Add NSObjectProtocol::is_member_of and some related "downcasting" hel…

…pers

crates/objc2/src/rc/id.rs

@@ -6,8 +6,11 @@ use core::panic::{RefUnwindSafe, UnwindSafe};
 use core::ptr::{self, NonNull};
 
 use super::AutoreleasePool;
-use crate::mutability::{IsIdCloneable, IsMutable};
 use crate::{ffi, ClassType, Message};
+use crate::{
+    mutability::{IsIdCloneable, IsMutable},
+    runtime::NSObjectProtocol,
+};
 
 /// A reference counted pointer type for Objective-C objects.
 ///
@@ -602,6 +605,25 @@ where
     }
 }
 
+impl<T: ClassType + NSObjectProtocol + 'static> Id<T> {
+    /// Tries to cast the object to the class `C` by checking whether the object
+    /// is a direct member of the class (through [`NSObjectProtocol::is_member_of`]).
+    pub fn member_cast<C>(this: Self) -> Result<Id<C>, Self>
+    where
+        C: ClassType + NSObjectProtocol + 'static,
+    {
+        if this.is_member_of::<C>() {
+            Ok(unsafe {
+                // SAFETY: This cast should be safe because the object is a direct member
+                // of class `C`.
+                Id::cast(this)
+            })
+        } else {
+            Err(this)
+        }
+    }
+}
+
 // TODO: Add ?Sized bound
 impl<T: IsIdCloneable> Clone for Id<T> {
     /// Makes a clone of the shared object.

crates/objc2/src/runtime/nsobject.rs

@@ -134,6 +134,14 @@ pub unsafe trait NSObjectProtocol {
         unsafe { msg_send![self, isKindOfClass: cls] }
     }
 
+    #[doc(hidden)]
+    fn __isMemberOfClass(&self, cls: &AnyClass) -> bool
+    where
+        Self: Sized + Message,
+    {
+        unsafe { msg_send![self, isMemberOfClass: cls] }
+    }
+
     /// Check if the object is an instance of the class, or one of it's
     /// subclasses.
     ///
@@ -155,6 +163,36 @@ pub unsafe trait NSObjectProtocol {
     // For example, something may have a return type of `NSString`, while
     // behind the scenes they really return `NSMutableString` and expect it to
     // not be modified.
+
+    /// Checks if the object is a member of the class.
+    ///
+    /// See [Apple's documentation][apple-doc] for more details on what you
+    /// may (and what you may not) do with this information.
+    ///
+    /// [apple-doc]: https://developer.apple.com/documentation/objectivec/1418956-nsobject/1418766-ismemberofclass
+    #[doc(alias = "isMemberOfClass")]
+    fn is_member_of<T: ClassType>(&self) -> bool
+    where
+        Self: Sized + Message,
+    {
+        self.__isMemberOfClass(T::class())
+    }
+
+    /// Tries to cast the object to the class `T` by checking whether the object
+    /// is a direct member of the class (through [`is_member_of`](`Self::is_member_of`)).
+    fn member_cast<T: ClassType>(&self) -> Option<&T>
+    where
+        Self: Sized + Message + 'static,
+        T: 'static,
+    {
+        if self.is_member_of::<T>() {
+            // SAFETY: self is a direct member of the specified (`T`) class.
+            // Therefore casting should be safe.
+            Some(unsafe { &*(self as *const Self).cast::<T>() })
+        } else {
+            None
+        }
+    }
 }
 
 crate::__inner_extern_protocol!(