feat(core): support easier, type-safe deferred calls

Author: goatfryed

godot-core/src/obj/gd.rs

@@ -444,6 +444,35 @@ impl<T: GodotClass> Gd<T> {
         })
     }
 
+    /// Runs the given Closure deferred.
+    /// The closure receives a reference to this object back.
+    /// This can be a type-safe alternative to [`Object::call_deferred`], but does not handle
+    /// dynamic dispatch, unless explicitly used.
+    /// If you need error propagation, use [`Self::try_apply_deferred`] instead.
+    pub fn apply_deferred<F>(&mut self, mut rust_function: F)
+    where
+        F: FnMut(Gd<T>) + 'static,
+    {
+        self.try_apply_deferred(move |this| {
+            rust_function(this);
+            Ok(())
+        })
+    }
+
+    /// See [`Self::apply_deferred`], this does not handle
+    pub fn try_apply_deferred<F>(&mut self, mut rust_function: F)
+    where
+        F: FnMut(Gd<T>) -> Result<(), ()> + 'static,
+    {
+        let this = self.clone();
+        Callable::from_local_fn(
+            // todo: naming ?
+            "apply_deferred",
+            move |_| rust_function(this.clone()).map(|_| Variant::nil()),
+        )
+        .call_deferred(&[]);
+    }
+
     /// Returns `Ok(cast_obj)` on success, `Err(self)` on error.
     // Visibility: used by DynGd.
     pub(crate) fn owned_cast<U>(self) -> Result<Gd<U>, Self>

itest/rust/src/object_tests/deferred_call_test.rs

@@ -62,9 +62,20 @@ impl INode for DeferredTestNode {
 fn calls_method_names_deferred(ctx: &crate::framework::TestContext) -> TaskHandle {
     let mut test_node = DeferredTestNode::new_alloc();
     ctx.scene_tree.clone().add_child(&test_node);
-    
+
     test_node.call_deferred("accept", &[]);
 
     let handle = test_node.bind().as_expectation_task();
     handle
 }
+
+#[itest(async)]
+fn calls_closure_deferred(ctx: &crate::framework::TestContext) -> TaskHandle {
+    let mut test_node = DeferredTestNode::new_alloc();
+    ctx.scene_tree.clone().add_child(&test_node);
+
+    test_node.apply_deferred(|mut this| this.bind_mut().accept());
+
+    let handle = test_node.bind().as_expectation_task();
+    handle
+}