2.下标支持泛型

1. 下标支持泛型

struct GenericDictionary<Key: Hashable, Value> {
    private var data: [Key: Value]
    
    init(data: [Key: Value]) {
        self.data = data
    }
    
    subscript<T>(key: Key) -> T? {
        return data[key] as? T
    }
    
}

使用：

let earth = GenericDictionary(data: ["name": "地球", "population": 1000, "moons": 1])

let name: String? = earth["name"]
let popu: Int? = earth["population"]
print(name,popu) // Optional("地球") Optional(1000)

下标类型同样支持泛型

extension GenericDictionary {
    subscript<Keys: Sequence>(keys: Keys) -> [Value] where Keys.Iterator.Element == Key {
        var values: [Value] = []
        for key in keys {
            if let value = data[key] {
                values.append(value)
            }
        }
        
        return values
    }
}

let nameAndMoons = earth[["name","moons"]]
print(nameAndMoons) // ["地球", 1]
let nameAndMoons2 = earth[Set(["moons","name"])]
print(nameAndMoons2) // ["地球", 1]