Android经验和架构

Delegated Properties

2017-05-23  本文已影响15人  lhyz

1.自定义代理

class Delegate {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        return "$thisRef,thanks you for delegating"
    }

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

上面的示例就是针对String这个类型的属性的代理类实现,用法如下

class Example {
    var p: String by Delegate()
}

在此类被使用的时候,对p变量的幅值和取值都会经过代理类

val e = Example()
    e.p = "a"
    print(e.p)

此时会依次打印出

a has been assigned to in Example@79fc0f2f
Example@79fc0f2f,thanks you for delegating

2.标准代理库

import kotlin.properties.Delegates

Kotlin标准库中实现了一个代理工厂类Delegates,可以很方便的被使用,以下是官网上介绍的几种

lazy (延迟赋值?)

val v: String by lazy {
    println("delegate")
    "hello"
}

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

可以看到的是lazy实现的是一种lambda调用方法。
lazy在变量第一次被赋值之前插入一段操作,因此最后的结果是输出了一次delegate,两次hello

observable (观察者?)

class User {
    var name: String by Delegates.observable("default") {
        property, oldValue, newValue ->
        println("$property $oldValue -> $newValue")
    }
}


fun main(args: Array<String>) {
    val user = User()
    user.name = "first"
    user.name = "second"
}

Delegates.observable定义两个参数,一个是初始值,一个就是监听值的修改的方法,使用lambda表达式的参数就是property, oldValue, newValue这三个,顾名思义即可。
因此输出的结果是:

var User.name: kotlin.String default -> first
var User.name: kotlin.String first -> second

3.存储属性map

class User(map: Map<String, Any?>) {
    val name: String by map
    val age: Int     by map
}


fun main(args: Array<String>) {
    val user = User(mapOf(
            "name" to "john",
            "age" to 11
    ))
    println(user.name)
    println(user.age)
}

使用map存储属性键值对进行代理初始化

4.代理类属性 总结

要实现一个代理类所要所的事如下:

如果不清楚具体写法,可以直接通过实现标准库中的接口来定义自己的代理类:下面就是标准库中给出的接口

interface ReadOnlyProperty<in R, out T> {
    operator fun getValue(thisRef: R, property: KProperty<*>): T
}
interface ReadWriteProperty<in R, T> {
    operator fun getValue(thisRef: R, property: KProperty<*>): T
    operator fun setValue(thisRef: R, property: KProperty<*>, value: T)
}

翻译规则(看看就行了)

class C {
    var prop: Type by MyDelegate()
}

// this code is generated by the compiler instead:
class C {
    private val prop$delegate = MyDelegate()
    var prop: Type
        get() = prop$delegate.getValue(this, this::prop)
        set(value: Type) = prop$delegate.setValue(this, this::prop, value)
}

*Providing a delegate

一个类似getValue的代理接口provideDelegate

class ResourceLoader<T>(id: ResourceID<T>) {
    operator fun provideDelegate(
            thisRef: MyUI,
            prop: KProperty<*>
    ): ReadOnlyProperty<MyUI, T> {
        checkProperty(thisRef, prop.name)
        // create delegate
    }

    private fun checkProperty(thisRef: MyUI, name: String) { ... }
}

fun <T> bindResource(id: ResourceID<T>): ResourceLoader<T> { ... }

class MyUI {
    val image by bindResource(ResourceID.image_id)
    val text by bindResource(ResourceID.text_id)
}

原因是如果要实现相同的效果,不用此方法而是使用getValue方法的话不太优雅,例如:

// Checking the property name without "provideDelegate" functionality
class MyUI {
    val image by bindResource(ResourceID.image_id, "image")
    val text by bindResource(ResourceID.text_id, "text")
}

fun <T> MyUI.bindResource(
        id: ResourceID<T>,
        propertyName: String
): ReadOnlyProperty<MyUI, T> {
   checkProperty(this, propertyName)
   // create delegate
}
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