Implements the node key for attribute #[init]

- Meant for fields of type: OnReady<Gd<T>> where T: Inherits<Node>
- This key accepts a single argument with the syntax: "NodePath"
- This key is mutually exclusive with the key `default`
- This key desugars into `default = OnReady::node("NodePath")`

  This feature required adding a `Gd<Node>` argument to the closure that initializes OnReady, which means that classes that use it must have an explicit `Base` field. (i.e. `base: Base<Node>`)

Author: Houtamelo

godot-core/src/obj/onready.rs

@@ -5,7 +5,10 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
  */
 
+use crate::builtin::NodePath;
+use crate::classes::Node;
 use crate::meta::GodotConvert;
+use crate::obj::{Gd, GodotClass, Inherits};
 use crate::registry::property::{PropertyHintInfo, Var};
 use std::mem;
 
@@ -17,9 +20,10 @@ use std::mem;
 ///
 /// `OnReady<T>` should always be used as a field. There are two modes to use it:
 ///
-/// 1. **Automatic mode, using [`new()`](Self::new).**<br>
-///    Before `ready()` is called, all `OnReady` fields constructed with `new()` are automatically initialized, in the order of
-///    declaration. This means that you can safely access them in `ready()`.<br><br>
+/// 1. **Automatic mode, using [`new()`](OnReady::new), [`from_base_fn()`](OnReady::from_base_fn) or
+///    [`node()`](OnReady::<Gd<T>>::node).**<br>
+///    Before `ready()` is called, all `OnReady` fields constructed with the above methods are automatically initialized,
+///    in the order of declaration. This means that you can safely access them in `ready()`.<br><br>
 /// 2. **Manual mode, using [`manual()`](Self::manual).**<br>
 ///    These fields are left uninitialized until you call [`init()`][Self::init] on them. This is useful if you need more complex
 ///    initialization scenarios than a closure allows. If you forget initialization, a panic will occur on first access.
@@ -36,21 +40,27 @@ use std::mem;
 /// [option]: std::option::Option
 /// [lazy]: https://docs.rs/once_cell/1/once_cell/unsync/struct.Lazy.html
 ///
-/// # Example
+/// # Requirements
+/// - The class must have an explicit `Base` field (i.e. `base: Base<Node>`).
+/// - The class must inherit `Node` (otherwise `ready()` would not exist anyway).
+///
+/// # Example - user-defined `init`
 /// ```
 /// use godot::prelude::*;
 ///
 /// #[derive(GodotClass)]
 /// #[class(base = Node)]
 /// struct MyClass {
+///    base: Base<Node>,
 ///    auto: OnReady<i32>,
 ///    manual: OnReady<i32>,
 /// }
 ///
 /// #[godot_api]
 /// impl INode for MyClass {
-///     fn init(_base: Base<Node>) -> Self {
+///     fn init(base: Base<Node>) -> Self {
 ///        Self {
+///            base,
 ///            auto: OnReady::new(|| 11),
 ///            manual: OnReady::manual(),
 ///        }
@@ -65,10 +75,54 @@ use std::mem;
 ///        assert_eq!(*self.manual, 22);
 ///     }
 /// }
+/// ```
+///
+/// # Example - macro-generated `init`
+/// ```
+/// use godot::prelude::*;
+///
+/// #[derive(GodotClass)]
+/// #[class(init, base = Node)]
+/// struct MyClass {
+///    base: Base<Node>,
+///    #[init(node = "ChildPath")]
+///    auto: OnReady<Gd<Node2D>>,
+///    #[init(default = OnReady::manual())]
+///    manual: OnReady<i32>,
+/// }
+///
+/// #[godot_api]
+/// impl INode for MyClass {
+///     fn ready(&mut self) {
+///        // self.node is now ready with the node found at path `ChildPath`.
+///        assert_eq!(self.auto.get_name(), "ChildPath".into());
+///
+///        // self.manual needs to be initialized manually.
+///        self.manual.init(22);
+///        assert_eq!(*self.manual, 22);
+///     }
+/// }
+/// ```
 pub struct OnReady<T> {
     state: InitState<T>,
 }
 
+impl<T: GodotClass + Inherits<Node>> OnReady<Gd<T>> {
+    /// Variant of [`OnReady::new()`], fetching the node located at `path` before `ready()`.
+    ///
+    /// This is the functional equivalent of the GDScript pattern `@onready var node = $NodePath`.
+    ///
+    /// # Panics
+    /// - If `path` does not point to a valid node.
+    ///
+    /// Note that the panic will only happen if and when the node enters the SceneTree for the first time
+    ///  (i.e.: it receives the `READY` notification).
+    pub fn node(path: impl Into<NodePath>) -> Self {
+        let path = path.into();
+        Self::from_base_fn(|base| base.get_node_as(path))
+    }
+}
+
 impl<T> OnReady<T> {
     /// Schedule automatic initialization before `ready()`.
     ///
@@ -82,6 +136,14 @@ impl<T> OnReady<T> {
     pub fn new<F>(init_fn: F) -> Self
     where
         F: FnOnce() -> T + 'static,
+    {
+        Self::from_base_fn(|_| init_fn())
+    }
+
+    /// Variant of [`OnReady::new()`], allowing access to `Base` when initializing.
+    pub fn from_base_fn<F>(init_fn: F) -> Self
+    where
+        F: FnOnce(&Gd<Node>) -> T + 'static,
     {
         Self {
             state: InitState::AutoPrepared {
@@ -126,7 +188,7 @@ impl<T> OnReady<T> {
     ///
     /// # Panics
     /// If the value is already initialized.
-    pub(crate) fn init_auto(&mut self) {
+    pub(crate) fn init_auto(&mut self, base: &Gd<Node>) {
         // Two branches needed, because mem::replace() could accidentally overwrite an already initialized value.
         match &self.state {
             InitState::ManualUninitialized => return, // skipped
@@ -147,7 +209,7 @@ impl<T> OnReady<T> {
         };
 
         self.state = InitState::Initialized {
-            value: initializer(),
+            value: initializer(base),
         };
     }
 }
@@ -214,9 +276,11 @@ impl<T: Var> Var for OnReady<T> {
 // ----------------------------------------------------------------------------------------------------------------------------------------------
 // Implementation
 
+type InitFn<T> = dyn FnOnce(&Gd<Node>) -> T;
+
 enum InitState<T> {
     ManualUninitialized,
-    AutoPrepared { initializer: Box<dyn FnOnce() -> T> },
+    AutoPrepared { initializer: Box<InitFn<T>> },
     AutoInitializing, // needed because state cannot be empty
     Initialized { value: T },
 }

godot-core/src/private.rs

@@ -114,8 +114,8 @@ pub struct ClassConfig {
 // ----------------------------------------------------------------------------------------------------------------------------------------------
 // Capability queries and internal access
 
-pub fn auto_init<T>(l: &mut crate::obj::OnReady<T>) {
-    l.init_auto();
+pub fn auto_init<T>(l: &mut crate::obj::OnReady<T>, base: &crate::obj::Gd<crate::classes::Node>) {
+    l.init_auto(base);
 }
 
 #[cfg(since_api = "4.3")]

godot-macros/src/class/derive_godot_class.rs

@@ -55,7 +55,7 @@ pub fn derive_godot_class(item: venial::Item) -> ParseResult<TokenStream> {
             }
         }
     } else {
-        quote! {}
+        TokenStream::new()
     };
 
     let deprecated_base_warning = if fields.has_deprecated_base {
@@ -226,15 +226,29 @@ fn make_user_class_impl(
     is_tool: bool,
     all_fields: &[Field],
 ) -> (TokenStream, bool) {
-    let onready_field_inits = all_fields
-        .iter()
-        .filter(|&field| field.is_onready)
-        .map(|field| {
-            let field = &field.name;
+    let onready_inits = {
+        let mut onready_fields = all_fields
+            .iter()
+            .filter(|&field| field.is_onready)
+            .map(|field| {
+                let field = &field.name;
+                quote! {
+                    ::godot::private::auto_init(&mut self.#field, &base);
+                }
+            });
+
+        if let Some(first) = onready_fields.next() {
             quote! {
-                ::godot::private::auto_init(&mut self.#field);
+                {
+                    let base = <Self as godot::obj::WithBaseField>::to_gd(self).upcast();
+                    #first
+                    #( #onready_fields )*
+                }
             }
-        });
+        } else {
+            TokenStream::new()
+        }
+    };
 
     let default_virtual_fn = if all_fields.iter().any(|field| field.is_onready) {
         let tool_check = util::make_virtual_tool_check();
@@ -267,7 +281,7 @@ fn make_user_class_impl(
             }
 
             fn __before_ready(&mut self) {
-                #( #onready_field_inits )*
+                #onready_inits
             }
 
             #default_virtual_fn
@@ -409,8 +423,37 @@ fn parse_fields(
             }
 
             // #[init(default = expr)]
-            let default = parser.handle_expr("default")?;
-            field.default = default;
+            if let Some(default) = parser.handle_expr("default")? {
+                field.default = Some(default);
+            }
+
+            // #[init(node = "NodePath")]
+            if let Some(node_path) = parser.handle_expr("node")? {
+                if !field.is_onready {
+                    return bail!(
+                        parser.span(),
+                        "The key `node` in attribute #[init] requires field of type `OnReady<T>`\n\
+				         Help: The syntax #[init(node = \"NodePath\")] is equivalent to \
+				         #[init(default = OnReady::node(\"NodePath\"))], \
+				         which can only be assigned to fields of type `OnReady<T>`"
+                    );
+                }
+
+                if field.default.is_some() {
+                    return bail!(
+				        parser.span(),
+				        "The key `node` in attribute #[init] is mutually exclusive with the key `default`\n\
+				         Help: The syntax #[init(node = \"NodePath\")] is equivalent to \
+				         #[init(default = OnReady::node(\"NodePath\"))], \
+				         both aren't allowed since they would override each other"
+			        );
+                }
+
+                field.default = Some(quote! {
+                    OnReady::node(#node_path)
+                });
+            }
+
             parser.finish()?;
         }