Composite in Swift
Composite is a structural design pattern that lets you compose objects into tree structures and then work with these structures as if they were individual objects.
Composite became a pretty popular solution for the most problems that require building a tree structure. Composite’s great feature is the ability to run methods recursively over the whole tree structure and sum up the results.
Complexity:
Popularity:
Usage examples: The Composite pattern is pretty common in Swift code. It’s often used to represent hierarchies of user interface components or the code that works with graphs.
Identification: If you have an object tree, and each object of a tree is a part of the same class hierarchy, this is most likely a composite. If methods of these classes delegate the work to child objects of the tree and do it via the base class/interface of the hierarchy, this is definitely a composite.
The following examples are available on
Swift Playgrounds.
Kudos to
Alejandro Mohamad for creating the Playground version.
Conceptual Example
This example illustrates the structure of the Composite 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
/// The base Component class declares common operations for both simple and
/// complex objects of a composition.
protocol Component {
/// The base Component may optionally declare methods for setting and
/// accessing a parent of the component in a tree structure. It can also
/// provide some default implementation for these methods.
var parent: Component? { get set }
/// In some cases, it would be beneficial to define the child-management
/// operations right in the base Component class. This way, you won't need
/// to expose any concrete component classes to the client code, even during
/// the object tree assembly. The downside is that these methods will be
/// empty for the leaf-level components.
func add(component: Component)
func remove(component: Component)
/// You can provide a method that lets the client code figure out whether a
/// component can bear children.
func isComposite() -> Bool
/// The base Component may implement some default behavior or leave it to
/// concrete classes.
func operation() -> String
}
extension Component {
func add(component: Component) {}
func remove(component: Component) {}
func isComposite() -> Bool {
return false
}
}
/// The Leaf class represents the end objects of a composition. A leaf can't
/// have any children.
///
/// Usually, it's the Leaf objects that do the actual work, whereas Composite
/// objects only delegate to their sub-components.
class Leaf: Component {
var parent: Component?
func operation() -> String {
return "Leaf"
}
}
/// The Composite class represents the complex components that may have
/// children. Usually, the Composite objects delegate the actual work to their
/// children and then "sum-up" the result.
class Composite: Component {
var parent: Component?
/// This fields contains the conponent subtree.
private var children = [Component]()
/// A composite object can add or remove other components (both simple or
/// complex) to or from its child list.
func add(component: Component) {
var item = component
item.parent = self
children.append(item)
}
func remove(component: Component) {
// ...
}
func isComposite() -> Bool {
return true
}
/// The Composite executes its primary logic in a particular way. It
/// traverses recursively through all its children, collecting and summing
/// their results. Since the composite's children pass these calls to their
/// children and so forth, the whole object tree is traversed as a result.
func operation() -> String {
let result = children.map({ $0.operation() })
return "Branch(" + result.joined(separator: " ") + ")"
}
}
class Client {
/// The client code works with all of the components via the base interface.
static func someClientCode(component: Component) {
print("Result: " + component.operation())
}
/// Thanks to the fact that the child-management operations are also
/// declared in the base Component class, the client code can work with both
/// simple or complex components.
static func moreComplexClientCode(leftComponent: Component, rightComponent: Component) {
if leftComponent.isComposite() {
leftComponent.add(component: rightComponent)
}
print("Result: " + leftComponent.operation())
}
}
/// Let's see how it all comes together.
class CompositeConceptual: XCTestCase {
func testCompositeConceptual() {
/// This way the client code can support the simple leaf components...
print("Client: I've got a simple component:")
Client.someClientCode(component: Leaf())
/// ...as well as the complex composites.
let tree = Composite()
let branch1 = Composite()
branch1.add(component: Leaf())
branch1.add(component: Leaf())
let branch2 = Composite()
branch2.add(component: Leaf())
branch2.add(component: Leaf())
tree.add(component: branch1)
tree.add(component: branch2)
print("\nClient: Now I've got a composite tree:")
Client.someClientCode(component: tree)
print("\nClient: I don't need to check the components classes even when managing the tree:")
Client.moreComplexClientCode(leftComponent: tree, rightComponent: Leaf())
}
}
Output.txt: Execution result
Client: I've got a simple component:
Result: Leaf
Client: Now I've got a composite tree:
Result: Branch(Branch(Leaf Leaf) Branch(Leaf Leaf))
Client: I don't need to check the components classes even when managing the tree:
Result: Branch(Branch(Leaf Leaf) Branch(Leaf Leaf) Leaf)
Real World Example
Example.swift: Real world example
import UIKit
import XCTest
protocol Component {
func accept<T: Theme>(theme: T)
}
extension Component where Self: UIViewController {
func accept<T: Theme>(theme: T) {
view.accept(theme: theme)
view.subviews.forEach({ $0.accept(theme: theme) })
}
}
extension UIView: Component {}
extension UIViewController: Component {}
extension Component where Self: UIView {
func accept<T: Theme>(theme: T) {
print("\t\(description): has applied \(theme.description)")
backgroundColor = theme.backgroundColor
}
}
extension Component where Self: UILabel {
func accept<T: LabelTheme>(theme: T) {
print("\t\(description): has applied \(theme.description)")
backgroundColor = theme.backgroundColor
textColor = theme.textColor
}
}
extension Component where Self: UIButton {
func accept<T: ButtonTheme>(theme: T) {
print("\t\(description): has applied \(theme.description)")
backgroundColor = theme.backgroundColor
setTitleColor(theme.textColor, for: .normal)
setTitleColor(theme.highlightedColor, for: .highlighted)
}
}
protocol Theme: CustomStringConvertible {
var backgroundColor: UIColor { get }
}
protocol ButtonTheme: Theme {
var textColor: UIColor { get }
var highlightedColor: UIColor { get }
/// other properties
}
protocol LabelTheme: Theme {
var textColor: UIColor { get }
/// other properties
}
/// Button Themes
struct DefaultButtonTheme: ButtonTheme {
var textColor = UIColor.red
var highlightedColor = UIColor.white
var backgroundColor = UIColor.orange
var description: String { return "Default Buttom Theme" }
}
struct NightButtonTheme: ButtonTheme {
var textColor = UIColor.white
var highlightedColor = UIColor.red
var backgroundColor = UIColor.black
var description: String { return "Night Buttom Theme" }
}
/// Label Themes
struct DefaultLabelTheme: LabelTheme {
var textColor = UIColor.red
var backgroundColor = UIColor.black
var description: String { return "Default Label Theme" }
}
struct NightLabelTheme: LabelTheme {
var textColor = UIColor.white
var backgroundColor = UIColor.black
var description: String { return "Night Label Theme" }
}
class CompositeRealWorld: XCTestCase {
func testCompositeRealWorld() {
print("\nClient: Applying 'default' theme for 'UIButton'")
apply(theme: DefaultButtonTheme(), for: UIButton())
print("\nClient: Applying 'night' theme for 'UIButton'")
apply(theme: NightButtonTheme(), for: UIButton())
print("\nClient: Let's use View Controller as a composite!")
/// Night theme
print("\nClient: Applying 'night button' theme for 'WelcomeViewController'...")
apply(theme: NightButtonTheme(), for: WelcomeViewController())
print()
print("\nClient: Applying 'night label' theme for 'WelcomeViewController'...")
apply(theme: NightLabelTheme(), for: WelcomeViewController())
print()
/// Default Theme
print("\nClient: Applying 'default button' theme for 'WelcomeViewController'...")
apply(theme: DefaultButtonTheme(), for: WelcomeViewController())
print()
print("\nClient: Applying 'default label' theme for 'WelcomeViewController'...")
apply(theme: DefaultLabelTheme(), for: WelcomeViewController())
print()
}
func apply<T: Theme>(theme: T, for component: Component) {
component.accept(theme: theme)
}
}
class WelcomeViewController: UIViewController {
class ContentView: UIView {
var titleLabel = UILabel()
var actionButton = UIButton()
override init(frame: CGRect) {
super.init(frame: frame)
setup()
}
required init?(coder decoder: NSCoder) {
super.init(coder: decoder)
setup()
}
func setup() {
addSubview(titleLabel)
addSubview(actionButton)
}
}
override func loadView() {
view = ContentView()
}
}
/// Let's override a description property for the better output
extension WelcomeViewController {
open override var description: String { return "WelcomeViewController" }
}
extension WelcomeViewController.ContentView {
override var description: String { return "ContentView" }
}
extension UIButton {
open override var description: String { return "UIButton" }
}
extension UILabel {
open override var description: String { return "UILabel" }
}
Output.txt: Execution result
Client: Applying 'default' theme for 'UIButton'
UIButton: has applied Default Buttom Theme
Client: Applying 'night' theme for 'UIButton'
UIButton: has applied Night Buttom Theme
Client: Let's use View Controller as a composite!
Client: Applying 'night button' theme for 'WelcomeViewController'...
ContentView: has applied Night Buttom Theme
UILabel: has applied Night Buttom Theme
UIButton: has applied Night Buttom Theme
