Add get/set to property attribute to specify custom getter/setter

gdnative-core/src/export/property.rs

@@ -1,4 +1,5 @@
 //! Property registration.
+use std::marker::PhantomData;
 
 use accessor::{Getter, RawGetter, RawSetter, Setter};
 use invalid_accessor::{InvalidGetter, InvalidSetter};
@@ -322,6 +323,115 @@ impl PropertyUsage {
     }
 }
 
+/// Placeholder type for exported properties with no backing field.
+///
+/// This is the go-to type whenever you want to expose a getter/setter to GDScript, which
+/// does not directly map to a field in your struct. Instead of adding a useless field
+/// of the corresponding type (which needs initialization, extra space, etc.), you can use
+/// an instance of this type as a placeholder.
+///
+/// `Property` is a zero-sized type (ZST) which has exactly one value: `Property::default()`.
+/// It implements most of the basic traits, which allows its enclosing struct to remain
+/// composable and derive those traits itself.
+///
+/// ## When to use `Property<T>` instead of `T`
+///
+/// The following table shows which combinations of `#[property]` attributes and field types are allowed.
+/// In this context, `get` and `set` behave symmetrically, so only one of the combinations is listed.
+/// Furthermore, `get_ref` can be used in place of `get`, when it appears with a path.
+///
+/// Field type ➡ <br> Attributes ⬇           | bare `T`                      | `Property<T>`
+/// ------------------------------------------|-------------------------------|-----------------------------
+/// `#[property]`                             | ✔️ default get + set       | ❌️
+/// `#[property(get, set)]` _(same as above)_ | ✔️ default get + set       | ❌️
+/// `#[property(get)]`                        | ✔️ default get (no set)    | ❌️
+/// `#[property(get="path")]`                 | ⚠️ custom get (no set)     | ✔️ custom get (no set)
+/// `#[property(get="path", set)]`            | ✔️ custom get, default set | ❌️
+/// `#[property(get="path", set="path")]`     | ⚠️ custom get + set        | ✔️ custom get + set
+///
+/// "⚠️" means that this attribute combination is allowed for bare `T`, but you should consider
+/// using `Property<T>`.
+///
+/// Since there is no default `get` or `set` in these cases, godot-rust will never access the field
+/// directly. In other words, you are not really exporting _that field_, but linking its name and type
+/// (but not its value) to the specified get/set methods.
+///
+/// To decide when to use which:
+/// * If you access your field as-is on the Rust side, use bare `T`.<br>
+///   With a `Property<T>` field on the other hand, you would need to _additionally_ add a `T` backing field.
+/// * If you don't need a backing field, use `Property<T>`.<br>
+///   This is the case whenever you compute a result dynamically, or map values between Rust and GDScript
+///   representations.
+///
+/// ## Examples
+///
+/// Read/write accessible:
+/// ```no_run
+/// # use gdnative::prelude::*;
+/// #[derive(NativeClass)]
+/// # #[no_constructor]
+/// struct MyObject {
+///     #[property]
+///     color: Color,
+/// }
+/// ```
+///
+/// Read-only:
+/// ```no_run
+/// # use gdnative::prelude::*;
+/// #[derive(NativeClass)]
+/// # #[no_constructor]
+/// struct MyObject {
+///     #[property(get)]
+///     hitpoints: f32,
+/// }
+/// ```
+///
+/// Read-write, with validating setter:
+/// ```no_run
+/// # use gdnative::prelude::*;
+/// # fn validate(s: &String) -> bool { true }
+/// #[derive(NativeClass)]
+/// # #[no_constructor]
+/// struct MyObject {
+///     #[property(get, set = "Self::set_name")]
+///     player_name: String,
+/// }
+///
+/// #[methods]
+/// impl MyObject {
+///     fn set_name(&mut self, _owner: TRef<Reference>, name: String) {
+///         if validate(&name) {
+///             self.player_name = name;
+///         }
+///     }
+/// }
+/// ```
+///
+/// Write-only, no backing field, custom setter:
+/// ```no_run
+/// # use gdnative::prelude::*;
+/// #[derive(NativeClass)]
+/// # #[no_constructor]
+/// struct MyObject {
+///     #[property(set = "Self::set_password")]
+///     password: Property<String>,
+/// }
+///
+/// #[methods]
+/// impl MyObject {
+///     fn set_password(&mut self, _owner: TRef<Reference>, password: String) {
+///         // securely hash and store password
+///     }
+/// }
+/// ```
+
+// Note: traits are mostly implemented to enable deriving the same traits on the enclosing struct.
+#[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
+pub struct Property<T> {
+    _marker: PhantomData<T>,
+}
+
 mod impl_export {
     use super::*;
 

gdnative-derive/src/lib.rs

@@ -218,6 +218,21 @@ pub fn profiled(meta: TokenStream, input: TokenStream) -> TokenStream {
 /// Call hook methods with `self` and `owner` before and/or after the generated property
 /// accessors.
 ///
+/// - `get` / `get_ref` / `set`
+///
+/// Configure getter/setter for property. All of them can accept a path to specify a custom
+/// property accessor. For example, `#[property(get = "Self::my_getter")]` will use
+/// `Self::my_getter` as the getter.
+///
+/// The difference of `get` and `get_ref` is that `get` will register the getter with
+/// `with_getter` function, which means your getter should return an owned value `T`, but
+/// `get_ref` use `with_ref_getter` to register getter. In this case, your custom getter
+/// should return a shared reference `&T`.
+///
+/// Situations with custom getters/setters and no backing fields require the use of the
+/// type [`Property<T>`][gdnative::export::Property]. Consult its documentation for
+/// a deeper elaboration of property exporting.
+///
 /// - `no_editor`
 ///
 /// Hides the property from the editor. Does not prevent it from being sent over network or saved in storage.
@@ -379,19 +394,21 @@ pub fn derive_native_class(input: TokenStream) -> TokenStream {
     let derive_input = syn::parse_macro_input!(input as DeriveInput);
 
     // Implement NativeClass for the input
-    native_script::derive_native_class(&derive_input).map_or_else(
+    let derived = native_script::derive_native_class(&derive_input).map_or_else(
         |err| {
             // Silence the other errors that happen because NativeClass is not implemented
             let empty_nativeclass = native_script::impl_empty_nativeclass(&derive_input);
             let err = err.to_compile_error();
 
-            TokenStream::from(quote! {
+            quote! {
                 #empty_nativeclass
                 #err
-            })
+            }
         },
         std::convert::identity,
-    )
+    );
+
+    TokenStream::from(derived)
 }
 
 #[proc_macro_derive(ToVariant, attributes(variant))]