Hands-On Design Patterns with Kotlin

The great combinator

We'll start a bit differently this time. Instead of composing our solution piece by piece, we'll look at the complete solution and decompose it. The code here will help you understand why:

class HappyMap<K, V>(private val map: MutableMap<K, V> =                                        mutableMapOf()) :       MutableMap<K, V> by map {    override fun put(key: K, value: V): V? {        return map.put(key, value).apply {            this?.let { println("Yay! $key") }        }    }}

The hardest part here is to understand the signature. What we need in the Decorator pattern is:

• To be able to receive the object we're decorating
• To keep a reference to it
• When our Decorator is called, we decide if we would like to change the behavior of the object we're holding, or to delegate the call

Since we need ...