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Composite in Swift
Composite is a structural design pattern that allows composing objects into a tree-like structure and work with the it as if it was a singular object.
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.
Usage of the pattern in Swift
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.
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
