一、类委托
interface DB{fun insert()
}
class SqliteDB : DB {override fun insert() {println(" SqliteDB insert")}
}class MySql : DB{override fun insert() {println(" MySql insert")}
}class OracleDB : DB{override fun insert() {println(" OracleDB insert")}
}class CreateDB(db:DB) : DB by dbfun main() {val db = CreateDB(MySql())db.insert()
}运行main()方法：
CreateDB类需要实现的接口（DB）方法委托给了具体实现类（MySql、OracleDB、SqliteDB）来实现，我认为这和设计模式中的策略模式如出一辙。
通过AndroidStudio的自带工具
Tools -> kotlin -> show kotlin Bytecode ，然后点Decompile，将字节码进行反编译得到java代码
也能佐证我们的想法：
二、委托属性
private var floatValue = 789.12
private var num by ::floatValuefun main() {println("num:$num")num = 567.88println("floatValue:$floatValue")
}运行结果：
这个示例中将属性num委托给了 属性floatValue，即对num进行get和set操作其实都作用在了floatValue上，因此会有上面的运行结果。
同样我们可以看下反编译后的代码：
让我们稍感惊讶的是，经过转换后甚至都没有定义num这个属性，只是增加了getNum和setNum两个方法来获取和改变floatValue的值。
三、自定义委托
方式1：
class Test3{var text:String by StringDelegate()
}class StringDelegate{operator fun getValue(owner:Any,property:KProperty<*>):String{return "delegate value"}operator fun setValue(owner:Any,property:KProperty<*>,value:String){println("value:$value")}
}fun main() {val t = Test3()println("text:${t.text}")t.text = "haha"
}要点：
- 想要成为可接受委托的类需要实现运算符重载方法，若委托类实现了getValue和setValue（即可读可写），则被委托属性text可用var修饰；若委托类只实现了getValue（即只读），则被委托属性text可用val修饰。
- getValue的返回值，和setValue的value参数值类型要与被委托属性类型一致。
- getValue和setValue中的owner参数必须是被委托属性text所属的类或其父类。
方式2：
class Test3{var text:String by StringDelegate()var text2:String by StringDelegate2()
}class StringDelegate{operator fun getValue(owner:Any,property:KProperty<*>):String{return "delegate value"}operator fun setValue(owner:Any,property:KProperty<*>,value:String){println("value:$value")}
}class StringDelegate2:ReadWriteProperty<Any,String>{override fun getValue(thisRef: Any, property: KProperty<*>): String {return "delegate value2"}override fun setValue(thisRef: Any, property: KProperty<*>, value: String) {println("value2:$value")}}fun main() {val t = Test3()println("text:${t.text}")t.text = "haha"println("text2:${t.text2}")t.text2 = "haha"
}要点：
- 这种方式的原理就是方式一，只不过是系统Api给我们提供了一种相对方便的使用方式。
- 此方式需要出入两个泛型，第一个必须是被委托属性text所属的类或其父类，第二个是被委托属性的类型
- 若委托类支持可读可写则继承ReadWriteProperty，若只支持可读则继承ReadOnlyProperty
此方式原理：
方式3：
在上面的基础之上还有另一种方式，可以根据属性的一些特征来返回一个合适的委托实现类，有点类似于抽象工厂模式，根据特定的需求返回合适的实现，如下：
class Test3{var text:String by StringDelegate()var text2:String by StringDelegate2()var text3:String by StringDelegateProvide()
}class StringDelegate{operator fun getValue(owner:Any,property:KProperty<*>):String{return "delegate value"}operator fun setValue(owner:Any,property:KProperty<*>,value:String){println("value:$value")}
}class StringDelegate2:ReadWriteProperty<Any,String>{override fun getValue(thisRef: Any, property: KProperty<*>): String {return "delegate value2"}override fun setValue(thisRef: Any, property: KProperty<*>, value: String) {println("value2:$value")}}class StringDelegate3:ReadWriteProperty<Any,String>{override fun getValue(thisRef: Any, property: KProperty<*>): String {return "delegate value3"}override fun setValue(thisRef: Any, property: KProperty<*>, value: String) {println("value3:$value")}}class StringDelegateProvide{operator fun provideDelegate(thisRef: Any,property: KProperty<*>):ReadWriteProperty<Any,String>{return if (property.name.contains("text3")){StringDelegate3()}else{StringDelegate2()}}
}fun main() {val t = Test3()println("text:${t.text}")t.text = "haha"println("text2:${t.text2}")t.text2 = "haha"println("text3:${t.text3}")
}四、使用案例
通过委托封装Mmkv存取值的过程
import com.blankj.utilcode.util.GsonUtils
import com.tencent.mmkv.MMKV
import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KPropertyopen class MMKVDelegate<T>(private val mmkv: MMKV,private val cls: Class<T>,private val key: String,private val defaultValue: T
) : ReadWriteProperty<Any?, T> {@Suppress("UNCHECKED_CAST")override fun getValue(thisRef: Any?, property: KProperty<*>): T {return when {cls.isEnum -> {val enumValues = cls.enumConstantsval enumValue = mmkv.decodeString(key, null)enumValues?.firstOrNull { it.toString() == enumValue } ?: defaultValue}cls.isData || MMKVDelegatedClass::class.java.isAssignableFrom(cls) -> runCatching {GsonUtils.fromJson(mmkv.decodeString(key), cls)}.getOrNull() ?: defaultValuecls.isAssignableFrom(String::class.java) -> mmkv.decodeString(key, defaultValue as String) as Tcls.isAssignableFrom(Int::class.java) -> mmkv.decodeInt(key, defaultValue as Int) as Tcls.isAssignableFrom(Boolean::class.java) -> mmkv.decodeBool(key, defaultValue as Boolean) as Tcls.isAssignableFrom(Float::class.java) -> mmkv.decodeFloat(key, defaultValue as Float) as Tcls.isAssignableFrom(Long::class.java) -> mmkv.decodeLong(key, defaultValue as Long) as Tcls.isAssignableFrom(Double::class.java) -> mmkv.decodeDouble(key, defaultValue as Double) as Tcls.isAssignableFrom(ByteArray::class.java) -> mmkv.decodeBytes(key, defaultValue as ByteArray) as Telse -> throw IllegalArgumentException("Unsupported type.")}}override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {val ok = when {cls.isEnum -> mmkv.encode(key, value?.toString())cls.isData || MMKVDelegatedClass::class.java.isAssignableFrom(cls) -> mmkv.encode(key,GsonUtils.toJson(value))cls.isAssignableFrom(String::class.java) -> mmkv.encode(key, value as String)cls.isAssignableFrom(Int::class.java) -> mmkv.encode(key, value as Int)cls.isAssignableFrom(Boolean::class.java) -> mmkv.encode(key, value as Boolean)cls.isAssignableFrom(Float::class.java) -> mmkv.encode(key, value as Float)cls.isAssignableFrom(Long::class.java) -> mmkv.encode(key, value as Long)cls.isAssignableFrom(Double::class.java) -> mmkv.encode(key, value as Double)cls.isAssignableFrom(ByteArray::class.java) -> mmkv.encode(key, value as ByteArray)else -> throw IllegalArgumentException("Unsupported type.")}if (!ok) {throw MMKVDelegateException.EncodeException()}}
}import kotlin.jvm.internal.Reflectionval <T> Class<T>.isData: Boolean get() = Reflection.getOrCreateKotlinClass(this).isData假设我们在项目中需要在本地记录一些App行为信息
object AppBehavior {private const val TAG = "AppBehavior"private const val MMKV_ID = "AppBehavior"//记录App启动次数var appStartTime: Int by SettingValue(Int::class.java,Key.APP_START_TIME,0)/*** APP行为的属性委托* 值存储成功时，自动发送UserLocalSettingChangedEvent事件* @param cls 属性的类型* @param key 存储到MMKV的key* @param defaultValue 从MMKV获取失败时，返回的默认值*/private class SettingValue<T>(private val cls: Class<T>,private val key: Key,private val defaultValue: T) : MMKVDelegate<T>(MMKV.mmkvWithID(MMKV_ID), cls, key.name, defaultValue) {override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {runCatching {super.setValue(thisRef, property, value)}.onFailure {Log.e(TAG, it.toString())if (it !is MMKVDelegateException) {throw it}}}}
}调用：
AppBehavior.appStartTime++
