在Swift 3中，Dictionary有一个keys属性。Keys有以下声明:

var keys: LazyMapCollection<Dictionary<Key, Value>, Key> { get }

仅包含字典键的集合。

注意，LazyMapCollection可以很容易地用Array的init(_:)初始化器映射到Array。

从NSDictionary到[String]

下面的iOS AppDelegate类片段展示了如何从NSDictionary中使用keys属性获得一个字符串数组([String]):

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    let string = Bundle.main.path(forResource: "Components", ofType: "plist")!
    if let dict = NSDictionary(contentsOfFile: string) as? [String : Int] {
        let lazyMapCollection = dict.keys
        
        let componentArray = Array(lazyMapCollection)
        print(componentArray)
        // prints: ["Car", "Boat"]
    }
    
    return true
}

从[String: Int]到[String]

下面的Playground代码以更通用的方式展示了如何使用keys属性从具有字符串键和整型值([String: Int])的字典中获得一个字符串数组([String]):

let dictionary = ["Gabrielle": 49, "Bree": 32, "Susan": 12, "Lynette": 7]
let lazyMapCollection = dictionary.keys

let stringArray = Array(lazyMapCollection)
print(stringArray)
// prints: ["Bree", "Susan", "Lynette", "Gabrielle"]

从[Int: String]到[String]

下面的Playground代码展示了如何从具有整型键和字符串值([Int: String])的字典中使用keys属性获得一个字符串数组([String]):

let dictionary = [49: "Gabrielle", 32: "Bree", 12: "Susan", 7: "Lynette"]
let lazyMapCollection = dictionary.keys
    
let stringArray = Array(lazyMapCollection.map { String($0) })
// let stringArray = Array(lazyMapCollection).map { String($0) } // also works
print(stringArray)
// prints: ["32", "12", "7", "49"]

来自Array Apple官方文档:

init(_:) -创建一个包含序列元素的数组。

宣言

Array.init<S>(_ s: S) where Element == S.Element, S : Sequence

参数

s -要转换为数组的元素序列。

讨论

You can use this initializer to create an array from any other type that conforms to the Sequence protocol...You can also use this initializer to convert a complex sequence or collection type back to an array. For example, the keys property of a dictionary isn’t an array with its own storage, it’s a collection that maps its elements from the dictionary only when they’re accessed, saving the time and space needed to allocate an array. If you need to pass those keys to a method that takes an array, however, use this initializer to convert that list from its type of LazyMapCollection<Dictionary<String, Int>, Int> to a simple [String].

func cacheImagesWithNames(names: [String]) {
    // custom image loading and caching
 }

let namedHues: [String: Int] = ["Vermillion": 18, "Magenta": 302,
        "Gold": 50, "Cerise": 320]
let colorNames = Array(namedHues.keys)
cacheImagesWithNames(colorNames)

print(colorNames)
// Prints "["Gold", "Cerise", "Magenta", "Vermillion"]"

斯威夫特和斯威夫特

componentArray = Array(dict.keys) // for Dictionary

componentArray = dict.allKeys // for NSDictionary

dict.keys.sorted() 

给出[String] https://developer.apple.com/documentation/swift/array/2945003-sorted

extension Array {
    public func toDictionary<Key: Hashable>(with selectKey: (Element) -> Key) -> [Key:Element] {
        var dict = [Key:Element]()
        for element in self {
            dict[selectKey(element)] = element
        }
        return dict
    }
}

数组从字典键在Swift

componentArray = [String] (dict.keys)