屬性委托本質(zhì)上就是代理模式
應(yīng)用參考：https://juejin.cn/post/7440362323992657956

• by

將某個屬性，委托給另一個對象進(jìn)行管理
將 Example 的 p 委托給 Delegate 進(jìn)行管理倒淫，通過操作符重載，實(shí)現(xiàn)自定義的 get/set 方法

class Delegate {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        return "$thisRef, thank you for delegating '${property.name}' to me!"
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
        println("$value has been assigned to '${property.name}' in $thisRef.")
    }
}

class Example {
    var p: String by Delegate()

    // setter  ->  setValue
    // getter  ->  getValue
}

fun main() {

    val e = Example()
    e.p = "AAA"
    println(e.p)
}

• by lazy

為什么 lazy 里面沒有定義 get/set 方法镜硕，也可以使用 by 關(guān)鍵字實(shí)現(xiàn)屬性代理功能返干？
因?yàn)?lazy 內(nèi)部實(shí)現(xiàn)了一個擴(kuò)展函數(shù)，并使用 operator 關(guān)鍵字實(shí)現(xiàn)了操作符重載财剖，所以能實(shí)現(xiàn)屬性代理的功能

截屏2025-02-01 14.55.23.png

class DelegatedProperty {
    val friends by lazy {
        loadFriendList()
    }

    private fun loadFriendList(): List<String> {
        return listOf("pony", "johnny", "jack")
    }
}

fun main(args: Array<String>) {
    println(DelegatedProperty().friends)
}

• by Delegates.observable

open class PropertyChangeAware {
    protected val changeSupport = PropertyChangeSupport("people")

    fun addPropertyChangeListener(listener: PropertyChangeListener) {
        changeSupport.addPropertyChangeListener(listener)
    }

    fun removePropertyChangeListener(listener: PropertyChangeListener) {
        changeSupport.removePropertyChangeListener(listener)
    }
}

class ObservableProperty2(
    private var propValue: Int,
    private val changeSupport: PropertyChangeSupport
) {
    operator fun getValue(p: People3, pro: KProperty<*>): Int = propValue
    operator fun setValue(p: People3, pro: KProperty<*>, newValue: Int) {
        val oldValue = propValue
        propValue = newValue
        changeSupport.firePropertyChange(pro.name, oldValue, newValue)
    }
}

class People3(val name: String, age: Int, salary: Int) : PropertyChangeAware() {
    //通過by實(shí)現(xiàn)屬性委托躺坟，委托給ObservableProperty2
    var age: Int by ObservableProperty2(age, changeSupport)
    var salary: Int by ObservableProperty2(salary, changeSupport)
}

fun testImp3() {
    val p3 = People3("AAA", 38, 74500)
    p3.addPropertyChangeListener(PropertyChangeListener { event ->
        //println("event source: ${event.source}")
        println("Property ${event.propertyName} changed from ${event.oldValue} to ${event.newValue}")
    })
    p3.age += 1
    p3.salary += 1000
}

fun main(args: Array<String>) {
    testImp3()
}