[swift-evolution] [Pitch] Overridable Members in Extensions

Jordan Rose jordan_rose at apple.com
Wed Feb 10 19:45:20 CST 2016

Hey, everyone. Here's a small feature with ABI implications, ready for feedback. In addition to comments on the proposal itself, I'm also interested in hearing how often this comes up for people:

- extending a class you don't own
- to add an overridable method
- where some of the overriders might be outside the current module




Overridable Members in Extensions

Proposal: SE-NNNN
Author: Jordan Rose <https://github.com/jrose-apple>
Status: Awaiting review
Review manager: TBD

Today, methods introduced in an extension of a class cannot override or be overridden unless the method is (implicitly or explicitly) marked @objc. This proposal lifts the blanket restriction while still enforcing safety.

Note: it's already plan-of-record that if the extension is in the same module as the class, the methods will be treated as if they were declared in the class itself. This proposal only applies to extensions declared in a different module.

This is used to add operations to system or library classes that you can customize in your own classes, as seen in the Apple AdaptivePhotos <https://developer.apple.com/library/ios/samplecode/AdaptivePhotos/Listings/AdaptiveCode_AdaptiveCode_UIViewController_PhotoContents_swift.html> sample code.

extension UIViewController {
  func containsPhoto(photo: Photo) -> Bool {
    return false
class ConversationViewController : UIViewController {
  // …
  override func containsPhoto(photo: Photo) -> Bool {
    return self.conversation.photos.contains(photo)
Additional motivation: parity with Objective-C. If Objective-C didn't allow this, we might not have done it, but right now the answer is "if your method is ObjC-compatible, just slap an attribute on it; otherwise you're out of luck", which isn't really a sound design choice.

 <https://github.com/jrose-apple/swift-evolution/tree/overridable-members-in-extensions#todays-workaround>Today's Workaround

If you know every class that needs a custom implementation of a method, you can use dynamic casts to get the same effect:

extension UIViewController {
  final func containsPhoto(photo: Photo) -> Bool {
    switch self {
    case is ListTableViewController:
      return true
    case let cvc as ConversationViewController:
      return cvc.conversation.photos.contains(photo)
      return false
But this is not possible if there may be subclasses outside of the module, and it either forces all of the implementations into a single method body or requires adding dummy methods to each class.

 <https://github.com/jrose-apple/swift-evolution/tree/overridable-members-in-extensions#proposed-solution>Proposed solution

This proposal lifts the restriction on non- at objc extension methods (and properties, and subscripts) by requiring an alternate dispatch mechanism that can be arbitrarily extended. To preserve safety and correctness, a new, narrower restriction will be put in place:

If an extension in module B is extending a class in module A, it may only override members added in module B.

Any other rule can result in two modules trying to add an override for the same method on the same class.

Note: This rule applies to @objc members as well as non- at objc members.
There is no restriction on extensions adding new overridable members. These members can be overridden by any extension in the same module (by the above rule) and by a subclass in any module, whether in the class declaration itself or in an extension in the same module.

 <https://github.com/jrose-apple/swift-evolution/tree/overridable-members-in-extensions#detailed-design>Detailed design

Besides safety, the other reason we didn't add this feature is because the Swift method dispatch mechanism uses a single virtual dispatch table for a class, which cannot be arbitrarily extended after the fact. The implementation would require an alternate dispatch mechanism that can be arbitrarily extended.

On Apple platforms this is implemented by the Objective-C method table; we would provide a simplified implementation of the same on Linux. For a selector we would use the mangled name of the original overridden method. These methods would still use Swift calling conventions; they're just being stored in the same lookup table as Objective-C methods.

 <https://github.com/jrose-apple/swift-evolution/tree/overridable-members-in-extensions#library-evolution>Library Evolution

As with any other method <https://github.com/apple/swift/blob/master/docs/LibraryEvolution.rst#classes>, it is legal to "move" an extension method up to an extension on the base class, as long as the original declaration is not removed entirely. The new entry point will forward over to the original entry point in order to preserve binary compatibility.

 <https://github.com/jrose-apple/swift-evolution/tree/overridable-members-in-extensions#impact-on-existing-code>Impact on existing code

No existing semantics will be affected. Dispatch for methods in extensions may get slower, since it's no longer using a direct call.

 <https://github.com/jrose-apple/swift-evolution/tree/overridable-members-in-extensions#alternatives-considered>Alternatives considered

 <https://github.com/jrose-apple/swift-evolution/tree/overridable-members-in-extensions#all-extension-methods-are-final>All extension methods are final

This is sound, and doesn't rule out the "closed class hierarchy" partial workaround described above. However, it does prevent some reasonable patterns that were possible in Objective-C, and it is something we've seen developers try to do (with or without @objc).

 <https://github.com/jrose-apple/swift-evolution/tree/overridable-members-in-extensions#objc-extension-methods-are-overridable-non-objc-methods-are-not>@objc extension methods are overridable, non- at objc methods are not

This is a practical answer, since it requires no further implementation work. We could require members in extensions to be explicitly annotated dynamic and final, respectively, so that the semantics are at least clear. However, it's not a very principled design choice: either overridable extension members are useful, or they aren't.

 <https://github.com/jrose-apple/swift-evolution/tree/overridable-members-in-extensions#future-extensions>Future extensions

The restriction that an extension cannot override a method from another module is intended for safety purposes, preventing two modules from each adding their own override. It's possible to make this a link-time failure rather than a compile-time failure by emitting a dummy symbol representing the (class, member) pair. Because of this, it may be useful to have an "I know what I'm doing" annotation that promises that no one else will add the same member; if it does happen then the program will fail to link.

(Indeed, we probably should do this anyway for @objc overrides, which run the risk of run-time collision because of Objective-C language semantics.)

If we ever have an "SPI" feature that allows public API to be restricted to certain clients, it would be reasonable to consider relaxing the safety restrictions for those clients specifically on the grounds that the library author trusts them to know what they're doing.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160210/fc0e2443/attachment.html>

More information about the swift-evolution mailing list