Visitor em Swift
O Visitor é um padrão de projeto comportamental que permite adicionar novos comportamentos à hierarquia de classes existente sem alterar nenhum código existente.
Leia por que os Visitors não podem ser simplesmente substituídos pela sobrecarga de método em nosso artigo Visitor e Double Dispatch.
Complexidade:
Popularidade:
Exemplos de uso: O Visitor não é um padrão muito comum devido à sua complexidade e aplicabilidade limitada.
The following examples are available on
Swift Playgrounds.
Kudos to
Alejandro Mohamad for creating the Playground version.
Exemplo conceitual
Este exemplo ilustra a estrutura do padrão de projeto Visitor. Ele se concentra em responder a estas perguntas:
- De quais classes ele consiste?
- Quais papéis essas classes desempenham?
- De que maneira os elementos do padrão estão relacionados?
Depois de aprender sobre a estrutura do padrão, será mais fácil entender o exemplo a seguir, com base em um caso de uso Swift do mundo real.
Example.swift: Exemplo conceitual
import XCTest
/// The Component interface declares an `accept` method that should take the
/// base visitor interface as an argument.
protocol Component {
func accept(_ visitor: Visitor)
}
/// Each Concrete Component must implement the `accept` method in such a way
/// that it calls the visitor's method corresponding to the component's class.
class ConcreteComponentA: Component {
/// Note that we're calling `visitConcreteComponentA`, which matches the
/// current class name. This way we let the visitor know the class of the
/// component it works with.
func accept(_ visitor: Visitor) {
visitor.visitConcreteComponentA(element: self)
}
/// Concrete Components may have special methods that don't exist in their
/// base class or interface. The Visitor is still able to use these methods
/// since it's aware of the component's concrete class.
func exclusiveMethodOfConcreteComponentA() -> String {
return "A"
}
}
class ConcreteComponentB: Component {
/// Same here: visitConcreteComponentB => ConcreteComponentB
func accept(_ visitor: Visitor) {
visitor.visitConcreteComponentB(element: self)
}
func specialMethodOfConcreteComponentB() -> String {
return "B"
}
}
/// The Visitor Interface declares a set of visiting methods that correspond to
/// component classes. The signature of a visiting method allows the visitor to
/// identify the exact class of the component that it's dealing with.
protocol Visitor {
func visitConcreteComponentA(element: ConcreteComponentA)
func visitConcreteComponentB(element: ConcreteComponentB)
}
/// Concrete Visitors implement several versions of the same algorithm, which
/// can work with all concrete component classes.
///
/// You can experience the biggest benefit of the Visitor pattern when using it
/// with a complex object structure, such as a Composite tree. In this case, it
/// might be helpful to store some intermediate state of the algorithm while
/// executing visitor's methods over various objects of the structure.
class ConcreteVisitor1: Visitor {
func visitConcreteComponentA(element: ConcreteComponentA) {
print(element.exclusiveMethodOfConcreteComponentA() + " + ConcreteVisitor1\n")
}
func visitConcreteComponentB(element: ConcreteComponentB) {
print(element.specialMethodOfConcreteComponentB() + " + ConcreteVisitor1\n")
}
}
class ConcreteVisitor2: Visitor {
func visitConcreteComponentA(element: ConcreteComponentA) {
print(element.exclusiveMethodOfConcreteComponentA() + " + ConcreteVisitor2\n")
}
func visitConcreteComponentB(element: ConcreteComponentB) {
print(element.specialMethodOfConcreteComponentB() + " + ConcreteVisitor2\n")
}
}
/// The client code can run visitor operations over any set of elements without
/// figuring out their concrete classes. The accept operation directs a call to
/// the appropriate operation in the visitor object.
class Client {
// ...
static func clientCode(components: [Component], visitor: Visitor) {
// ...
components.forEach({ $0.accept(visitor) })
// ...
}
// ...
}
/// Let's see how it all works together.
class VisitorConceptual: XCTestCase {
func test() {
let components: [Component] = [ConcreteComponentA(), ConcreteComponentB()]
print("The client code works with all visitors via the base Visitor interface:\n")
let visitor1 = ConcreteVisitor1()
Client.clientCode(components: components, visitor: visitor1)
print("\nIt allows the same client code to work with different types of visitors:\n")
let visitor2 = ConcreteVisitor2()
Client.clientCode(components: components, visitor: visitor2)
}
}
Output.txt: Resultados da execução
The client code works with all visitors via the base Visitor interface:
A + ConcreteVisitor1
B + ConcreteVisitor1
It allows the same client code to work with different types of visitors:
A + ConcreteVisitor2
B + ConcreteVisitor2
Exemplo do mundo real
Example.swift: Exemplo do mundo real
import Foundation
import XCTest
protocol Notification: CustomStringConvertible {
func accept(visitor: NotificationPolicy) -> Bool
}
struct Email {
let emailOfSender: String
var description: String { return "Email" }
}
struct SMS {
let phoneNumberOfSender: String
var description: String { return "SMS" }
}
struct Push {
let usernameOfSender: String
var description: String { return "Push" }
}
extension Email: Notification {
func accept(visitor: NotificationPolicy) -> Bool {
return visitor.isTurnedOn(for: self)
}
}
extension SMS: Notification {
func accept(visitor: NotificationPolicy) -> Bool {
return visitor.isTurnedOn(for: self)
}
}
extension Push: Notification {
func accept(visitor: NotificationPolicy) -> Bool {
return visitor.isTurnedOn(for: self)
}
}
protocol NotificationPolicy: CustomStringConvertible {
func isTurnedOn(for email: Email) -> Bool
func isTurnedOn(for sms: SMS) -> Bool
func isTurnedOn(for push: Push) -> Bool
}
class NightPolicyVisitor: NotificationPolicy {
func isTurnedOn(for email: Email) -> Bool {
return false
}
func isTurnedOn(for sms: SMS) -> Bool {
return true
}
func isTurnedOn(for push: Push) -> Bool {
return false
}
var description: String { return "Night Policy Visitor" }
}
class DefaultPolicyVisitor: NotificationPolicy {
func isTurnedOn(for email: Email) -> Bool {
return true
}
func isTurnedOn(for sms: SMS) -> Bool {
return true
}
func isTurnedOn(for push: Push) -> Bool {
return true
}
var description: String { return "Default Policy Visitor" }
}
class BlackListVisitor: NotificationPolicy {
private var bannedEmails = [String]()
private var bannedPhones = [String]()
private var bannedUsernames = [String]()
init(emails: [String], phones: [String], usernames: [String]) {
self.bannedEmails = emails
self.bannedPhones = phones
self.bannedUsernames = usernames
}
func isTurnedOn(for email: Email) -> Bool {
return bannedEmails.contains(email.emailOfSender)
}
func isTurnedOn(for sms: SMS) -> Bool {
return bannedPhones.contains(sms.phoneNumberOfSender)
}
func isTurnedOn(for push: Push) -> Bool {
return bannedUsernames.contains(push.usernameOfSender)
}
var description: String { return "Black List Visitor" }
}
class VisitorRealWorld: XCTestCase {
func testVisitorRealWorld() {
let email = Email(emailOfSender: "some@email.com")
let sms = SMS(phoneNumberOfSender: "+3806700000")
let push = Push(usernameOfSender: "Spammer")
let notifications: [Notification] = [email, sms, push]
clientCode(handle: notifications, with: DefaultPolicyVisitor())
clientCode(handle: notifications, with: NightPolicyVisitor())
}
}
extension VisitorRealWorld {
/// Client code traverses notifications with visitors and checks whether a
/// notification is in a blacklist and should be shown in accordance with a
/// current SilencePolicy
func clientCode(handle notifications: [Notification], with policy: NotificationPolicy) {
let blackList = createBlackList()
print("\nClient: Using \(policy.description) and \(blackList.description)")
notifications.forEach { item in
guard !item.accept(visitor: blackList) else {
print("\tWARNING: " + item.description + " is in a black list")
return
}
if item.accept(visitor: policy) {
print("\t" + item.description + " notification will be shown")
} else {
print("\t" + item.description + " notification will be silenced")
}
}
}
private func createBlackList() -> BlackListVisitor {
return BlackListVisitor(emails: ["banned@email.com"],
phones: ["000000000", "1234325232"],
usernames: ["Spammer"])
}
}
Output.txt: Resultados da execução
Client: Using Default Policy Visitor and Black List Visitor
Email notification will be shown
SMS notification will be shown
WARNING: Push is in a black list
Client: Using Night Policy Visitor and Black List Visitor
Email notification will be silenced
SMS notification will be shown
WARNING: Push is in a black list
