<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 9, 2016, at 12:26 PM, Russ Bishop via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=us-ascii" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">An official proposal PR has been opened on this: <a href="https://github.com/apple/swift-evolution/pull/198" class="">https://github.com/apple/swift-evolution/pull/198</a><div class=""><br class=""></div><div class="">It includes clarifications, a few changes, and some better examples.</div></div></div></blockquote><br class=""></div><div>FWIW, I’ve implemented much of this proposal as a series of compiler cleanups. It introduces a few tweaks to the _ObjectiveCBridgeable protocol that were necessary to generalize (and de-special-case) the NSString/NSArray/NSDictionary/NSSet bridging, but it’s still considered an implementation detail. So, some comments on the proposal itself…</div><div><br class=""></div><div><br class=""></div><div><pre style="box-sizing: border-box; overflow: auto; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; margin-top: 0px; margin-bottom: 16px; line-height: 1.45; padding: 16px; background-color: rgb(247, 247, 247); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-wrap: normal; color: rgb(51, 51, 51);" class=""><code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; padding: 0px; margin: 0px; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal; border: 0px; display: inline; line-height: inherit; word-wrap: normal;" class="">/// A type adopting `ObjectiveCBridgeable` will be exposed
/// to Objective-C as the type `ObjectiveCType`
public protocol ObjectiveCBridgeable: _ObjectiveCBridgeable {
associatedtype ObjectiveCType : AnyObject
associatedtype _ObjectiveCType = ObjectiveCType</code></pre><div class="">I think we should just say that ObjectiveCBridgeable replaces _ObjectiveCBridgeable, and only have the first associated type. (You actually wanted a typealias anyway, I think).</div><div class=""><br class=""></div><div class=""><pre style="box-sizing: border-box; overflow: auto; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; margin-top: 0px; margin-bottom: 16px; line-height: 1.45; padding: 16px; background-color: rgb(247, 247, 247); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-wrap: normal; color: rgb(51, 51, 51);" class=""><code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; padding: 0px; margin: 0px; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal; border: 0px; display: inline; line-height: inherit; word-wrap: normal;" class=""> /// Returns `true` iff instances of `Self` can be converted to
/// Objective-C. Even if this method returns `true`, a given
/// instance of `Self._ObjectiveCType` may, or may not, convert
/// successfully to `Self`.
///
/// A default implementation returns `true`.
@warn_unused_result
static func isBridgedToObjectiveC() -> Bool</code></pre><div class="">It’s probably worth saying why someone might override this method: usually, it’s because the Swift type is generic and it only makes sense to bridge for some type arguments. Granted, there is no way to communicate this information to the compiler, which is a bit of a hole in the design. For example, Array<T> only bridges to NSArray when T is itself representable in Objective-C. We really need conditional conformances to for this part of the feature to work properly.</div></div><div class=""><br class=""></div><div class=""><pre style="box-sizing: border-box; overflow: auto; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; margin-top: 0px; margin-bottom: 16px; line-height: 1.45; padding: 16px; background-color: rgb(247, 247, 247); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-wrap: normal; color: rgb(51, 51, 51);" class=""><code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; padding: 0px; margin: 0px; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal; border: 0px; display: inline; line-height: inherit; word-wrap: normal;" class=""> /// Try to construct a value of the Self type from
/// an Objective-C object of the bridged class type.
///
/// If the conversion fails this initializer returns `nil`.
init?(bridgedFromObjectiveC: ObjectiveCType)</code></pre><div class="">FWIW, implementing this required a new “unconditional” entry point used by the compiler:</div></div><div class=""><br class=""></div><div class=""><div class=""> /// Bridge from an Objective-C object of the bridged class type to a</div><div class=""> /// value of the Self type.</div><div class=""> ///</div><div class=""> /// This bridging operation is used for unconditional bridging when</div><div class=""> /// interoperating with Objective-C code, either in the body of an</div><div class=""> /// Objective-C thunk or when calling Objective-C code, and may</div><div class=""> /// defer complete checking until later. For example, when bridging</div><div class=""> /// from `NSArray` to `Array<Element>`, we can defer the checking</div><div class=""> /// for the individual elements of the array.</div><div class=""> ///</div><div class=""> /// \param source The Objective-C object from which we are</div><div class=""> /// bridging. This optional value will only be `nil` in cases where</div><div class=""> /// an Objective-C method has returned a `nil` despite being marked</div><div class=""> /// as `_Nonnull`/`nonnull`. In most such cases, bridging will</div><div class=""> /// generally force the value immediately. However, this gives</div><div class=""> /// bridging the flexibility to substitute a default value to cope</div><div class=""> /// with historical decisions, e.g., an existing Objective-C method</div><div class=""> /// that returns `nil` to for "empty result" rather than (say) an</div><div class=""> /// empty array. In such cases, when `nil` does occur, the</div><div class=""> /// implementation of `Swift.Array`'s conformance to</div><div class=""> /// `_ObjectiveCBridgeable` will produce an empty array rather than</div><div class=""> /// dynamically failing.</div><div class=""> static func _unconditionallyBridgeFromObjectiveC(source: _ObjectiveCType?)</div><div class=""> -> Self</div></div><div class=""><br class=""></div><div class="">It can get a default implementation, and should be a non-failable initializer.</div><div class=""><br class=""></div><div class=""><pre style="box-sizing: border-box; overflow: auto; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; margin-top: 0px; margin-bottom: 16px; line-height: 1.45; padding: 16px; background-color: rgb(247, 247, 247); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-wrap: normal; color: rgb(51, 51, 51);" class=""><code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; padding: 0px; margin: 0px; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal; border: 0px; display: inline; line-height: inherit; word-wrap: normal;" class=""> static func _getObjectiveCType() -> Any.Type {
return ObjectiveCType.self
}</code></pre><div class="">This was an implementation hack; it’s gone now.</div></div><div class=""><br class=""></div><div class=""><ol style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""><li style="box-sizing: border-box;" class="">Expose the protocol <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ObjectiveCBridgeable</code><ol style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 0px; list-style-type: lower-roman;" class=""><li style="box-sizing: border-box;" class="">Any type adopting <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ObjectiveCBridgeable</code> will gain conformance to <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">_ObjectiveCBridgeable</code></li></ol></li></ol><div class="">As noted earlier, I think there should just be one protocol here, ObjectiveCBridgeable.</div><div class=""><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><span style="background-color: rgb(255, 255, 255);" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>4. </span><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">The </span><code style="color: rgb(51, 51, 51); box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ObjectiveCType</code><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""> </span><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">must be defined in Swift. If a</span><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""> </span><code style="color: rgb(51, 51, 51); box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">-swift.h</code><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""> </span><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">header is generated, it will include a</span><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""> </span><code style="color: rgb(51, 51, 51); box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">SWIFT_BRIDGED()</code><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">macro where the parameter indicates the Swift type with which the</span><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""> </span><code style="color: rgb(51, 51, 51); box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ObjectiveCType</code><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""> </span><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">bridges. The macro will be applied to the</span><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""> </span><code style="color: rgb(51, 51, 51); box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ObjectiveCType</code><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""> </span><span style="color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">and any subclasses.</span></div></div><div class=""><div class=""><br class=""></div></div><div class="">This is unnecessarily restrictive, and eliminates the “make my Objective-C class bridge into a Swift value type” case that (for example) the compiler already does for String/Array/Dictionary/Set. I think there are two cases:</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>(a) The ObjectiveCType is defined in Objective-C. It must be an Objective-C class with the attribute swift_bridge(“Bar”), where “Bar” is the name of the bridged Swift type.</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>(b) The ObjectiveCType is defined in Swift, in which case it must be an @objc class. When emitting the generated header, the appropriate swift_bridge attribute will be added to the @interface declaration.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><ol style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""><li style="box-sizing: border-box;" class="">(This was #5) It is an error for bridging to be ambiguous.</li><li style="box-sizing: border-box;" class=""><ol style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 0px; list-style-type: lower-roman;" class=""><li style="box-sizing: border-box;" class="">A Swift type may bridge to an Objective-C base class, then provide different subclass instances at runtime but no other Swift type may bridge to that base class or any of its subclasses.</li><li style="box-sizing: border-box;" class="">The compiler must emit a diagnostic when it detects two Swift types attempting to bridge to the same <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ObjectiveCType</code>.</li></ol></li></ol></div><div class="">This is a tricky area. Currently, Int/Float/Double/Bool/CGFloat/UInt all have _ObjectiveCBridgeable conformances, although those conformances only really kick in at runtime (e.g., when dynamically casting an [AnyObject] or [NSNumber] to [Int] or [Double] with as? or matching a switch case). They would run afoul of this rule. However, this rule does generally make sense: if two Swift types have the same ObjectiveCType, we won’t know how to map an Objective-C API back into Swift. Those numeric types only work because they are trivially mapped between Swift and (Objective-)C; they don’t need to go through the _ObjectiveCBridgeable conformance. </div><div class=""><br class=""></div><div class="">On the other hand, the greater ambiguity problem is if there are two conformances to ObjectiveCBridgeable on the same type; that’s already covered by Swift’s rules about multiple conformances.</div><div class=""><br class=""></div><div class=""><ol style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""><li style="box-sizing: border-box;" class="">(This was #6) The Swift type and <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ObjectiveCType</code> must be defined in the same module</li></ol></div></div><div>Yes, absolutely. If the ObjectiveCType comes from Objective-C, then it must come from the same-named Objective-C module.</div><div><br class=""></div><div>Under “Alternatives considered”, there is a typo “fesible”.</div><div><br class=""></div><div>I’ve been assuming we’re only allowed to bridge with Objective-C classes (whether they are defined in Swift or Objective-C), but Swift also has @objc enums now… I assume you don’t intend @objc enums to be part of this, so I think it makes sense to be explicit about the limitation to classes.</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Doug</div><div><br class=""></div><br class=""></body></html>