Kotlin基础 — 委托-白红宇

类委托(代理模式)

类的两种代理方法

interface Base {    fun print()}class BaseImpl(val x: Int) : Base {    override fun print() { print(x) }}// 第一种：这里直接使用关键字 by 进行代理class Derived(base: Base) : Base by base// 第二种：class Derived(base: Base) : Base {    val base : Base    init {        this.base = base    }    override fun print() {        base.print() // base是BaseImpl的实例对象    }}// 测试class Main {	fun main() {	    val b = BaseImpl(10)	    Derived(b).print() // prints 10	}}复制代码

属性委托

语法结构： val/var <property name>: <Type> by <expression>

两种实现方法：

方法一：可以按需继承 ReadOnlyProperty、ReadWriteProperty 两个接口中的一个；

// 读取属性public interface ReadOnlyProperty
      
        {    public operator fun getValue(thisRef: R, property: KProperty<*>): T}// 读写属性public interface ReadWriteProperty
       
         {	// 关键字 operator 重载操作符    public operator fun getValue(thisRef: R, property: KProperty<*>): T    public operator fun setValue(thisRef: R, property: KProperty<*>, value: T)}复制代码

示例代码：

class Delegate
      
        : ReadWriteProperty
       
         {	// 重写了getValue()方法    override fun getValue(thisRef: Any?, property: KProperty<*>): T {        return ...    }    override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {    }    }class Main {	var p: String by Delegate
        
         ()	fun test() {		val main = Main()		print(main.p)  // 会调用到Delegate中的getValue()方法		main.p = "Elson" // 会调用到Delegate中的setValue()方法	}}复制代码

方法二：自己定义，但方法参数必须和 1 中接口的方法参数一致；

class Delegate
      
        {	// 使用关键字 operator 来实现重载    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {        return "$thisRef, thank you for delegating '${property.name}' to me!"    }     operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {        println("$value has been assigned to '${property.name}' in $thisRef.")    }}复制代码

示例代码：

class Main {	var p: String by Delegate
      
       () //属性 p 被代理	fun test() {		val main = Main()		print(main.p)  // 会调用到Delegate中的getValue()方法		main.p = "Elson" // 会调用到Delegate中的setValue()方法	}}复制代码

标准委托

1. 延迟属性 Lazy

示例代码：

val lazyValue: String by lazy {    println("computed!")    "Hello"}fun main(args: Array
      
       ) {    println(lazyValue) //第一次调用lazyValue时，会先初始化lazy{}作用区间的代码，即打印一个"computed!"，然后返回一个"Hello"字符串并赋值给lazyValue；    println(lazyValue) //第二次调用lazyValue时，直接将"Hello"赋值给lazyValue；可参考下面的源码}复制代码

源码：Lazy.kt 文件

public fun 
      
        lazy(initializer: () -> T): Lazy
       
         = SynchronizedLazyImpl(initializer)private object UNINITIALIZED_VALUE@JvmVersionprivate class SynchronizedLazyImpl
        
         (initializer: () -> T, lock: Any? = null) : Lazy
         
          , Serializable {    private var initializer: (() -> T)? = initializer    @Volatile private var _value: Any? = UNINITIALIZED_VALUE    // final field is required to enable safe publication of constructed instance    private val lock = lock ?: this    override val value: T        get() {            val _v1 = _value            if (_v1 !== UNINITIALIZED_VALUE) {                @Suppress("UNCHECKED_CAST")                return _v1 as T //第二次执行操作时，这里直接返回对象            }            return synchronized(lock) {                val _v2 = _value                if (_v2 !== UNINITIALIZED_VALUE) {                    @Suppress("UNCHECKED_CAST") (_v2 as T)                }                else {                    val typedValue = initializer!!()//第一次使用时执行这里，将lazy大括号内的代码进行初始化,并返回最后一行的数据并赋值给typedValue                    _value = typedValue                     initializer = null                    typedValue                }            }        }    override fun isInitialized(): Boolean = _value !== UNINITIALIZED_VALUE    override fun toString(): String = if (isInitialized()) value.toString() else "Lazy value not initialized yet."    private fun writeReplace(): Any = InitializedLazyImpl(value)}复制代码

2. 可观察属性 Observable

示例代码：

class User {	// 方法一：    var name: String by Delegates.observable("
    
     ") {        prop, old, new ->        println("$old -> $new")    }	// 方法二：Delegates.observable()实际上就是对ObservableProperty的一层封装	var name : String by object : ObservableProperty
     
      ("
      
       ") {        override fun afterChange(property: KProperty<*>, oldValue: String, newValue: String) {	        println("$oldValue -> $newValue")        }    }}fun main() {    val user = User()    user.name = "first"    user.name = "second"}复制代码

输出结果 < no name > -> first first -> second

源码：

public inline fun 
    
      observable(initialValue: T, crossinline onChange: (property: KProperty<*>, oldValue: T, newValue: T) -> Unit):        ReadWriteProperty
     
       = object : ObservableProperty
      
       (initialValue) {            override fun afterChange(property: KProperty<*>, oldValue: T, newValue: T) = onChange(property, oldValue, newValue)}public inline fun 
       
         vetoable(initialValue: T, crossinline onChange: (property: KProperty<*>, oldValue: T, newValue: T) -> Boolean):        ReadWriteProperty
        
          = object : ObservableProperty
         
          (initialValue) { // 需要返回一个boolean值，判断是否需要发射，默认为true override fun beforeChange(property: KProperty<*>, oldValue: T, newValue: T): Boolean = onChange(property, oldValue, newValue)}public abstract class ObservableProperty
          
           (initialValue: T) : ReadWriteProperty
           
             { private var value = initialValue protected open fun beforeChange(property: KProperty<*>, oldValue: T, newValue: T): Boolean = true protected open fun afterChange (property: KProperty<*>, oldValue: T, newValue: T): Unit {} // 代理后，自动被调用 public override fun getValue(thisRef: Any?, property: KProperty<*>): T { return value } // 代理后，自动被调用 public override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) { val oldValue = this.value // 判断是否需要发射，默认为true if (!beforeChange(property, oldValue, value)) { return } this.value = value afterChange(property, oldValue, value) }}复制代码

3. 把属性储存在映射中

这部分不知道怎么看源码

class User(val map: Map
    
     ) {    val name: String by map    val age: Int     by map}val user = User(mapOf(    "name" to "John Doe",    "age"  to 25))复制代码

目录

类委托(代理模式)

属性委托

两种实现方法：

标准委托

1. 延迟属性 Lazy

2. 可观察属性 Observable

3. 把属性储存在映射中