对于(U)Int8 / 16/32/64类型,是否可以将Swifts自动数值桥接复制到Foundation(NSNumber)？

(以下答案基于使用Swift 2.2和XCode 7.3.)

正如我在思考是否发布或只是跳过这个问题一样,我在Swift源代码中偶然发现了swift/stdlib/public/core/BridgeObjectiveC.swift,特别是协议_ObjectiveCBridgeable.我之前在Swiftdoc.org时曾经简要地注意过该协议,但是在后者的当前(空)蓝图中,我从来没有考虑过它.使用来自Swift源的_ObjectiveCBridgeable的蓝图,我们可以迅速让一些自定义类型的本地符合它.

在继续之前,请注意_ObjectiveCBridgeable是一个内部/隐藏协议(_UnderscorePreFixedProtocol),因此在即将推出的Swift版本中,基于它的解决方案可能会在没有警告的情况下中断.

启用Int64桥接到Foundation类NSnumber

作为示例,扩展Int64以符合_ObjectiveCBridgeable,并随后测试这个非常简单的修复是否足以从Int64到Foundation类NSnumber保持的隐式类型转换(桥接).

import Foundation

extension Int64: _ObjectiveCBridgeable {

    public typealias _ObjectiveCType = NSnumber

    public static func _isBridgedToObjectiveC() -> Bool {
        return true
    }

    public static func _getObjectiveCType() -> Any.Type {
        return _ObjectiveCType.self
    }

    public func _bridgeToObjectiveC() -> _ObjectiveCType {
        return NSnumber(longLong: self)
    }

    public static func _forceBridgeFromObjectiveC(source: _ObjectiveCType,inout result: Int64?) {
        result = source.longLongValue
    }

    public static func _conditionallyBridgeFromObjectiveC(source: _ObjectiveCType,inout result: Int64?) -> Bool {
        self._forceBridgeFromObjectiveC(source,result: &result)
        return true
    }
}

测试：

/* Test case: scalar */
let fooInt: Int = 42
let fooInt64: Int64 = 42
var fooAnyObj : AnyObject

fooAnyObj = fooInt    // OK,natively
fooAnyObj = fooInt64  // OK! _ObjectiveCBridgeable confoRMANce successful

/* Test case: array */
let fooIntArr: [Int] = [42,23]
let fooInt64Arr: [Int64] = [42,23]
var fooAnyObjArr : [AnyObject]

fooAnyObjArr = fooIntArr    // OK,natively
fooAnyObjArr = fooInt64Arr  // OK! _ObjectiveCBridgeable confoRMANce successful

因此,与_ObjectiveCBridgeable的一致性确实足以启用自动的分配桥接到相应的Foundation类;在这种情况下,NSnumber(在Swift中,__ NSCFnumber).

启用Int8,UInt8,Int16,UInt16,Int32,(Int64)和UInt64桥接到NSnumber

使用下面的NSnumber转换表,可以很容易地修改Int64到_ObjectiveCBridgeable的上述一致性,以涵盖任何Swift原生整数类型.

/* NSnumber initializer:               NSnumber native Swift type property
   --------------------------------    -----------------------------------
   init(char: <Int8>)                  .charValue
   init(unsignedChar: <UInt8>)         .unsignedCharValue
   init(short: <Int16>)                .shortValue
   init(unsignedShort: <UInt16>)       .unsignedShortValue
   init(int: <Int32>)                  .intValue
   init(unsignedInt: <UInt32>)         .unsignedIntValue
   init(longLong: <Int64>)             .longLongValue
   init(unsignedLongLong: <UInt64>)    .unsignedLongLongValue              */