Conversion operator in swift

问题

Is it possible to write custom conversion (casting) operator in swift? Especially I'm looking for enums conversion, ex:

enum MyEnum : Int {
    case Case1 = 0
    case Case2
    func __conversion() -> String { // doesn't work since Swift 1.0
        switch self {
        case Case1: return "Case 1"
        case Case2: return "Case 2"
        }
    }
}

let enumStr: String = MyEnum.Case1

Of course, I can convert to String with explicit method, but I would like to have implicit mechanism.

回答1:

Disclaimer/TL;DR! This answer pertains to the technical question as to whether we can possibly implement implicit bridging mechanisms between different Swift types ourself. The answer is: for some cases, yes, but only in a limited sense and by means of "hacks": do not use this is production code!

Swift internal protocol abuse: we may implement implicit mechanisms to Obj-C objects (e.g. `NSNumber`, `NSString` ...)

As MartinR writes in his comment, custom conversion methods are not present for (native) Swift.

For the technical discussion we can, however, (ab)use the internal protocol _ObjectiveCBridgeable to allow implicit bridging from your enum to Obj-C objects, in this case e.g. NSString. For a more detailed Q&A of the subject of the internal protocol _ObjectiveCBridgeable, see

Is it possible to replicate Swifts automatic numeric value bridging to Foundation (NSNumber) for (U)Int8/16/32/64 types?

Before proceeding, I'll quote a disclaimer from my answer in thread above:

... note that _ObjectiveCBridgeable is an internal/hidden protocol (_UnderScorePreFixedProtocol), so solutions based on it might break without warning in upcoming Swift versions.

Example #1: implementing implicit bridging of your enum to NSString

First lets add a failable initializer to your enum, allowing (attempted) initialization by String instances:

import Foundation

enum MyEnum: Int {
    case Case1 = 0
    case Case2

    init?(string: String) {
        switch string {
        case "Case 1": self = .Case1
        case "Case 2": self = .Case2
        default: return nil
        }
    }
}

Next up, let MyEnum conform to _ObjectiveCBridgeable, as described in more detail in the thread linked to above

extension MyEnum: _ObjectiveCBridgeable {

    typealias _ObjectiveCType = NSString

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

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

    func _bridgeToObjectiveC() -> _ObjectiveCType {
        return NSString(string: "Case \(self.rawValue+1)")
    }

    static func _forceBridgeFromObjectiveC(source: _ObjectiveCType, inout result: MyEnum?) {
        result = MyEnum(string: source as String)
    }

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

With the conformance above, we can now make use of implicit bridging from MyEnum instances to NSString

/* example usage */
var myCase: MyEnum = .Case1
var enumNSstr: NSString = myCase // MyEnum -> NSString, implicit

print(enumNSstr) // Case 1

enumNSstr = "Case 2"

// NSString -> MyEnum, by type conversion (castable)
myCase = (enumNSstr as MyEnum) ?? .Case1
print(myCase) // Case 2

Example #2: implementing implicit bridging of your enum to a custom Swift native type

We may even abuse the _ObjectiveCBridgeable protocol further, using its (deep backend) mechanisms to implement implicit bridging between two native Swift types, with the limitation that the type bridged to must be a reference type (specifically: instances of the type must be representable by AnyObject, hence the reference type limitation).

Let MyEnum be as defined above, but additionally, define a reference (class) type Foo, and conform MyEnum to _ObjectiveCBridgeable with the bridged to type, _ObjectiveCType being set to Foo.

class Foo {
    var bar: String
    init(bar: String) { self.bar = bar }
}

extension MyEnum: _ObjectiveCBridgeable {

    typealias _ObjectiveCType = Foo

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

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

    func _bridgeToObjectiveC() -> _ObjectiveCType {
        return Foo(bar: "Case \(self.rawValue+1)")
    }

    static func _forceBridgeFromObjectiveC(source: _ObjectiveCType, inout result: MyEnum?) {
        result = MyEnum(string: source.bar)
    }

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

We can now make use of implicit bridging from MyEnum instances to Foo

/* example usage */
var myCase: MyEnum = .Case1
var myFoo: Foo = myCase // MyEnum -> Foo, implicit
print(myFoo.bar) // Case 1

myFoo.bar = "Case 2"

// Foo -> MyEnum, by type conversion (castable)
myCase = (myFoo as? MyEnum) ?? .Case1
print(myCase) // Case 2

Finally note that you may, for any given type (say, MyEnum), naturally only implement implicit bridging to a single other (reference) type; since you can only conform to _ObjectiveCType once (for a unique type for the typealias _ObjectiveCType), otherwise yielding a compile time error for redundant protocol conformance.

The above is tested for Swift 2.2.

来源：https://stackoverflow.com/questions/38477905/conversion-operator-in-swift

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swift