Фасад — це структурний патерн, який надає простий (але урізаний) інтерфейс до складної системи об’єктів, бібліотеки або фреймворку.
Крім того, що Фасад дозволяє знизити загальну складність програми, він також допомагає винести код, який залежить від зовнішньої системи, в одне місце.
Застосування: Патерн часто зустрічається в клієнтських додатках, написаних на Swift, які використовують складні бібліотеки або API.
Ознаки застосування патерна: Фасад впізнається у класі, який має простий інтерфейс, але делегує основну частину роботи іншим класам. Найчастіше, фасади самі стежать за життєвим циклом об’єктів складної системи.
Цей приклад показує структуру патерна Фасад, а саме — з яких класів він складається, які ролі ці класи виконують і як вони взаємодіють один з одним.
Після ознайомлення зі структурою, вам буде легше сприймати наступний приклад, що розглядає реальний випадок використання патерна в світі Swift.
Example.swift: Приклад структури патерна
import XCTest
/// The Facade class provides a simple interface to the complex logic of one or
/// several subsystems. The Facade delegates the client requests to the
/// appropriate objects within the subsystem. The Facade is also responsible for
/// managing their lifecycle. All of this shields the client from the undesired
/// complexity of the subsystem.
class Facade {
private var subsystem1: Subsystem1
private var subsystem2: Subsystem2
/// Depending on your application's needs, you can provide the Facade with
/// existing subsystem objects or force the Facade to create them on its
/// own.
init(subsystem1: Subsystem1 = Subsystem1(),
subsystem2: Subsystem2 = Subsystem2()) {
self.subsystem1 = subsystem1
self.subsystem2 = subsystem2
}
/// The Facade's methods are convenient shortcuts to the sophisticated
/// functionality of the subsystems. However, clients get only to a fraction
/// of a subsystem's capabilities.
func operation() -> String {
var result = "Facade initializes subsystems:"
result += " " + subsystem1.operation1()
result += " " + subsystem2.operation1()
result += "\n" + "Facade orders subsystems to perform the action:\n"
result += " " + subsystem1.operationN()
result += " " + subsystem2.operationZ()
return result
}
}
/// The Subsystem can accept requests either from the facade or client directly.
/// In any case, to the Subsystem, the Facade is yet another client, and it's
/// not a part of the Subsystem.
class Subsystem1 {
func operation1() -> String {
return "Sybsystem1: Ready!\n"
}
// ...
func operationN() -> String {
return "Sybsystem1: Go!\n"
}
}
/// Some facades can work with multiple subsystems at the same time.
class Subsystem2 {
func operation1() -> String {
return "Sybsystem2: Get ready!\n"
}
// ...
func operationZ() -> String {
return "Sybsystem2: Fire!\n"
}
}
/// The client code works with complex subsystems through a simple interface
/// provided by the Facade. When a facade manages the lifecycle of the
/// subsystem, the client might not even know about the existence of the
/// subsystem. This approach lets you keep the complexity under control.
class Client {
// ...
static func clientCode(facade: Facade) {
print(facade.operation())
}
// ...
}
/// Let's see how it all works together.
class FacadeConceptual: XCTestCase {
func testFacadeConceptual() {
/// The client code may have some of the subsystem's objects already
/// created. In this case, it might be worthwhile to initialize the
/// Facade with these objects instead of letting the Facade create new
/// instances.
let subsystem1 = Subsystem1()
let subsystem2 = Subsystem2()
let facade = Facade(subsystem1: subsystem1, subsystem2: subsystem2)
Client.clientCode(facade: facade)
}
}
Output.txt: Результат виконання
Facade initializes subsystems: Sybsystem1: Ready!
Sybsystem2: Get ready!
Facade orders subsystems to perform the action:
Sybsystem1: Go!
Sybsystem2: Fire!
Життєвий приклад
Example.swift: Життєвий приклад
import XCTest
/// Facade Design Pattern
///
/// Intent: Provides a simplified interface to a library, a framework, or any
/// other complex set of classes.
class FacadeRealWorld: XCTestCase {
/// In the real project, you probably will use third-party libraries. For
/// instance, to download images.
///
/// Therefore, facade and wrapping it is a good way to use a third party API
/// in the client code. Even if it is your own library that is connected to
/// a project.
///
/// The benefits here are:
///
/// 1) If you need to change a current image downloader it should be done
/// only in the one place of a project. A number of lines of the client code
/// will stay work.
///
/// 2) The facade provides an access to a fraction of a functionality that
/// fits most client needs. Moreover, it can set frequently used or default
/// parameters.
func testFacedeRealWorld() {
let imageView = UIImageView()
print("Let's set an image for the image view")
clientCode(imageView)
print("Image has been set")
XCTAssert(imageView.image != nil)
}
fileprivate func clientCode(_ imageView: UIImageView) {
let url = URL(string: "www.example.com/logo")
imageView.downloadImage(at: url)
}
}
private extension UIImageView {
/// This extension plays a facede role.
func downloadImage(at url: URL?) {
print("Start downloading...")
let placeholder = UIImage(named: "placeholder")
ImageDownloader().loadImage(at: url,
placeholder: placeholder,
completion: { image, error in
print("Handle an image...")
/// Crop, cache, apply filters, whatever...
self.image = image
})
}
}
private class ImageDownloader {
/// Third party library or your own solution (subsystem)
typealias Completion = (UIImage, Error?) -> ()
typealias Progress = (Int, Int) -> ()
func loadImage(at url: URL?,
placeholder: UIImage? = nil,
progress: Progress? = nil,
completion: Completion) {
/// ... Set up a network stack
/// ... Downloading an image
/// ...
completion(UIImage(), nil)
}
}
Output.txt: Результат виконання
Let's set an image for the image view
Start downloading...
Handle an image...
Image has been set
