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如何定义协议扩展中可设置和获取的变量

computed-properties swift-protocols swift

1

Leem · 技术社区 · 6 年前

我想在Swift中实现抽象类,我知道在Swift中没有抽象类的概念。但我知道我们可以通过使用协议在Swift中模仿这个概念。例如,我试过:

// With protocol, I can define functions that concrete class have to implement
protocol ProductProvider {
   func getProductNumber() -> Int
}

// with protocol extension, I can define shared (computed) properties and functions among concrete classes that comply with this protocol
extension ProductProvider {
  var maxProductCount: Int {
        return 10
  }
}

extension ProductProvider {
  var maxProductCount: Int {
        set(newValue) {
           // set to what if I couldn't define private stored variable in extension to hold the most recent set value ?
        }
        get{
           // how to return the most recent value set?
        }
  }
}

我的问题在上述代码的注释中。如何在swift4协议扩展中设置和获取变量?如果这是不可能的,什么是可行的变通办法?

2 回复 | 直到 6 年前

1

2

Oni_01 5 年前

举个例子:

protocol AbstractObject {

    var maxProductCount: Int { get set }
}

struct ConformObject: AbstractObject {

    var maxProductCount: Int
}

所以现在可以在默认实现中使用变量

extension AbstractObject {

    mutating func addOne() -> Int {
        self.maxProductCount += 1
        return self.maxProductCount
    }
}

2

1

Ehsan Saddique 6 年前

除了讨论它是否是实现所需目标的正确方法之外,还可以使用对象关联。

public final class ObjectAssociation<T: AnyObject> {

    private let policy: objc_AssociationPolicy

    /// - Parameter policy: An association policy that will be used when linking objects.
    public init(policy: objc_AssociationPolicy = .OBJC_ASSOCIATION_RETAIN_NONATOMIC) {

        self.policy = policy
    }

    /// Accesses associated object.
    /// - Parameter index: An object whose associated object is to be accessed.
    public subscript(index: AnyObject) -> T? {

        get { return objc_getAssociatedObject(index, Unmanaged.passUnretained(self).toOpaque()) as! T? }
        set { objc_setAssociatedObject(index, Unmanaged.passUnretained(self).toOpaque(), newValue, policy) }
    }
}

extension SomeType {

    private static let association = ObjectAssociation<NSObject>()

    var simulatedProperty: NSObject? {

        get { return SomeType.association[self] }
        set { SomeType.association[self] = newValue }
    }
}

资料来源: https://stackoverflow.com/a/43056053/1811810