Kotlin上手（一）

标准函数with

with函数接收两个参数，第一个参数是任意类型的对象，第二个是Lambda表达式。with函数会在Lambda表达式中提供第一个参数的上下文，并使用Lambda表达式中的最后一行代码作为返回值返回。

fun test() { val list = listOf("Apple", "Banana", "Orange") val result = with(StringBuilder()) { append("开始吃水果") for (fruit in list) { append(fruit).append("\n") } append("水果都被吃完了") toString() } println(result) }

标准函数run

run函数不能直接调用，一定要调用某个对象的run函数才行，run函数只接收一个Lambda表达式作为参数，并且会在Lambda表达式中提供上下文，并把Lambda表达式的最后一行代码作为返回值返回

val list = listOf("Apple", "Banana", "Orange") val result = StringBuilder().run { append("开始吃水果\n") for (fruit in list) { append(fruit).append("\n") } append("水果被吃完") toString() } println(result)

标准函数apply

同样是在某个对象上调用，并接收一个Lambda表达式作为参数，但无法指定返回值

val intent = Intent(context,MainActivity::class.java).apply { putExtra("param1","1") putExtra("param2","2") }

延迟初始化

private lateinit var adapter: RecyclerListAdapter private fun init() { adapter = RecyclerListAdapter() }

判断是否已经完成了初始化

if (!::adapter.isInitiallized) { adapter = RecyclerListAdapter() }

密封类

它是一个可继承的类，Kotlin编译器会自动检查密封类有哪些子类，并强制要求将每一个子类所对应的条件全部处理。密封类及其子类只能定义在同一个文件的顶层位置，不能嵌套在其他类中。

sealed class Result class Success(val msg: String) : Result() class Failure(val error: Exception) : Result() fun getResultMsg(result: Result) = when (result) { is Success -> result.msg is Failure -> "Error is ${result.error.message}" } sealed class MsgViewHolder(view: View): RecyclerView.ViewHolder(view) class LeftViewHolder(view: View): MsgViewHolder(view) class RightViewHolder(view: View): MsgViewHolder(view) class MsgAdapter(val msgList: List<String>): RecyclerView.Adapter<MsgViewHolder>() { override fun onBindViewHolder(holder: MsgViewHolder, position: Int) { val msg = msgList[position] when (holder) { is LeftViewHolder -> holder.leftMsg = msg.leftText is RightViewHolder -> holder.rightMsg = msg.rightText } } }

扩展函数

fun Any.log() { Log.d("测试日志", this.toString()) } fun test1() { "Kotlin扩展函数真牛".log() }

运算符重载

class Money(val value: Int) { operator fun plus(money: Money): Money { val sum = value + money.value return Money(sum) } } fun test2() { val money1 = Money(5) val money2 = Money(10) val money3 = money1 + money2 println(money3.value) }

多重重载

class Money(val value: Int) { operator fun plus(money: Money): Money { val sum = value + money.value return Money(sum) } operator fun plus(newValue: Int): Money { val sum = value + newValue return Money(sum) } } fun test2() { val money1 = Money(5) val money2 = Money(10) val money3 = money1 + money2 val money4 = money3 + 20 println(money4.value) println(money3.value) }