methods. The first two methods are required for all struct variant types. By convention, implementors should also handle the last method as well, but this is not a strict requirement.

Field Names

Implementors may choose to handle Enum::index_of, Enum::name_at, and Enum::field[_mut] for tuple variants by considering stringified usizes to be valid names (such as "3"). This isn’t wrong to do, but the convention set by the derive macro is that it isn’t supported. It’s preferred that these strings be converted to their proper usize representations and the Enum::field_at[_mut] methods be used instead.