验证用户名的代码

func authenticate(user: String) {    // make sure usernames are at least three characters    guard user.trimmingCharacters(in: .whitespacesAndNewlines).count >= 3 else {        print("Username \(user) is too short.")        return    }    // make sure usernames contain no invalid characters    let illegalCharacters = ["@", "-", "&", "."]    guard illegalCharacters.contains(where: user.contains) == false else {        print("Username \(user) contains illegal characters.")        return    }    // Proceed with authentication…}``` swift上面代码的缺点：在其他地方使用会重复``` swiftextension String {    func isValidUsername() -> Bool {        guard self.trimmingCharacters(in: .whitespacesAndNewlines).count >= 3 else {            return false        }        let illegalCharacters = ["@", "-", "&", "."]        guard illegalCharacters.contains(where: self.contains) == false else {            return false        }        return true    }}复制代码

上面代码的缺点：需要在每个地方调用isValidUsername()

struct User: Equatable {    let value: String    init?(string: String) {        guard string.trimmingCharacters(in: .whitespacesAndNewlines).count >= 3 else {            return nil        }        let illegalCharacters = ["@", "-", "&", "."]        guard illegalCharacters.contains(where: string.contains) == false else {            return nil        }        self.value = string    }}func authenticate(user: User) {    // Proceed with authentication…}复制代码

值对象：value objects should be both immutable and equatable, but they also add in validation as part of their creation. This means that if you’re handed a value object you know for sure it’s already passed validation – you don’t need to re-validate it, because if it were valid then it couldn’t exist.

设计：参考系统的 URL(string: "This ain't it chief") 好处：安全，扩展性强

注：contains与allSatisfy的区别

// 有一个满足就为truefunc contains(where predicate: (Element) throws -> Bool) rethrows -> Bool// 所以满足就为truefunc allSatisfy(_ predicate: (Element) throws -> Bool) rethrows -> Bool复制代码

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