关于kotlin中的by关键字的用法，kotlin官方文档属性委托这一节讲得很清楚。

  简单来说就是这样的，假设存在一个接口Component如下：

interface Component {
	fun method1(): IntArray
	fun method2(a: Int)
	fun method3(a: Int, str: String)
}

  那么对于实现该接口的方法，可以这样：

class Decorator(private val component: Component): Component {
	override fun method1(): IntArray = component.method1()
	override fun method2(a: Int) = component.method2(a)
	override fun method3(a: Int, str: String) = component.method3(a, str)
}

  但也可以通过by关键字更简单地实现：

class Decorator(private val component: Component): Component by component

  这两段代码功能一致。

  于是我们便能利用这一功能来更方便地实现装饰器模式，现在我们来实现三个装饰器类，分别对Component的三个方法进行装饰。

interface Component {
	fun method1(): IntArray
	fun method2(a: Int)
	fun method3(a: Int, str: String)
}

class Decorator1(
	private val component: Component,
	private inline val f0: ()->Unit = {},
	private inline val f1: (arr: IntArray)->IntArray = {arr -> arr}
): Component by component {
	override fun method1(): IntArray {
		f0()
		return f1(component.method1())
	}
//	相当于自动实现了
//	override fun method2(a: Int) = component.method2(a)
//	override fun method3(a: Int, str: String) = component.method3(a, str)
}

class Decorator2(
	private val component: Component,
	private inline val f0: (Int)->Unit = {},
	private inline val f1: (Int)->Unit = {}
): Component by component {
	override fun method2(a: Int) {
		f0(a)
		component.method2(a)
		f1(a)
	}
//	override fun method1(): IntArray = component.method1()
//	override fun method3(a: Int, str: String) = component.method3(a, str)
}

class Decorator3(
	private val component: Component,
	private inline val f0: (Int, String)->Unit = {_, _ -> },
	private inline val f1: (Int, String)->Unit = {_, _ -> }
): Component by component {
	override fun method3(a: Int, str: String) {
		f0(a, str)
		component.method3(a, str)
		f1(a, str)
	}
//	override fun method1(): IntArray = component.method1()
//	override fun method2(a: Int) = component.method2(a)
}

  在主函数中调用这三个装饰器。

fun main() {

	val obj1 = object: Component{
		override fun method1(): IntArray = IntArray(5){it * it}
		override fun method2(a: Int) = println("a^2 is ${a * a}")
		override fun method3(a: Int, str: String) = println("a is a, and str is \"$str\"")
	}

	val obj2 = object: Component{
		override fun method1(): IntArray = IntArray(10){it}
		override fun method2(a: Int) = println("a - 3 is ${a - 3}")
		override fun method3(a: Int, str: String) = println("say \"$str\" to number a = $a")
	}

	val dcrt1: Component = Decorator1(obj1, {}){ arr ->
		println("old arr is ${arr.contentToString()}")
		return@Decorator1 IntArray(10) {it * it *it}
	}

	val dcrt2: Component = Decorator2(obj1){ a ->
		println("a is $a")
	}

	val dcrt3: Component = Decorator3(obj2, { a, str ->
		println("say \"$str\" to java $a times")
	}, {a, str ->
		println("and say \"$str\" to kotlin $a times")
	})

	val dcrt4: Component = Decorator2(Decorator3(obj2){ a, str ->
		println("say \"$str\" to jetBrains $a times")
	}){a ->
		println("a + 3 is ${a + 3}")
	}

	display(dcrt1, 10, "Hello world!")
	display(dcrt2, 15, "Hello kotlin!")
	display(dcrt3, 20, "Hello, java!")
	display(dcrt4, 25, "Hello, jetBrains!")

}

fun display(component: Component, a: Int, str: String) {
	with(component) {
		println("--------------------------------")
		println(method1().contentToString())
		method2(a)
		method3(a, str)
		println("--------------------------------")
		println()
	}
}

  运行结果：