【22】kotlin 屬性代理

案列

package com.yzdzy.kotlin.chapter4.delegates

import kotlin.reflect.KProperty

class Delegates {
    val hello by lazy {
        "helloWorld"
    }
    val hello2 by X()
    var hello3 by X()

}

class X {
    private var value: String? = null
    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        println("getValue:$thisRef->${property.name}")
        return value ?: "";
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
        print("setValue $thisRef->${property.name} = $value")
        this.value = value;
    }
}

fun main(args: Array<String>) {
    val delegates = Delegates()
    print(delegates.hello)
    print(delegates.hello2)
    print(delegates.hello3)
    delegates.hello3 = "value of hello3"
    println(delegates.hello3)

}

 輸出

helloWorldgetValue:com.yzdzy.kotlin.chapter4.delegates.Delegates@4eec7777->hello2
getValue:com.yzdzy.kotlin.chapter4.delegates.Delegates@4eec7777->hello3
setValue com.yzdzy.kotlin.chapter4.delegates.Delegates@4eec7777->hello3 = value of hello3getValue:com.yzdzy.kotlin.chapter4.delegates.Delegates@4eec7777->hello3
value of hello3

 

定義方法

• -val/var<property name>:<Type> by <expression>

• 代理者需要實現相應的setValue/getValue方法

• lazy原理就是屬性代理