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Bridge Numeric Types to NSNumber and Cocoa Structs to NSValue

Introduction

A handful of Swift numeric types are bridged to NSNumber when passed into Objective-C object contexts. We should extend this bridging behavior to all Swift numeric types. We should also bridge common Cocoa structs such as NSRange by boxing them into NSValue objects.

Swift-evolution thread: here

Motivation

SE-0116 changed how Objective-C's id and untyped collections import into Swift to use the Any type. This makes it much more natural to pass in Swift value types such as String and Array, but introduces the hazard of passing in types that don't bridge well to Objective-C objects. Particularly problematic are number types; whereas Int, UInt, and Double will automatically bridge as NSNumber, other-sized numeric types fall back to opaque boxing:

let i = 17
let plist = ["seventeen": i]
// OK
try! JSONSerialization.data(withJSONObject: plist)

let j: UInt8 = 38
let brokenPlist = ["thirty-eight": j]
// Will throw because `j` didn't bridge to a JSON type
try! JSONSerialization.data(withJSONObject: brokenPlist)

We had shied away from enabling this bridging for all numeric types in the Swift 1.x days, among other reasons because we allowed implicit bridging conversions in both directions from Swift value types to NS objects and back, which meant that you could slowly and brokenly convert between any two numeric types transitively via NSNumber if we allowed this. We killed the implicit conversions completely with SE-0072 so that is no longer a concern, so expanding the bridging behavior should no longer be a major problem, since it must now always be explicitly asked for.

There are also many Cocoa APIs that accept NSArray and NSDictionary objects with members that are NSValue-boxed structs. Matt Neuberg highlights Core Animation as an example in this bug report. With id-as-Any, it's natural to expect this to work:

anim.toValue = CGPoint.zero

However, the CGPoint value does not box as a meaningful Objective-C object, so this currently breaks Core Animation at runtime despite compiling successfully. It would be more idiomatic to bridge these types to NSValue.

Proposed solution

All of Swift's number types should be made to bridge to NSNumber when used as objects in Objective-C:

  • Int8
  • Int16
  • Int32
  • Int64
  • UInt8
  • UInt16
  • UInt32
  • UInt64
  • Float
  • Double

Cocoa structs with existing NSValue factory and property support should be made to bridge to NSValue when used as objects:

  • NSRange
  • CGPoint
  • CGVector
  • CGSize
  • CGRect
  • CGAffineTransform
  • UIEdgeInsets
  • UIOffset
  • CATransform3D
  • CMTime
  • CMTimeRange
  • CMTimeMapping
  • MKCoordinate
  • MKCoordinateSpan
  • SCNVector3
  • SCNVector4
  • SCNMatrix4

Detailed design

Bridged NSNumber and NSValue objects must be castable back to their original Swift value types. NSValue normally preserves the type information of its included struct in its objCType property. We can check the objCType of an NSValue instance when attempting to cast back to a specific bridged struct type. Note that, although NSValue has factory initializers and accessors for each of the above struct types, the bridging implementation ought to stick to NSValue's core valueWithBytes:objCType: and getValue: API, to avoid potential availability issues with the type-specific methods.

NSNumber is a bit trickier, since Cocoa's implementation does not generally guarantee to remember the exact number type an instance was constructed from. When we bridge Swift number types to NSNumber, though, we use specific NSNumber subclasses to preserve the original Swift type, and in these cases we can check the exact Swift type in dynamic casts. For NSNumbers from Cocoa, we can say that casting an NSNumber to a Swift number type succeeds if the value of the NSNumber is exactly representable as the target type. This is imperfect, since it means that an NSNumber can potentially be cast to a different type from the original value.

Impact on existing code

This change has no static source impact, but changes the dynamic behavior of the Objective-C bridge. From Objective-C's perspective, values that used to bridge as opaque objects will now come in as semantically meaningful Objective-C objects. This should be a safe change, since existing code should not be relying on the behavior of opaque bridged objects. From Swift's perspective, values should still be able to round-trip from concrete number and struct types to Any to id to Any and back by dynamic casting. The ability to reliably distinguish the exact number type that an NSNumber was constructed from would be lost.

Alternatives considered

We can of course do nothing and leave the behavior as-is.

NSValue also carries factories for valueWithPointer: and valueWithNonretainedObject:. Maybe we could bridge UnsafePointer and Unmanaged this way, but we probably shouldn't.

Instead of implementing NSValue bridging in the overlay, Zach Waldowski suggests using Objective-C's __attribute__((objc_boxable)), which enables autoboxing of a struct in ObjC with @(...) syntax, to also instruct Swift's Clang importer to synthesize a bridge to NSValue automatically for types annotated with the attribute. However, this attribute hasn't been widely adopted in Apple SDKs.