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社区首页 >问答首页 >用开关控制流从函数推断返回类型

问用开关控制流从函数推断返回类型
EN

Stack Overflow用户
提问于 2019-02-23 20:38:16
回答 1查看 59关注 0票数 1

我有一套属性/协议(背景是here，但我认为它是多余的)

类类型如下所示：

代码语言：javascript
复制
struct AdjustmentTypes {
    internal class BaseType<T>: Hashable {

        static func == (lhs: AdjustmentTypes.BaseType<T>, rhs: AdjustmentTypes.BaseType<T>) -> Bool {
            return lhs.name == rhs.name
        }

        typealias A = T

        var hashValue: Int { return name.hashValue }

        let name: String
        let defaultValue: T
        let min: T
        let max: T
        var value: T

        init(name: String, defaultValue: T, min: T, max: T) {
            self.name = name
            self.defaultValue = defaultValue
            self.min = min
            self.max = max
            self.value = defaultValue
        }
    }

    class FloatType: BaseType<CGFloat> { }

    class IntType: BaseType<Int> { }
}

我使用类型擦除来删除类型，这样我就可以将它们存储在Set中，并且我已经构建了一些帮助方法来简化我的Set工具：

代码语言：javascript
复制
class AdjustmentsSet {

    private var adjustmentsSet: Set<AnyHashable> = []

    func insert(_ adjustment: AnyHashable) {
        adjustmentsSet.insert(adjustment)
    }

    func remove(_ adjustment: AnyHashable) {
        adjustmentsSet.remove(adjustment)
    }

    func contains(_ adjustment: AnyHashable) -> Bool {
        return adjustmentsSet.contains(adjustment)
    }

    var count: Int { return adjustmentsSet.count }
}

var adjustmentsSet = AdjustmentsSet()

我现在要做的是向我的Set管理类添加一些帮助程序，以便能够检索具有正确类型的属性，例如，如果我这样做了：

代码语言：javascript
复制
let brightness = Brightness().make()
adjustments.get(brightness)

它应该返回nil，但是如果我返回了：

代码语言：javascript
复制
adjustments.insert(brightness)
adjustments.get(brightness)

现在我应该将值取回，作为它的正确类型AdjustmentTypes.FloatType。

我想到了这样的一个Switch语句：

代码语言：javascript
复制
class AdjustmentsSet {

    // ...

    func get(_ adjustment: AnyHashable) -> Any? {
        guard let untyped = adjustmentsSet.first(where: { $0 == adjustment }) else { return nil }
        switch adjustment {
        case _ as AdjustmentTypes.FloatType: return untyped as! AdjustmentTypes.FloatType
        case _ as AdjustmentTypes.IntType: return untyped as! AdjustmentTypes.IntType
        default: return nil
        }
    }
}

但是，当然，致命的缺陷是返回Any，而不是预期的类型。

如何推断返回值的类型并返回正确的类型？

完整的例子，只需把它放在操场上：

代码语言：javascript
复制
// Generic conforming protocol to AnyHashable
protocol AnyAdjustmentProtocol {
    func make() -> AnyHashable
}

protocol AdjustmentProtocol: AnyAdjustmentProtocol {
    associatedtype A
    func make() -> A
}

struct AdjustmentTypes {
    internal class BaseType<T>: Hashable {

        static func == (lhs: AdjustmentTypes.BaseType<T>, rhs: AdjustmentTypes.BaseType<T>) -> Bool {
            return lhs.name == rhs.name
        }

        typealias A = T

        var hashValue: Int { return name.hashValue }

        let name: String
        let defaultValue: T
        let min: T
        let max: T
        var value: T

        init(name: String, defaultValue: T, min: T, max: T) {
            self.name = name
            self.defaultValue = defaultValue
            self.min = min
            self.max = max
            self.value = defaultValue
        }
    }

    class FloatType: BaseType<CGFloat> { }

    class IntType: BaseType<Int> { }
}

struct AnyAdjustmentType<A>: AdjustmentProtocol, Hashable {
    static func == (lhs: AnyAdjustmentType<A>, rhs: AnyAdjustmentType<A>) -> Bool {
        return lhs.hashValue == rhs.hashValue
    }

    private let _make: () -> AnyHashable
    private let hashClosure:() -> Int

    var hashValue: Int {
        return hashClosure()
    }

    init<T: AdjustmentProtocol & Hashable>(_ adjustment: T) where T.A == A {
        _make = adjustment.make
        hashClosure = { return adjustment.hashValue }
    }
    func make() -> AnyHashable {
        return _make()
    }
}

struct Brightness: AdjustmentProtocol, Hashable {
    func make() -> AnyHashable {
        return AdjustmentTypes.FloatType(name: "Brightness", defaultValue: 0, min: 0, max: 1)
    }
}
struct WhiteBalance: AdjustmentProtocol, Hashable {
    func make() -> AnyHashable {
        return AdjustmentTypes.IntType(name: "White Balance", defaultValue: 4000, min: 3000, max: 7000)
    }
}

let brightness = Brightness().make()
let whiteBalance = WhiteBalance().make()

class AdjustmentsSet {

    private var adjustmentsSet: Set<AnyHashable> = []

    func insert(_ adjustment: AnyHashable) {
        adjustmentsSet.insert(adjustment)
    }

    func remove(_ adjustment: AnyHashable) {
        adjustmentsSet.remove(adjustment)
    }

    func contains(_ adjustment: AnyHashable) -> Bool {
        return adjustmentsSet.contains(adjustment)
    }

    var count: Int { return adjustmentsSet.count }
}

var adjustmentsSet = AdjustmentsSet()
ios
swift
generics
casting
EN

回答 1

Stack Overflow用户

回答已采纳

发布于 2019-02-23 21:59:26

您需要将该方法重写为泛型，并使用足够的类型信息调用它，也就是说，您需要预先知道希望该方法返回的类型。

我也不确定AnyHashables是否是理想的。没有什么可以阻止您添加String、Ints和其他可用于调整集的随机类型。

代码语言：javascript
复制
var adjustmentsSet = AdjustmentsSet()
adjustmentsSet.insert("1") // compiles just fine!

或者，您可以使用和传递您的AdjustmentTypes并用泛型方法重写AdjustmentsSet类：

代码语言：javascript
复制
class AdjustmentsSet {

    private var adjustmentsSet: Set<AnyHashable> = []

    func insert<T>(_ adjustment: AdjustmentTypes.BaseType<T>) {
        adjustmentsSet.insert(adjustment)
    }

    func remove<T>(_ adjustment: AdjustmentTypes.BaseType<T>) {
        adjustmentsSet.remove(adjustment)
    }

    func contains<T>(_ adjustment: AdjustmentTypes.BaseType<T>) -> Bool {
        return adjustmentsSet.contains(adjustment)
    }

    func get<T>(_ adjustment: AdjustmentTypes.BaseType<T>) -> AdjustmentTypes.BaseType<T>? {
        return (adjustmentsSet.compactMap { $0 as? AdjustmentTypes.BaseType<T> }).first(where: { $0 == adjustment })
    }

    var count: Int { return adjustmentsSet.count }
}

接下来，make()方法也应该是更强类型的，因为您没有传递AnyHashables。我实现了这样的亮度和白平衡：

代码语言：javascript
复制
extension AdjustmentTypes {
    static let Brightness = AdjustmentTypes.FloatType(name: "Brightness", defaultValue: 0, min: 0, max: 1)
    static let WhiteBalance = AdjustmentTypes.IntType(name: "White Balance", defaultValue: 4000, min: 3000, max: 7000)
}

还利用Swift中的类型别名和结构，使您的调整类型系统具有价值语义：

代码语言：javascript
复制
struct AdjustmentTypes {

    struct BaseType<T>: Hashable {

        static func == (lhs: AdjustmentTypes.BaseType<T>, rhs: AdjustmentTypes.BaseType<T>) -> Bool {
            return lhs.name == rhs.name
        }

        typealias A = T

        var hashValue: Int { return name.hashValue }

        let name: String
        let defaultValue: T
        let min: T
        let max: T
        var value: T

        init(name: String, defaultValue: T, min: T, max: T) {
            self.name = name
            self.defaultValue = defaultValue
            self.min = min
            self.max = max
            self.value = defaultValue
        }
    }

    typealias FloatType = BaseType<CGFloat>
    typealias IntType = BaseType<Int>
}

最后，您可以按预期使用调整集：

代码语言：javascript
复制
var brightness = AdjustmentTypes.Brightness
brightness.value = 0.5

var adjustmentsSet = AdjustmentsSet()
adjustmentsSet.insert(brightness)

let retrievedBrightness = adjustmentsSet.get(AdjustmentTypes.Brightness)! // strongly typed!
retrievedBrightness.value // 0.5
AdjustmentTypes.Brightness.value // 0.0

整个操场：

代码语言：javascript
复制
struct AdjustmentTypes {

    struct BaseType<T>: Hashable {

        static func == (lhs: AdjustmentTypes.BaseType<T>, rhs: AdjustmentTypes.BaseType<T>) -> Bool {
            return lhs.name == rhs.name
        }

        typealias A = T

        var hashValue: Int { return name.hashValue }

        let name: String
        let defaultValue: T
        let min: T
        let max: T
        var value: T

        init(name: String, defaultValue: T, min: T, max: T) {
            self.name = name
            self.defaultValue = defaultValue
            self.min = min
            self.max = max
            self.value = defaultValue
        }
    }

    typealias FloatType = BaseType<CGFloat>
    typealias IntType = BaseType<Int>
}

extension AdjustmentTypes {
    static let Brightness = AdjustmentTypes.FloatType(name: "Brightness", defaultValue: 0, min: 0, max: 1)
    static let WhiteBalance = AdjustmentTypes.IntType(name: "White Balance", defaultValue: 4000, min: 3000, max: 7000)
}

class AdjustmentsSet {

    private var adjustmentsSet: Set<AnyHashable> = []

    func insert<T>(_ adjustment: AdjustmentTypes.BaseType<T>) {
        adjustmentsSet.insert(adjustment)
    }

    func remove<T>(_ adjustment: AdjustmentTypes.BaseType<T>) {
        adjustmentsSet.remove(adjustment)
    }

    func contains<T>(_ adjustment: AdjustmentTypes.BaseType<T>) -> Bool {
        return adjustmentsSet.contains(adjustment)
    }

    func get<T>(_ adjustment: AdjustmentTypes.BaseType<T>) -> AdjustmentTypes.BaseType<T>? {
        return (adjustmentsSet.compactMap { $0 as? AdjustmentTypes.BaseType<T> }).first(where: { $0 == adjustment })
    }

    var count: Int { return adjustmentsSet.count }
}

var brightness = AdjustmentTypes.Brightness
brightness.value = 0.5

var adjustmentsSet = AdjustmentsSet()
adjustmentsSet.insert(brightness)

let retrievedBrightness = adjustmentsSet.get(AdjustmentTypes.Brightness)! // strongly typed!
retrievedBrightness.value // 0.5
AdjustmentTypes.Brightness.value // 0.0

希望这有帮助，祝你的项目好运！

票数 2
EN
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：

https://stackoverflow.com/questions/54845983

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