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

Author: goatfryed

godot-core/src/obj/gd.rs

@@ -9,6 +9,7 @@ use std::fmt::{Debug, Display, Formatter, Result as FmtResult};
 use std::ops::{Deref, DerefMut};
 
 use godot_ffi as sys;
+use godot_ffi::is_main_thread;
 use sys::{static_assert_eq_size_align, SysPtr as _};
 
 use crate::builtin::{Callable, NodePath, StringName, Variant};
@@ -444,6 +445,28 @@ impl<T: GodotClass> Gd<T> {
         })
     }
 
+    /// Runs the given Closure deferred.
+    ///
+    /// This can be a type-safe alternative to [`classes::Object::call_deferred`], but does not handle dynamic dispatch, unless explicitly used.
+    /// This must be used on the main thread.
+    #[cfg(since_api = "4.2")]
+    pub fn apply_deferred<F>(&mut self, mut rust_function: F)
+    where
+        F: FnMut(&mut T) + 'static,
+        T: GodotClass + Bounds<Declarer = bounds::DeclUser>,
+    {
+        assert!(
+            is_main_thread(),
+            "apply_deferred must be called on main thread"
+        );
+        let this = self.clone();
+        let callable = Callable::from_local_fn("apply_deferred", move |_| {
+            rust_function(this.clone().bind_mut().deref_mut());
+            Ok(Variant::nil())
+        });
+        callable.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

@@ -67,9 +67,20 @@ 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,focus)]
+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(DeferredTestNode::accept);
+
+    let handle = test_node.bind().as_expectation_task();
+    handle
+}