编码通用 String 对象给出 nil Swift

Question

我有一个 UserDefaults class 来处理存储、删除和提取存储对象的默认值。这是完整的 class，简洁明了，我相信：

现在问题出在存储函数上。我似乎无法对 Encodable String 对象进行编码。我知道我可以将该对象存储为默认值，但这会破坏此 MVDefaults 处理通用对象的目的。

我在这里遗漏了什么吗？

import Foundation

enum MVDefaultsKey: String {
    case requestToken = "defaultsRequestToken"
}

/// The class that has multiple class functions for handling defaults.
/// Also has the helper class functions for handling auth tokens.
class MVDefaults {

    // MARK: - Functions

    /// Stores token.
    class func store<T: Encodable>(_ object: T, key: MVDefaultsKey) {
        let encoder = JSONEncoder()
        let encoded = try? encoder.encode(object)
        UserDefaults.standard.set(encoded, forKey: key.rawValue)
    }

    /// Removes the stored token
    class func removeDefaultsWithKey(_ key: MVDefaultsKey) {
        UserDefaults.standard.removeObject(forKey: key.rawValue)
    }

    /// Returns stored token (optional) if any.
    class func getObjectWithKey<T: Decodable>(_ key: MVDefaultsKey, type: T.Type) -> T? {
        guard let savedData = UserDefaults.standard.data(forKey: key.rawValue) else {
            return nil
        }

        let object = try? JSONDecoder().decode(type, from: savedData)

        return object
    }
}

Answer 1

想想字符串 "hello" 编码为 JSON 会是什么样子。它看起来像：

"hello"

不会吧？

这不是有效的 JSON（根据 here）！您不能将字符串直接编码为 JSON，也不能直接将字符串解码。

比如这段代码

let string = try! JSONDecoder().decode(String.self, from: "\"hello\"".data(using: .utf8)!)

会产生错误

JSON text did not start with array or object and option to allow fragments not set.

和

let data = try! JSONEncoder().encode("Hello")

会产生错误：

Top-level String encoded as string JSON fragment.

此处的工作只是使用 UserDefaults 提供的专用 set 方法存储您的字符串。你仍然可以使用你的泛型方法，不过，你只需要检查类型并进行转换：

if let str = object as? String {
    UserDefaults.standard.set(str, forKey: key)
} else if let int = object as? Int {
    ...

Answer 2

虽然 Sweeper 的评论很有帮助并且应该是答案（因为没有他我将无法得出我自己的答案），但我仍然会继续分享我对默认值所做的工作 class，使其处理非 JSON 编码对象（例如 String、Int、Array 等）。

这里是：

MVDefaults.swift

import Foundation

enum MVDefaultsKey: String {
    case requestToken = "defaultsRequestToken"
    case someOtherKey = "defaultsKeyForTests"
}

/// The class that has multiple class functions for handling defaults.
/// Also has the helper class functions for handling auth tokens.
class MVDefaults {

    // MARK: - Functions

    /// Stores object.
    class func store<T: Encodable>(_ object: T, key: MVDefaultsKey) {
        let encoder = JSONEncoder()
        let encoded = try? encoder.encode(object)

        if encoded == nil {
            UserDefaults.standard.set(object, forKey: key.rawValue)
            return
        }

        UserDefaults.standard.set(encoded, forKey: key.rawValue)
    }

    /// Removes the stored object
    class func removeDefaultsWithKey(_ key: MVDefaultsKey) {
        UserDefaults.standard.removeObject(forKey: key.rawValue)
    }

    /// Returns stored object (optional) if any.
    class func getObjectWithKey<T: Decodable>(_ key: MVDefaultsKey, type: T.Type) -> T? {
        if let savedData = UserDefaults.standard.data(forKey: key.rawValue) {
            let object = try? JSONDecoder().decode(type, from: savedData)
            return object
        }

        return UserDefaults.standard.object(forKey: key.rawValue) as? T
    }
}

这是我为测试默认方法而编写的测试（规范）。全部通过！ ✅

MVDefaultsSpec.swift

@testable import Movieee
import Foundation
import Quick
import Nimble

class MVDefaultsSpec: QuickSpec {
    override func spec() {
        describe("Tests for MVDefaults") {

            context("Tests the whole MVDefaults.") {

                it("tests the store and retrieve function for any Codable object") {

                    let data = stubbedResponse("MovieResult")
                    expect(data).notTo(beNil())
                    let newMovieResult = try? JSONDecoder().decode(MovieResult.self, from: data!)

                    MVDefaults.store(newMovieResult, key: .someOtherKey)

                    let retrievedObject = MVDefaults.getObjectWithKey(.someOtherKey, type: MovieResult.self)

                    // Assert
                    expect(retrievedObject).notTo(beNil())

                    // Assert
                    expect(retrievedObject?.movies?.first?.id).to(equal(newMovieResult?.movies?.first?.id))
                }

                it("tests the store and retrieve function for String") {

                    let string = "Ich bin ein Berliner"

                    MVDefaults.store(string, key: .someOtherKey)

                    let retrievedObject = MVDefaults.getObjectWithKey(.someOtherKey, type: String.self)

                    // Assert
                    expect(retrievedObject).notTo(beNil())

                    // Assert
                    expect(retrievedObject).to(equal(string))
                }

                it("tests the removal function") {

                    MVDefaults.removeDefaultsWithKey(.someOtherKey)

                    let anyRetrievedObject = UserDefaults.standard.object(forKey: MVDefaultsKey.someOtherKey.rawValue)
                    let stringRetrievedObject = MVDefaults.getObjectWithKey(.someOtherKey, type: String.self)

                    // Assert
                    expect(anyRetrievedObject).to(beNil())

                    // Assert
                    expect(stringRetrievedObject).to(beNil())
                }

                it("tests the store and retrieve function for Bool") {

                    let someBool: Bool = true

                    MVDefaults.store(someBool, key: .someOtherKey)

                    let retrievedObject = MVDefaults.getObjectWithKey(.someOtherKey, type: Bool.self)

                    // Assert
                    expect(retrievedObject).to(equal(someBool))

                    // Assert
                    expect(retrievedObject).notTo(beNil())
                }
            }
        }
    }
}

编码通用 String 对象给出 nil Swift

Encoding generic String object gives nil Swift

generics

ios

swift

codable