# Swift中where的用法
###### tags: `Swift`

你可以在許多情境裡使用where，對型別進行限制，從而讓程式碼更簡潔。

## extension
當陣列中的元素是UIColor時，才可以調用toCgColors函式，將UIColor轉換為CGColor

extension Array where Element == UIColor {
    func toCgColors() -> [CGColor] {
        var cgColors: [CGColor] = []
        for color in self {
            cgColors.append(color.cgColor)
        }
        return cgColors
    }
}

在extension中的init，針對泛型去限制類別，判斷型別為合法的類別才做動作

extension String {
    init(collection: T) where T.Element == String {
        self = collection.joined(separator: ",")
    }
}

let companies = String(collection: ["Apple", "Google", "Tesla"])
print(companies) // "Apple, Google, Tesla"

## for loop

let numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

for number in numbers where number % 3 == 0 {
    print(number) // 0, 3, 6, 9
}

## switch case

enum Action {
    case createUser(age: Int)
    case createPost
    case logout
}

func printAction(action: Action) {
    switch action {
    case .createUser(let age) where age < 21:
        print("Young and wild!")
    case .createUser:
        print("Older and wise!")
    case .createPost:
        print("Creating a post")
    case .logout:
        print("Logout")
    }
}

printAction(action: Action.createUser(age: 18)) // Young and wild
printAction(action: Action.createUser(age: 25)) // Older and wise

## guard

let input:String? = "Answer"
guard let str = input where str == "Answer" else {
    print("False")
}
print("True")

除了guard let外，if let和while let的unwrap也同樣適用

## first 和 contains

// first
let names = ["Henk", "John", "Jack"]
let firstJname = names.first(where: { (name) -> Bool in
    return name.first == "J"
}) // Returns John

// contains
let fruits = ["Banana", "Apple", "Kiwi"]
let containsBanana = fruits.contains(where: { (fruit) in
    return fruit == "Banana"
}) // Returns true