Kotlin把代码写少的Tips

Kotlin已经被google列为Android官方的一级开发语言，此文章是在自己学习观看google io 2017 和阅读Kotlin官方文档的时候，记录下Kotlin的一些write less的Tips.

• Tip1：定义函数

#常规写法：
fun sum(a: Int, b: Int): Int {
    return a + b
}
#Write less:
fun sum(a:Int, b:Int) = a + b

• **Tip2:使用when关键字 **

#相对于java常规的switch case，when提供了更加灵活的方式：
fun describe(obj: Any): String =
when (obj) {
    1          -> "One"
    "Hello"    -> "Greeting"
    is Long    -> "Long"
    !is String -> "Not a string"
    else       -> "Unknown"
}

fun main(args: Array<String>) {
    println(describe(1))
    println(describe("Hello"))
    println(describe(1000L))
    println(describe(2))
    println(describe("other"))
}

• Tip3:for循环中使用in,step,downTo

#组合使用in,step,downTo
fun main(args: Array<String>) {
    for (x in 1..10 step 4) {
        print(x)
    }
    print(" - ")
    for (x in 9 downTo 0 step 3) {
        print(x)
    }
}
Result:159 - 9630

• Tip4:使用lambda表达式来操作集合

#在集合中使用filter,map,sortBy等关键字
fun main(args: Array<String>) {
    val fruits = listOf("banana", "avocado", "apple", "kiwi")
    fruits
    .filter { it.startsWith("a") }
    .sortedBy { it }
    .map { it.toUpperCase() }
    .forEach { println(it) }
}
Result:
APPLE
AVOCADO

• Tip5:条件语句

when (x) {
    is Foo -> ...
    is Bar -> ...
    else   -> ...
}

• Tip6:构造只读list，map

val list = listOf("a", "b", "c")
val map = mapOf("a" to 1, "b" to 2, "c" to 3)

• Tip7:构造单例

object Resource {
    val name = "Name"
}

• Tip8:null情况的赋值处理

val data = ...
val email = data["email"] ?: throw IllegalStateException("Email is missing!")

• Tip9:在return中使用when关键字

fun transform(color: String): Int {
    return when (color) {
        "Red" -> 0
        "Green" -> 1
        "Blue" -> 2
        else -> throw IllegalArgumentException("Invalid color param value")
    }
}

• Tip10:使用with关键字调用多个方法

class Turtle {
    fun penDown()
    fun penUp()
    fun turn(degrees: Double)
    fun forward(pixels: Double)
}

val myTurtle = Turtle()
with(myTurtle) { //draw a 100 pix square
    penDown()
    for(i in 1..4) {
        forward(100.0)
        turn(90.0)
    }
    penUp()
}

• Tip11:if else的简写方法

#普通写法
var max: Int
if (a > b) {
    max = a
} else {
    max = b
}
#简写
val max = if (a > b) a else b

• Tip12:成员变量的get和set方法

var stringRepresentation: String
   get() = this.toString()
   set(value) {
       setDataFromString(value)
   }

#简写get
val isEmpty get() = this.size == 0  // has type Boolean

• Tip13:重载操作符

operator fun ViewGroup.get(index : Int): View? = getChildAt(index)
operator fun ViewGroup.minusAssign(child: View) = removeView(child)
operator fun ViewGroup.plusAssign(child: View) = addView(child)

#重载[] -  +,你就可以使用下面的方法来操作view

val views = //...
val first = views[0]
views -= first
view += first

• Tip14:使用inline函数

#java的方式
MainActivity.java
SQLiteDatabase db = //.....
db.beginTransacation();
try {
    db.delete("user","first_name = ?",new String[] {"jake"};
    db.setTransactionSuccessful();
} finally {
    db.endTransaction();
}

#kotlin的inline函数
##Database.kt
inline fun SQLiteDatabase.transaction(body : () -> Unit ) {
    beginTransaction()
    try {
          body()
          setTransaction()
    } finally {
          endTransaction()
    }
}
##MainActivity.kt
val db = //....
db.transaction = {
        db.delete("users", "first_name = ?",arrayOf("jake"))
}

• Tip15:使用Delegate来观察值变化

private val name by Delegate.observable("jane")  {old, new, prop - >
        println("Name changed form $old to $new )
}