Kotlin作用域函数之间的区别和使用场景详解

val adam = Person("Adam").apply { 
 age = 20
 city = "London"
}
println(adam)

val adam = Person("Adam")
adam.age = 20
adam.city = "London"
println(adam)

fun main() {
 val str = "Hello"
 // this
 str.run {
 println("The receiver string length: $length")
 //println("The receiver string length: ${this.length}") // 和上句效果相同
 }

 // it
 str.let {
 println("The receiver string's length is ${it.length}")
 }
}

val adam = Person("Adam").apply { 
 age = 20  // 和 this.age = 20 或者 adam.age = 20 一样
 city = "London"
}
println(adam)

fun getRandomInt(): Int {
 return Random.nextInt(100).also {
 writeToLog("getRandomInt() generated value $it")
 }
}
val i = getRandomInt()

fun getRandomInt(): Int {
 return Random.nextInt(100).also { value ->
 writeToLog("getRandomInt() generated value $value")
 }
}
val i = getRandomInt()

val numberList = mutableListOf<Double>()
numberList.also { println("Populating the list") }
 .apply {
 add(2.71)
 add(3.14)
 add(1.0)
 }
 .also { println("Sorting the list") }
 .sort()

fun getRandomInt(): Int {
 return Random.nextInt(100).also {
 writeToLog("getRandomInt() generated value $it")
 }
}

val i = getRandomInt()

val numbers = mutableListOf("one", "two", "three")
val countEndsWithE = numbers.run { 
 add("four")
 add("five")
 count { it.endsWith("e") }
}
println("There are $countEndsWithE elements that end with e.")

val numbers = mutableListOf("one", "two", "three")
with(numbers) {
 val firstItem = first()
 val lastItem = last() 
 println("First item: $firstItem, last item: $lastItem")
}

val numbers = mutableListOf("one", "two", "three", "four", "five")
val resultList = numbers.map { it.length }.filter { it > 3 }
println(resultList)

val numbers = mutableListOf("one", "two", "three", "four", "five")
numbers.map { it.length }.filter { it > 3 }.let { 
 println(it)
 // 如果需要可以调用更多函数
} 

val numbers = mutableListOf("one", "two", "three", "four", "five")
numbers.map { it.length }.filter { it > 3 }.let(::println)

val str: String? = "Hello" 
//processNonNullString(str) // 编译错误：str 可能为空
val length = str?.let { 
 println("let() called on $it")
 processNonNullString(it) // 编译通过：'it' 在 '?.let { }' 中必不为空
 it.length
}

val numbers = listOf("one", "two", "three", "four")
val modifiedFirstItem = numbers.first().let { firstItem ->
 println("The first item of the list is '$firstItem'")
 if (firstItem.length >= 5) firstItem else "!"   firstItem   "!"
}.toUpperCase()
println("First item after modifications: '$modifiedFirstItem'")

val numbers = mutableListOf("one", "two", "three")
with(numbers) {
 println("'with' is called with argument $this")
 println("It contains $size elements")
}

val numbers = mutableListOf("one", "two", "three")
val firstAndLast = with(numbers) {
 "The first element is ${first()},"  
 " the last element is ${last()}"
}
println(firstAndLast)

val service = MultiportService("https://example.kotlinlang.org", 80)
val result = service.run {
 port = 8080
 query(prepareRequest()   " to port $port")
}
// 同样的代码如果用 let() 函数来写:
val letResult = service.let {
 it.port = 8080
 it.query(it.prepareRequest()   " to port ${it.port}")
}

val hexNumberRegex = run {
 val digits = "0-9"
 val hexDigits = "A-Fa-f"
 val sign = " -"
 Regex("[$sign]?[$digits$hexDigits] ")
}
for (match in hexNumberRegex.findAll(" 1234 -FFFF not-a-number")) {
 println(match.value)
}

val adam = Person("Adam").apply {
 age = 32
 city = "London" 
}
println(adam)

val numbers = mutableListOf("one", "two", "three")
numbers
 .also { println("The list elements before adding new one: $it") }
 .add("four")