swift - 如何声明 Dictionary 符合协议(protocol)

Question

我已经定义了一个协议(protocol):

public protocol VariableTable {
    subscript(key:String) -> Decimal? { get set }
}

这仅仅表示 VariableTable必须为 String->Decimal 提供下标运算符。

显然，Dictionary<String, Decimal>符合那个要求。我如何让编译器知道这一点？

extension Dictionary<String, Decimal> : VariableTable {}

产量:

Constrained extension must be declared on the unspecialized generic type 'Dictionary' with constraints specified by a 'where' clause

其中:

extension Dictionary : VariableTable where Key == String, Value == Decimal {}

或:

 extension Dictionary : VariableTable where Element == (String, Decimal) {}

导致错误:

Extension of type 'Dictionary' with constraints cannot have an inheritance clause

Answer 1

这在 Swift 3.0 中是不可能的。

但是如果您只关心这个 VariableTable 下标，您可以将字典包装在另一种符合协议(protocol)的类型中，例如:

public protocol VariableTableProtocol {
    subscript(key:String) -> Decimal? { get set }
}

final class VariableTable: VariableTableProtocol {
    fileprivate var dictionary: [String: Decimal] = [:]

    subscript(key: String) -> Decimal? {
       get {
           return dictionary[key]
       }
       set {
          dictionary[key] = newValue
       }
    }
}