Prototype is a creational design pattern that allows cloning objects, even complex ones, without coupling to their specific classes.
All prototype classes should have a common interface that makes it possible to copy objects even if their concrete classes are unknown. Prototype objects can produce full copies since objects of the same class can access each other’s private fields.
This example illustrates the structure of the Prototype design pattern and focuses on the following questions:
What classes does it consist of?
What roles do these classes play?
In what way the elements of the pattern are related?
After learning about the pattern’s structure it’ll be easier for you to grasp the following example, based on a real-world Swift use case.
Example.swift: Conceptual example
import XCTest
/// Swift has built-in cloning support. To add cloning support to your class,
/// you need to implement the NSCopying protocol in that class and provide the
/// implementation for the `copy` method.
class BaseClass: NSCopying, Equatable {
private var intValue = 1
private var stringValue = "Value"
required init(intValue: Int = 1, stringValue: String = "Value") {
self.intValue = intValue
self.stringValue = stringValue
}
/// MARK: - NSCopying
func copy(with zone: NSZone? = nil) -> Any {
let prototype = type(of: self).init()
prototype.intValue = intValue
prototype.stringValue = stringValue
print("Values defined in BaseClass have been cloned!")
return prototype
}
/// MARK: - Equatable
static func == (lhs: BaseClass, rhs: BaseClass) -> Bool {
return lhs.intValue == rhs.intValue && lhs.stringValue == rhs.stringValue
}
}
/// Subclasses can override the base `copy` method to copy their own data into
/// the resulting object. But you should always call the base method first.
class SubClass: BaseClass {
private var boolValue = true
func copy() -> Any {
return copy(with: nil)
}
override func copy(with zone: NSZone?) -> Any {
guard let prototype = super.copy(with: zone) as? SubClass else {
return SubClass() // oops
}
prototype.boolValue = boolValue
print("Values defined in SubClass have been cloned!")
return prototype
}
}
/// The client code.
class Client {
// ...
static func someClientCode() {
let original = SubClass(intValue: 2, stringValue: "Value2")
guard let copy = original.copy() as? SubClass else {
XCTAssert(false)
return
}
/// See implementation of `Equatable` protocol for more details.
XCTAssert(copy == original)
print("The original object is equal to the copied object!")
}
// ...
}
/// Let's see how it all works together.
class PrototypeConceptual: XCTestCase {
func testPrototype_NSCopying() {
Client.someClientCode()
}
}
Output.txt: Execution result
Values defined in BaseClass have been cloned!
Values defined in SubClass have been cloned!
The original object is equal to the copied object!
Real World Example
Example.swift: Real world example
import XCTest
class PrototypeRealWorld: XCTestCase {
func testPrototypeRealWorld() {
let author = Author(id: 10, username: "Ivan_83")
let page = Page(title: "My First Page", contents: "Hello world!", author: author)
page.add(comment: Comment(message: "Keep it up!"))
/// Since NSCopying returns Any, the copied object should be unwrapped.
guard let anotherPage = page.copy() as? Page else {
XCTFail("Page was not copied")
return
}
/// Comments should be empty as it is a new page.
XCTAssert(anotherPage.comments.isEmpty)
/// Note that the author is now referencing two objects.
XCTAssert(author.pagesCount == 2)
print("Original title: " + page.title)
print("Copied title: " + anotherPage.title)
print("Count of pages: " + String(author.pagesCount))
}
}
private class Author {
private var id: Int
private var username: String
private var pages = [Page]()
init(id: Int, username: String) {
self.id = id
self.username = username
}
func add(page: Page) {
pages.append(page)
}
var pagesCount: Int {
return pages.count
}
}
private class Page: NSCopying {
private(set) var title: String
private(set) var contents: String
private weak var author: Author?
private(set) var comments = [Comment]()
init(title: String, contents: String, author: Author?) {
self.title = title
self.contents = contents
self.author = author
author?.add(page: self)
}
func add(comment: Comment) {
comments.append(comment)
}
/// MARK: - NSCopying
func copy(with zone: NSZone? = nil) -> Any {
return Page(title: "Copy of '" + title + "'", contents: contents, author: author)
}
}
private struct Comment {
let date = Date()
let message: String
}
Output.txt: Execution result
Original title: My First Page
Copied title: Copy of 'My First Page'
Count of pages: 2
