bevy_reflect_derive/field_attributes.rs
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//! Contains code related to field attributes for reflected types.
//!
//! A field attribute is an attribute which applies to particular field or variant
//! as opposed to an entire struct or enum. An example of such an attribute is
//! the derive helper attribute for `Reflect`, which looks like: `#[reflect(ignore)]`.
use crate::{
attribute_parser::terminated_parser, custom_attributes::CustomAttributes,
REFLECT_ATTRIBUTE_NAME,
};
use quote::ToTokens;
use syn::{parse::ParseStream, Attribute, LitStr, Meta, Token, Type};
mod kw {
syn::custom_keyword!(ignore);
syn::custom_keyword!(skip_serializing);
syn::custom_keyword!(default);
syn::custom_keyword!(remote);
}
pub(crate) const IGNORE_SERIALIZATION_ATTR: &str = "skip_serializing";
pub(crate) const IGNORE_ALL_ATTR: &str = "ignore";
pub(crate) const DEFAULT_ATTR: &str = "default";
/// Stores data about if the field should be visible via the Reflect and serialization interfaces
///
/// Note the relationship between serialization and reflection is such that a member must be reflected in order to be serialized.
/// In boolean logic this is described as: `is_serialized -> is_reflected`, this means we can reflect something without serializing it but not the other way round.
/// The `is_reflected` predicate is provided as `self.is_active()`
#[derive(Default, Clone, Copy, PartialEq, Eq)]
pub(crate) enum ReflectIgnoreBehavior {
/// Don't ignore, appear to all systems
#[default]
None,
/// Ignore when serializing but not when reflecting
IgnoreSerialization,
/// Ignore both when serializing and reflecting
IgnoreAlways,
}
impl ReflectIgnoreBehavior {
/// Returns `true` if the ignoring behavior implies member is included in the reflection API, and false otherwise.
pub fn is_active(self) -> bool {
match self {
ReflectIgnoreBehavior::None | ReflectIgnoreBehavior::IgnoreSerialization => true,
ReflectIgnoreBehavior::IgnoreAlways => false,
}
}
/// The exact logical opposite of `self.is_active()` returns true iff this member is not part of the reflection API whatsoever (neither serialized nor reflected)
pub fn is_ignored(self) -> bool {
!self.is_active()
}
}
/// Controls how the default value is determined for a field.
#[derive(Default, Clone)]
pub(crate) enum DefaultBehavior {
/// Field is required.
#[default]
Required,
/// Field can be defaulted using `Default::default()`.
Default,
/// Field can be created using the given function name.
///
/// This assumes the function is in scope, is callable with zero arguments,
/// and returns the expected type.
Func(syn::ExprPath),
}
/// A container for attributes defined on a reflected type's field.
#[derive(Default, Clone)]
pub(crate) struct FieldAttributes {
/// Determines how this field should be ignored if at all.
pub ignore: ReflectIgnoreBehavior,
/// Sets the default behavior of this field.
pub default: DefaultBehavior,
/// Custom attributes created via `#[reflect(@...)]`.
pub custom_attributes: CustomAttributes,
/// For defining the remote wrapper type that should be used in place of the field for reflection logic.
pub remote: Option<Type>,
}
impl FieldAttributes {
/// Parse all field attributes marked "reflect" (such as `#[reflect(ignore)]`).
pub fn parse_attributes(attrs: &[Attribute]) -> syn::Result<Self> {
let mut args = FieldAttributes::default();
attrs
.iter()
.filter_map(|attr| {
if !attr.path().is_ident(REFLECT_ATTRIBUTE_NAME) {
// Not a reflect attribute -> skip
return None;
}
let Meta::List(meta) = &attr.meta else {
return Some(syn::Error::new_spanned(attr, "expected meta list"));
};
// Parse all attributes inside the list, collecting any errors
meta.parse_args_with(terminated_parser(Token![,], |stream| {
args.parse_field_attribute(stream)
}))
.err()
})
.reduce(|mut acc, err| {
acc.combine(err);
acc
})
.map_or(Ok(args), Err)
}
/// Parses a single field attribute.
fn parse_field_attribute(&mut self, input: ParseStream) -> syn::Result<()> {
let lookahead = input.lookahead1();
if lookahead.peek(Token![@]) {
self.parse_custom_attribute(input)
} else if lookahead.peek(kw::ignore) {
self.parse_ignore(input)
} else if lookahead.peek(kw::skip_serializing) {
self.parse_skip_serializing(input)
} else if lookahead.peek(kw::default) {
self.parse_default(input)
} else if lookahead.peek(kw::remote) {
self.parse_remote(input)
} else {
Err(lookahead.error())
}
}
/// Parse `ignore` attribute.
///
/// Examples:
/// - `#[reflect(ignore)]`
fn parse_ignore(&mut self, input: ParseStream) -> syn::Result<()> {
if self.ignore != ReflectIgnoreBehavior::None {
return Err(input.error(format!(
"only one of {:?} is allowed",
[IGNORE_ALL_ATTR, IGNORE_SERIALIZATION_ATTR]
)));
}
input.parse::<kw::ignore>()?;
self.ignore = ReflectIgnoreBehavior::IgnoreAlways;
Ok(())
}
/// Parse `skip_serializing` attribute.
///
/// Examples:
/// - `#[reflect(skip_serializing)]`
fn parse_skip_serializing(&mut self, input: ParseStream) -> syn::Result<()> {
if self.ignore != ReflectIgnoreBehavior::None {
return Err(input.error(format!(
"only one of {:?} is allowed",
[IGNORE_ALL_ATTR, IGNORE_SERIALIZATION_ATTR]
)));
}
input.parse::<kw::skip_serializing>()?;
self.ignore = ReflectIgnoreBehavior::IgnoreSerialization;
Ok(())
}
/// Parse `default` attribute.
///
/// Examples:
/// - `#[reflect(default)]`
/// - `#[reflect(default = "path::to::func")]`
fn parse_default(&mut self, input: ParseStream) -> syn::Result<()> {
if !matches!(self.default, DefaultBehavior::Required) {
return Err(input.error(format!("only one of {:?} is allowed", [DEFAULT_ATTR])));
}
input.parse::<kw::default>()?;
if input.peek(Token![=]) {
input.parse::<Token![=]>()?;
let lit = input.parse::<LitStr>()?;
self.default = DefaultBehavior::Func(lit.parse()?);
} else {
self.default = DefaultBehavior::Default;
}
Ok(())
}
/// Parse `@` (custom attribute) attribute.
///
/// Examples:
/// - `#[reflect(@(foo = "bar"))]`
/// - `#[reflect(@(min = 0.0, max = 1.0))]`
fn parse_custom_attribute(&mut self, input: ParseStream) -> syn::Result<()> {
self.custom_attributes.parse_custom_attribute(input)
}
/// Parse `remote` attribute.
///
/// Examples:
/// - `#[reflect(remote = path::to::RemoteType)]`
fn parse_remote(&mut self, input: ParseStream) -> syn::Result<()> {
if let Some(remote) = self.remote.as_ref() {
return Err(input.error(format!(
"remote type already specified as {}",
remote.to_token_stream()
)));
}
input.parse::<kw::remote>()?;
input.parse::<Token![=]>()?;
self.remote = Some(input.parse()?);
Ok(())
}
/// Returns `Some(true)` if the field has a generic remote type.
///
/// If the remote type is not generic, returns `Some(false)`.
///
/// If the field does not have a remote type, returns `None`.
pub fn is_remote_generic(&self) -> Option<bool> {
if let Type::Path(type_path) = self.remote.as_ref()? {
type_path
.path
.segments
.last()
.map(|segment| !segment.arguments.is_empty())
} else {
Some(false)
}
}
}