代理属性
实例
import kotlin.reflect.KProperty
/**
* Created by doctorq on 2017/6/6.
* 代理属性
*/
class Example {
var p: String by Delegate()
}
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 $thisRef.'")
}
}
fun main(args: Array<String>) {
val e = Example()
println(e.p)
e.p =
"test"
}
执行结果:
Example@1a6c5a9e, thank you for delegating ‘p’ to me! test has been assigned to ‘p in Example@1a6c5a9e.’
从执行结果可以看出，调用e.p的时候实际调用的是代理类的getValue()方法，而调用e.p="test"实际调用的是代理类的SetValue()方法。
标准代理
lazy
val lazyValue:
String by lazy {
println(
"computed!")
"Hello"
}
fun main(args:
Array<String>) {
println(lazyValue)
println(
"==========")
println(lazyValue)
}
上面使用的是lazy代理，我们知道lazy定义的方法会在第一次调用时初始化，后续调用就会沿用之前的值。所以上面代码的输出如下：
computed!
Hello
observable
实时关注属性的变化
import kotlin
.properties
.Delegates
class QUser {
var name:
String by Delegates
.observable(
"<no name>") {
prop, old,
new ->
println(
"$old -> $new")
}
}
fun main(args:
Array<String>) {
val user
= QUser()
user
.name
= "first"
println(
"==========")
user
.name
= "second"
}
执行输出：
first -> second
使用Map存储属性
class MyUser(
val map: Map<String, Any?>) {
val name: String by map
val age: Int by map
}
fun main(args: Array<String>) {
val user = MyUser(mapOf(
"name" to "Doctorq",
"age" to 28
))
println(user.name)
println(user.age)
}
MyUser类中的2个属性被代理到map中对应的键值。
