Kotlin：Delegation Pattern 和 by 關鍵字

Kotlin 的 by keyword 讓我們可以很輕鬆地實作 Delegation Pattern。首先，先讓我們看一下，一般實作 Delegation Pattern 的程式碼。我們看到 Window.area() 委託呼叫給 bound.area()，就沒有做其他的事情了。當 Shape 的 methods 多的時候，Window 就要實作很多這種的程式碼。

interface Shape {
    fun area(): Int
}

class Rectangle(private val width: Int, private val height: Int) : Shape {
    override fun area(): Int = width * height
}

class Window(private val bound: Shape) : Shape {
    override fun area(): Int = bound.area()
}

fun main() {
    val shape = Window(Rectangle(6, 5))
    println("area is ${shape.area()}")
}

// Output:
// area is 30

當我們使用 Kotlin 的 by keyword 時，我們就不需要再實作這些委託的程式碼。這使得程式碼相當地精簡，如下。

interface Shape {
    fun area(): Int
}

class Rectangle(private val width: Int, private val height: Int) : Shape {
    override fun area(): Int = width * height
}

class Window(private val bound: Shape) : Shape by bound

fun main() {
    val shape = Window(Rectangle(6, 5))
    println("area is ${shape.area()}")
}

// Output:
// area is 30

by keyword 也允許我們使用自己的實作。

interface Shape {
    fun area(): Int
    fun printArea()
}

class Rectangle(private val width: Int, private val height: Int) : Shape {
    override fun area(): Int = width * height

    override fun printArea() {
        println("Rectangle area is ${area()}")
    }
}

class Window(private val bound: Shape) : Shape by bound {
    override fun printArea() {
        println("Window area is ${area()}")
    }
}

fun main() {
    Window(Rectangle(6, 5)).printArea()
}

// Output:
// Window area is 30

然而，與繼承不同的是，委託的物件無法呼叫我們自己實作的方法。如下程式碼中，當我們呼叫 Window.printArea() 時，它會委託給 Rectangle.printArea()，然後在其裡面呼叫的會是 Rectangle.area() 而不是 Window.area()。

interface Shape {
    fun area(): Int
    fun printArea()
}

class Rectangle(private val width: Int, private val height: Int) : Shape {
    override fun area(): Int = width * height

    override fun printArea() {
        println("Rectangle area is ${area()}")
    }
}

class Window(private val bound: Shape) : Shape by bound {
    override fun area(): Int = bound.area() * 10
}

fun main() {
    Window(Rectangle(6, 5)).printArea()
}

// Output:
// Rectangle area is 30