Swift - 暂时将模型切换为只读

Question

我有一个电话号码模型，如下所示：

import UIKit
import Foundation

struct PhoneValidation : OptionSet {
    let rawValue: Int
    
    static let phoneInValid = PhoneValidation(rawValue: 1 << 0)
    static let phoneValid = PhoneValidation(rawValue: 1 << 1)
    static let smsValidationAttempted = PhoneValidation(rawValue: 1 << 2)
    static let smsValidationFailed = PhoneValidation(rawValue: 1 << 3)
    static let smsValidationSuccessful = PhoneValidation(rawValue: 1 << 4)      // OTP is successfully validated in backend. The field should be non-editable in this duration
    static let smsValidationOTPTriggered = PhoneValidation(rawValue: 1 << 5)    // OTP validation triggered. The field should be non-editable in this duration
}

class PhonesViewModel: NSCopying {

    public var phoneType: PhoneNumberType = PhoneNumberType.mobile
    
    public var phone: String?
    
    public var code: String?
    
    public var countryCode: String?
    
    public var isValid : PhoneValidation?
    
    func copy(with zone: NSZone? = nil) -> Any {
        let copy = PhonesViewModel()
        copy.phoneType = phoneType
        copy.phone = phone
        copy.code = code
        copy.countryCode = countryCode
        copy.isValid = isValid
        return copy
    }
}

正如您在上面看到的，手机模型可以在不同状态之间转换。 SMS 验证适用于少数国家，少数国家不适用。因此，我计划在 SMS 验证适用于某个国家/地区并且正在进行验证时设置 smsValidationOTPTriggered 状态。

我需要的是，在设置状态 smsValidationOTPTriggered 或 smsValidationSuccessful 时，我不希望应用程序的任何模块修改模型的值（phoneType、phone、code、countryCode）。换句话说，我希望模型在模型中设置这两种状态时切换到只读模式，并希望在尝试修改时通知模块错误或异常。

对于我在这里想要实现的目标，是否已有最佳实践可用？在提出这个问题之前我已经搜索过，但没有找到。我怎样才能做到这一点？

谢谢， 拉吉·帕万·古姆达尔

Answer 1

您可以使用名为“popsicle immutability”的技术。对象最初是可变的，但可以“冻结”。禁止对冻结对象进行修改。在您的情况下，当 isValid 属性具有值 smsValidationOTPTriggered 或 smsValidationSuccessful 时，PhonesViewModel 被冻结。

让我们添加Freezable 协议，以满足对象的要求，这些对象可以变得不可变并符合PhonesViewModel：

protocol Freezable: class {
    var isFrozen: Bool { get }
}

extension PhonesViewModel: Freezable {
    var isFrozen: Bool {
        isValid == .smsValidationOTPTriggered || isValid == .smsValidationSuccessful
    }
}

现在我们必须在分配属性时为isFrozen 值添加验证。它可以添加到属性观察器中，例如：

...
public var phone: String? {
    didSet {
        validate()
    }
}
...
private func validate() {
    assert(!isFrozen)
}

或者使用属性包装器：

@propertyWrapper
struct Guarded<Value> {
    private var value: Value

    init(wrappedValue: Value) {
        value = wrappedValue
    }

    @available(*, unavailable)
    var wrappedValue: Value {
        get { fatalError("only works on instance properties of classes that conforms to Freezable protocol") }
        set { fatalError("only works on instance properties of classes that conforms to Freezable protocol") }
    }

    static subscript<EnclosingSelf: Freezable>(
        _enclosingInstance object: EnclosingSelf,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Value>,
        storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Self>
    ) -> Value {
        get {
            object[keyPath: storageKeyPath].value
        }
        set {
            precondition(!object.isFrozen, "Object \(object) is frozen! Modifications are forbidden")
            object[keyPath: storageKeyPath].value = newValue
        }
    }
}

所以你的班级看起来像：

class PhonesViewModel: NSCopying {

    @Guarded
    public var phoneType: PhoneNumberType = PhoneNumberType.mobile

    @Guarded
    public var phone: String?

    @Guarded
    public var code: String?

    @Guarded
    public var countryCode: String?

    @Guarded
    public var isValid : PhoneValidation?

    func copy(with zone: NSZone? = nil) -> Any {
        let copy = PhonesViewModel()
        copy.phoneType = phoneType
        copy.phone = phone
        copy.code = code
        copy.countryCode = countryCode
        copy.isValid = isValid
        return copy
    }
}

Answer 2

这样的事情怎么样，我认为最好为您的情况使用属性包装器！以下不是一个确切的解决方案，但可以修改/更改以满足您的需要 导入 UIKit

 enum PhoneNumberType {
    case mobile
}

enum PhoneValidation {
    case phoneInValid
    case phoneValid
    case smsValidationAttempted
    case smsValidationFailed
    case smsValidationSuccessful
    case smsValidationOTPTriggered
}

struct PhonesViewModel {
    public var phoneType: PhoneNumberType = PhoneNumberType.mobile
    public var phone: String?
    public var code: String?
    public var countryCode: String?
    public var phoneValidation : PhoneValidation?
    
    func validate(value: [PhoneValidation]) -> Bool {
        //add proper check here
        return false 
    }
}


@propertyWrapper
struct Wrapper {
    private(set) var value: PhonesViewModel? = nil
    var validators: [PhoneValidation] = []
    
    var wrappedValue: PhonesViewModel? {
        get { value }
        set {
            if let model = newValue, model.validate(value: validators) {
                value = newValue
                print("Value assigned")
            } else {
                print("Value not assigned")
            }
        }
    }
}

struct SomeOtherClass {
    @Wrapper(validators: [PhoneValidation.phoneInValid])
    var model: PhonesViewModel?
}

var a = SomeOtherClass()
a.model = PhonesViewModel()
a.model = PhonesViewModel()