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Make Optional Requirements Objective-C-only

Introduction

Swift currently has support for "optional" requirements in Objective-C protocols, to match with the corresponding feature of Objective-C. We don't want to make optional requirements a feature of Swift protocols (for reasons described below), nor can we completely eliminate the notion of the language (for different reasons also described below). Therefore, to prevent confusion about our direction, this proposal requires an explicit '@objc' attribute on each optional requirement to indicate that this is an Objective-C compatibility feature.

Swift-evolution threads:

Motivation

Having optional only work for Objective-C requirements is very weird: it feels like a general feature with a compiler bug that prevents it from working generally. However, we don't want to make it a feature of Swift protocols and we can't eliminate it (see alternatives considered), so we propose to rename the keyword to make it clear that this feature is intended only for compatibility with Objective-C.

Proposed solution

Require an explicit @objc attribute on each optional requirement:

@objc protocol NSTableViewDelegate {
  @objc optional func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int) -> NSView? // correct

  optional func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat  // error: 'optional' requirements are an Objective-C compatibility feature; add '@objc'
}

Impact on existing code

Code that declares @objc protocols with optional requirements will need to be changed to add the @objc attribute. However, it is trivial for the migrator to update the code and for the compiler to provide Fix-Its, so the actual impact on users should be small. Moreover, explicitly writing @objc on optional requirements has always been permitted.

Alternatives considered

It's a fairly common request to make optional requirements work in Swift protocols (as in the aforementioned threads). However, such proposals have generally met with resistance because optional requirements have significant overlap with other protocol features: "default" implementations via protocol extensions and protocol inheritance. For the former case, the author of the protocol can provide a "default" implementation via a protocol extension that encodes the default case (rather than putting it at the call site). In the latter case, the protocol author can separate the optional requirements into a different protocol that a type can adopt to opt-in to whatever behavior they customize. While not exactly the same as optional requirements, which allow one to perform per-requirement checking to determine whether the type implemented that requirement, the gist of the threads is that doing so is generally considered an anti-pattern: one would be better off factoring the protocol in a different way. Therefore, we do not propose to make optional requirements work for Swift protocols.

The second alternative would be to eliminate optional requirements entirely from the language. The primary challenge here is Cocoa interoperability, because Cocoa's protocols (primarily delegates and data sources) have a large number of optional requirements that would have to be handled somehow in Swift. These optional requirements would have to be mapped to some other construct in Swift, but the code generation model must remain the same because the Cocoa frameworks rely on the ability to ask the question "was this requirement implemented by the type?" in Objective-C code at run time.

The most popular approach to try to map optional requirements into existing Swift constructs is to turn an optional method requirement into a property of optional closure type. For example, this Objective-C protocol:

@protocol NSTableViewDelegate
@optional
- (nullable NSView *)tableView:(NSTableView *)tableView viewForTableColumn:(NSTableColumn *)tableColumn row:(NSInteger)row;
- (CGFloat)tableView:(NSTableView *)tableView heightOfRow:(NSInteger)row;
@end

which currently imports into Swift as:

@objc protocol NSTableViewDelegate {
  optional func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int) -> NSView?
  optional func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat
}

would become, e.g.,

@objc protocol NSTableViewDelegate {
  var tableView: ((NSTableView, viewFor: NSTableColumn, row: Int) -> NSView?)? { get }
  var tableView: ((NSTableView, heightOfRow: Int) -> CGFloat)? { get }
}

Unfortunately, this introduces an overloaded property named tableView. To really make this work, we would need to introduce the ability for a property to have a compound name, which would also let us take the labels out of the function type:

@objc protocol NSTableViewDelegate {
  var tableView(_:viewFor:row:): ((NSTableView, NSTableColumn, Int) -> NSView?)? { get }
  var tableView(_:heightOfRow:): ((NSTableView, Int) -> CGFloat)? { get }
}

By itself, that is a good feature. However, we're not done, because we would need yet another extension to the language: one would want to be able to provide a method in a class that is used to conform to a property in the protocol, e.g.,

class MyDelegate : NSObject, NSTableViewDelegate {
  func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int) -> NSView? { ... }
  func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat { ... }
}

Indeed, the Objective-C implementation model effectively requires us to satisfy these property-of-optional-closure requirements with methods so that Objective-C clients can use -respondsToSelector:. In other words, one would not be able to implement these requirements in by copy-pasting from the protocol to the implementing class:

class MyDelegate : NSObject, NSTableViewDelegate {
  // Note: The Objective-C entry points for these would return blocks, which is incorrect
  var tableView(_:viewFor:row:): ((NSTableView, NSTableColumn, Int) -> NSView?)? { return ...   }
  var tableView(_:heightOfRow:): ((NSTableView, Int) -> CGFloat)? { return ... }
}

That is both a strange technical restriction that would be limited to Objective-C protocols and a serious usability problem: the easiest way to stub out the contents of your type when it conforms to a given protocol is to copy the declarations from the protocol into your type, then fill in the details. This change would break that usage scenario badly.

There have been other ideas to eliminate optional requirements. For example, Objective-C protocols could be annotated with attributes that say what the default implementation for each optional requirement is (to be used only in Swift), but such a massive auditing effort is impractical. There is a related notion of caller-site default implementations that was not well-received due to its complexity.

Initially, this proposal introduce a new keyword objcoptional. However, that keyword was really ugly. Thank you to Xiaodi Wu for the suggestion to require an explicit @objc!