1.官方文档
英文：
https://kotlinlang.org/docs/reference/delegation.html
https://kotlinlang.org/docs/reference/delegated-properties.html
中文：
https://www.kotlincn.net/docs/reference/delegation.html
https://www.kotlincn.net/docs/reference/delegated-properties.html
2.委托作用
用来实现代理/委托模式，可以在不修改原来代码及软件结构的情况下，对原有功能扩展或者修改。也可以在设计时直接使用委托模式方便扩展。
3.类委托
3.1 示例
interface __base1__ { fun f2() }
class base1Impl : __base1__ {
override fun f2() {
Log.e(TAG_DELEGATE,"base1Impl.f2()")
}
}
class delegate1(var impl : __base1__) : __base1__ by impl
fun delegate_test1(){
Log.e(TAG_DELEGATE,"====================================== delegate_test_ 1 ")
var impl = base1Impl()
var delegate1 = delegate1(impl)
delegate1.f2()
var d2 = delegate1(object : __base1__{
override fun f2() {
Log.e(TAG_DELEGATE,"delegate1.companion.impl.f2()")
}
})
d2.f2()
}结果
2019-09-13 09:46:04.165 25360-25360/com.example.kotlin E/delegate: ====================================== delegate_test_ 1
2019-09-13 09:46:04.166 25360-25360/com.example.kotlin E/delegate: base1Impl.f2()
2019-09-13 09:46:04.166 25360-25360/com.example.kotlin E/delegate: delegate1.companion.impl.f2()
其中：
- __base1__是接口，
- base1Impl 是__base1__的实现
- delegate1 是个委托，它在声明实现接口__base1__，但是并没有实现任何函数。是把它交给它的一个成员impl处理。这个impl就是base1Impl
- kotlin中使用关键字 by 声明委托。如下：
class delegate1(var impl : __base1__) : __base1__ by impl
3.2 接口能被委托，类不可以
open class deleagte_base2
open class delegate_sub1 : deleagte_base2()
class delegate1(base1 : deleagte_base2) : deleagte_base2 by base1 //error 只有接口能被委托，类不可以 abstract class AB1{
abstract fun f1()
}
class D2 (var ab : AB1) : AB1 by ab //error,抽象类也不可以委托。
结果： 第3、8 行编译不过。
3.3 不要重写委托接口的方法
不要重写委托接口的方法，否则调用的是重写版本的，不是委托对象的。
interface __I3__ { fun f3() }
class I3Impl : __I3__{
override fun f3() {
Log.e(TAG_DELEGATE,"I3Impl.f3()")
}
}
class delegate3(i3 : __I3__) : __I3__ by i3 {
override fun f3() { //不要重写委托类的方法，否则调用的是重写的，不是委托的。
Log.e(TAG_DELEGATE,"delegate3.f3()")
}
}
fun delegate_test3(){
Log.e(TAG_DELEGATE,"====================================== delegate_test_ 3 ")
var im3 = I3Impl()
var delegate3 = delegate3(im3)
delegate3.f3()
}结果
2019-09-13 10:55:35.772 27606-27606/com.example.kotlin E/delegate: ====================================== delegate_test_ 3
2019-09-13 10:55:35.772 27606-27606/com.example.kotlin E/delegate: delegate3.f3()
delegate3 中的f3被执行，并不是 I3Impl 的f3 被执行。
4.变量委托
成员变量、全局变量、局部变量 的管理都可以委托给其它特定对象。
4.1 kotlin语言包里自带的变量委托
| 委托 | 作用 |
lazy | 不可变量(val)的延迟初始化 |
observable | 观察变量的变化 |
| vetoable | 观察变量的变化，且在改值时有否定权 |
| notNull | 变量是否为空，为空是不可使用。 |
| map | 变量委托给一个map管理 |
4.2 自带变量委托示例
//成员变量使用委托属性
class delegate4{ val name : String by lazy(mode = LazyThreadSafetyMode.NONE){
//mode = LazyThreadSafetyMode.PUBLICATION 表示关闭同步锁，默认开启同步锁的
Log.e(TAG_DELEGATE,"lazy called ")
"lazy"
} var value : Int by Delegates.observable(initialValue = ){
prop,old,new ->
Log.e(TAG_DELEGATE,"${prop.name} onChanged : old = $old,new = $new")
} var count : Int by Delegates.vetoable(initialValue = ){
prop,old,new ->
Log.e(TAG_DELEGATE,"${prop.name} onChanged : old = $old,new = $new ")
false //return false 表示不接受修改，本次修改被否决
} var str : String by Delegates.notNull() //在未初始化前使用，IllegalStateException。 } fun delegate_test4(){
Log.e(TAG_DELEGATE,"====================================== delegate_test_ 4 ") var d4 = delegate4() d4.value = Log.e(TAG_DELEGATE,"d4.name = ${d4.name}") d4.value = d4.count = d4.count = Log.e(TAG_DELEGATE,"d4.count = ${d4.count}") // Log.e(TAG_DELEGATE,"d4.str = ${d4.str}") //error d4.str未初始化。 }
结果
2019-09-13 15:13:25.209 8175-8175/com.example.kotlin E/delegate: ====================================== delegate_test_ 4
2019-09-13 15:13:25.212 8175-8175/com.example.kotlin E/delegate: value onChanged : old = 0,new = 99
2019-09-13 15:13:25.212 8175-8175/com.example.kotlin E/delegate: lazy called
2019-09-13 15:13:25.212 8175-8175/com.example.kotlin E/delegate: d4.name = lazy
2019-09-13 15:13:25.213 8175-8175/com.example.kotlin E/delegate: value onChanged : old = 99,new = 1024
2019-09-13 15:13:25.213 8175-8175/com.example.kotlin E/delegate: count onChanged : old = 8,new = 2046
2019-09-13 15:13:25.213 8175-8175/com.example.kotlin E/delegate: count onChanged : old = 8,new = 328
2019-09-13 15:13:25.213 8175-8175/com.example.kotlin E/delegate: d4.count = 8
map 示例
class D9(val map8 : Map<String,Any?>){
val name : String by map8
val id : Int by map8
var T =
//var age : Int by T //error,Int 并没有提供委托函数。
var age : Int = T
var mmap = mutableMapOf<String,Any?>()
var desc : String by mmap
init{
mmap.put("desc","done")
}
override fun toString(): String {
return "name = $name,id = $id,desc = $desc,age = $age"
}
fun reset(){
mmap.put("desc","null")
this.age =
this::age
::age
}
}
fun delegate_test9(){
Log.e(TAG_DELEGATE,"====================================== delegate_test_9 ")
var map = mutableMapOf<String,Any?>(
"name" to "sia"
,"age" to
,"id" to
)
var d9 = D9(map)
Log.e(TAG_DELEGATE,"d9 = $d9")
//d9本身的name和id不可修改，但是它的值来自外部map，当map改变时，它们也改了。
map.put("name","nick")
map.put("id",)
Log.e(TAG_DELEGATE,"d9 = $d9")
d9.reset()
Log.e(TAG_DELEGATE,"d9 = $d9")
}结果
2019-09-13 15:13:25.215 8175-8175/com.example.kotlin E/delegate: ====================================== delegate_test_9
2019-09-13 15:13:25.216 8175-8175/com.example.kotlin E/delegate: d9 = name = sia,id = 3,desc = done,age = 200
2019-09-13 15:13:25.216 8175-8175/com.example.kotlin E/delegate: d9 = name = nick,id = 8,desc = done,age = 200
2019-09-13 15:13:25.217 8175-8175/com.example.kotlin E/delegate: d9 = name = nick,id = 8,desc = null,age = 0
5.变量委托的注意事项
5.1 指定初始值时不能使用委托
fun delegate_test5(){
Log.e(TAG_DELEGATE,"====================================== delegate_test_5 ")
val noinit1 : Int = 10 by Delegates.notNull() //error,指定初始值时不能使用委托
var noinit2 : Int by Delegates.notNull() //ok
noinit2 =
val size = by lazy { Log.e(TAG_DELEGATE,"init"); } //error,size已经初始化。
class d8{
var value = by Delegates.notNull<Int>() //error,value已经初始化。
}
}结果：第 4、8、10 行编译不过
5.2 类型可以省略
fun delegate_test6(){
Log.e(TAG_DELEGATE,"====================================== delegate_test_6 ")
val notype1 by lazy { }
val notype2 by lazy { Log.e(TAG_DELEGATE,"init"); "notype2" }
Log.e(TAG_DELEGATE,"notype1 = $notype1 , notype2 = $notype2")
}结果
2019-09-13 15:41:07.935 8824-8824/com.example.kotlin E/delegate: ====================================== delegate_test_6
2019-09-13 15:41:07.936 8824-8824/com.example.kotlin E/delegate: init
2019-09-13 15:41:07.936 8824-8824/com.example.kotlin E/delegate: notype1 = 3 , notype2 = notype2
5.3 不能同时指定多个委托
fun delegate_test7(){
Log.e(TAG_DELEGATE,"====================================== delegate_test_7 ")
val notype1 by lazy { 3 } ,Delegates.notNull() //error
val SIZE by lazy { Log.e(TAG_DELEGATE,"init"); }
Log.e(TAG_DELEGATE,"SIZE = $SIZE,不能同时指定多个委托")
}结果： 第4行编译不过