Add SubclassOf trait

crates/objc2/src/class_type.rs

@@ -109,3 +109,32 @@ pub unsafe trait ClassType: Message {
         unsafe { msg_send_id![Self::class(), alloc] }
     }
 }
+
+/// TODO
+pub unsafe trait SubclassOf<Super: ClassType> {}
+
+unsafe impl<T> SubclassOf<T> for T {}
+
+unsafe impl<T: ClassType> SubclassOf<T::Super> for T where T::Super: ClassType {}
+
+unsafe impl<T: ClassType> SubclassOf<<T::Super as ClassType>::Super> for T
+where
+    T::Super: ClassType,
+    <T::Super as ClassType>::Super: ClassType,
+{
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::foundation::{NSMutableString, NSObject, NSString};
+
+    fn assert_subclass_of<T: SubclassOf<C>, C: ClassType>() {}
+
+    #[test]
+    fn test_subclass_of() {
+        assert_subclass_of::<NSString, NSObject>();
+        assert_subclass_of::<NSMutableString, NSObject>();
+        assert_subclass_of::<NSMutableString, NSString>();
+    }
+}

crates/objc2/src/lib.rs

@@ -195,7 +195,7 @@ pub use objc_sys as ffi;
 #[doc(no_inline)]
 pub use objc2_encode::{Encode, EncodeArguments, Encoding, RefEncode};
 
-pub use crate::class_type::ClassType;
+pub use crate::class_type::{ClassType, SubclassOf};
 pub use crate::message::{Message, MessageArguments, MessageReceiver};
 pub use crate::protocol::{ConformsTo, ProtocolType};
 pub use crate::verify::VerificationError;