Delegation

Implementation by Delegation

                  interface Base {
    fun print()
}
​
class BaseImpl(val x: Int) : Base {
    override fun print() { print(x) }
}
​
class Derived(b: Base) : Base by b
​
fun main() {
    val b = BaseImpl(10)
    Derived(b).print()
}
                

                    protocol Base {
    func print()
}

class BaseImpl: Base {
    private var x = 0
    init(x: Int) {
        self.x = x
    }
    
    func print() { Swift.print(x) }
}

class Derived: Base {
    private var b: Base
    init(_ b: Base) {
        self.b = b
    }
    
    func print() {
        b.print()
    }
}


let b = BaseImpl(x: 10)
Derived(b).print()

                  

Overriding a member of an interface implemented by delegation

                  interface Base {
    fun printMessage()
    fun printMessageLine()
}
​
class BaseImpl(val x: Int) : Base {
    override fun printMessage() { print(x) }
    override fun printMessageLine() { println(x) }
}
​
class Derived(b: Base) : Base by b {
    override fun printMessage() { print("abc") }
}
​
fun main() {
    val b = BaseImpl(10)
    Derived(b).printMessage()
    Derived(b).printMessageLine()
}
                

                    protocol Base {
    func printMessage()
    func printMessageLine()
}

class BaseImpl: Base {
    private var x = 0
    init(x: Int) {
        self.x = x
    }
    
    func printMessage() { print(x) }
    func printMessageLine() { print(x) }
}

class Derived: Base {
    private var b: Base
    init(_ b: Base) {
        self.b = b
    }
    
    func printMessage() { b.printMessage() }
    func printMessageLine() { print("abc") }
}


let b = BaseImpl(x: 10)
Derived(b).printMessage()
Derived(b).printMessageLine()

                  

                  interface Base {
    val message: String
    fun print()
}
​
class BaseImpl(val x: Int) : Base {
    override val message = "BaseImpl: x = $x"
    override fun print() { println(message) }
}
​
class Derived(b: Base) : Base by b {
    // This property is not accessed from b's implementation of `print`
    override val message = "Message of Derived"
}
​
fun main() {
    val b = BaseImpl(10)
    val derived = Derived(b)
    derived.print()
    println(derived.message)
}
                

                    protocol Base {
    var message: String { get }
    func print()
}

class BaseImpl: Base {
    private var x = 0
    init(x: Int) {
        self.x = x
    }
    
    var message: String {
        get { "BaseImpl: x = \(x)" }
    }
    
    func print() { Swift.print(message) }
}

class Derived: Base {
    private var b: Base
    init(_ b: Base) {
        self.b = b
    }
    
    let message = "Message of Derived"
    
    func print() { b.print() }
}

let b = BaseImpl(x: 10)
let derived = Derived(b)
derived.print()
print(derived.message)