Example练习-高级部分

• Scope Functions

  • let

    fun customPrint(s: String) {
    print(s.uppercase())
    }
    fun main() {
    val empty = "test".let {
       customPrint(it)
       it.isEmpty()
    }
    println("$empty")
    fun printNonNull(str:String?) {
       println("Printing $str")
       str?.let {
          print("\t")
          customPrint(it)
       }
    }
    fun printIfBothNonNull(strOne: String?, strTwo: String?) {
       strOne?.let { firstString ->
           strTwo?.let { secondString -> 
              customPrint("$firstString : $secondString")
              println()
           }
       }
    }
    printNonNull(null)
    printNonNull("my string") 
    printIfBothNonNull("First","Second") 
    printIfBothNonNull("1",null)
    }

  • run,通过this取object

    fun main() {
    fun getNullableLength(ns:String?) {
     println("for \"$ns\":")
     ns?.run {
         println("\tis empty? " + isEmpty())
         println("\tlength = $length")
         length
     }
    }
    getNullableLength(null)
    getNullableLength("")
    getNullableLength("some string with Kotlin")
    }
    

  • with

    class Configuration(var host: String, var port: Int) 
    fun main() {
    val configuration = Configuration(host = "127.0.0.1", port = 9000) 
    
    with(configuration) {
      println("$host:$port")
    }
    // instead of:
    println("${configuration.host}:${configuration.port}")    
    }
    

  • apply，自动返回调用对象本身

    data class Person(var name: String, var age:Int, var about:String) {
    constructor():this("",0,"")
    }
    fun main() {
    val jake = Person()
    val stringDescription = jake.apply {
       name = "Jake"
       age = 30
       about = "Android developer"
    }.toString()
    println(stringDescription)
    }
    

  • also,也是执行代码块后返回对象，通过it进行引用，主要用于额外的操作

    data class Person(var name: String, var age: Int, var about: String) {
           constructor() : this("", 0, "")
    }
    fun writeCreationLog(p: Person) {
    println("A new person ${p.name} was created.")
    }         
    fun main() {
    val jake = Person("Jake",30,"Android developer")
      .also {
          writeCreationLog(it)
      }
    }
    

• 代理

  interface SoundBehavior {
    fun makeSound()
  }
  class ScreamBehavior(val n:String): SoundBehavior {
    override fun makeSound() = println("${n.toUpperCase()} !!!")
  }
  class RockAndRollBehavior(val n:String): SoundBehavior {
    override fun makeSound() = println("I'm The King of Rock 'N' Roll: $n")
  }
  class TomAraya(n:String): SoundBehavior by ScreamBehavior(n)
  class ElvisPresley(n:String): SoundBehavior by ScreamBehavior(n)
  fun main() {
    val tomAraya = TomAraya("Thrash Metal")
    tomAraya.makeSound()
    val elvisPresley = ElvisPresley("Dancin to the Jailhouse Rock")
    elvisPresley.makeSound()
  }
  

  ```kotlin //代理属性 class Example { var p:String by Delegate() override fun toString() = “Example Class” } class Delegate() { operator fun getValue(thisRef: Any?, prop: KProperty<*>): String {

   return "$thisRef, thank you for delegating '${prop.name}' to me!"
  

  } operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: String) {

   println("$value has been assigned to ${prop.name} in $thisRef")
  

  } } fun main() { val e = Example() println(e.p) e.p = “NEW” } //Standard Delegates class LazySample { init { println(“created!”) } val lazyStr: String by lazy { println(“computed!”) “my lazy” } }

fun main() { val sample = LazySample() println(“lazyStr = ${sample.lazyStr}”)//需要计算缓存结果 println(“ = ${sample.lazyStr}”)//直接取结果 }

-  解构
```kotlin
fun findMinMax(list: List<Int>): Pair<Int,Int> {
    //do the math
    return Pair(50, 100)
}
fun main() {
    val (x,y,z) = listOf(2,34,4)
    val map = mapOf("Alice" to 21, "Bob" to 25)
    for((name, age) in map) {
        println("$name is $age years old")
    }
    val (min,max) = findMinMax(listOf(100,90,60))
}

data class User(val username:String, val email:String)
fun getUser() = User("Mary", "mary@somewhere.com")
fun main() {
    val user = getUser()
    val (username, email) = user
    println(username == user.component1())
    val (_, emailAddress) = getUser()
}

class Pair<K, V>(val first: K, val second: V) {
    operator fun compenent1(): K {
        return first
    }
    operator fun component2(): V {
        return second
    }
}

fun main() {
    val (num, name) = Pair(1,"one")
    println("num = $num, name = $name")
}