Scope函數(shù)翻譯過來就叫作用域函數(shù)吧闻坚，Kotlin中的作用域函數(shù)包含let, run with, apply, also五個(gè).
作用域函數(shù)方便我們更加容易操作對(duì)象本身舀患，使得代碼更加易讀求厕。舉個(gè)例子
定一個(gè)Person類
class Person(var name : String, var age : Int, var country : String) {
override fun toString(): String {
return "Person(name='$name', age=$age, city='$country')"
}
}
通常操作如下
val person = Person("jay",22,"china")
person.country = "usa"
Log.i("tag", person.toString())
無非是更改國家后再輸出該對(duì)象的值柬唯，如果使用作用域中函數(shù)效果是
Person("jay",22,"china").let {
it.country = "usa"
Log.i("tag", it.toString())
}
上面使用作用域函數(shù)代碼會(huì)更加清晰.
下面來說下每個(gè)作用域函數(shù)的區(qū)別. 主要有兩個(gè)
1债朵，引用到的context對(duì)象不同
2畸陡，返回值不同
下面分別來看
1，引用到的context對(duì)象不同
context有兩種架忌，一個(gè)是this一個(gè)是it.
fun main() {
val str = "Hello"
// this
str.run {
println("The receiver string length: $length")
//println("The receiver string length: ${this.length}") // does the same
}
// it
str.let {
println("The receiver string's length is ${it.length}")
}
}
run, with, apply是使用的是this作為context對(duì)象吞彤，因此在函數(shù)中，對(duì)象本身可以像它類中方法一樣使用叹放，你也可以忽略掉this的調(diào)用
val adam = Person("Adam").apply {
age = 20 // same as this.age = 20 or adam.age = 20
city = "London"
}
println(adam)
let和also以lambda參數(shù)形式傳入饰恕，它的context是it(前提是沒有指定參數(shù)名字).
fun getRandomInt(): Int {
return Random.nextInt(100).also {
writeToLog("getRandomInt() generated value $it")
}
}
val i = getRandomInt()
另外也可以自定義參數(shù)名字
fun getRandomInt(): Int {
return Random.nextInt(100).also { value ->
writeToLog("getRandomInt() generated value $value")
}
}
val i = getRandomInt()
2，返回值不同
apply和also返回的context對(duì)象
let,run和with返回的是lambda執(zhí)行的結(jié)果
來看看also調(diào)用的情況
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()
在返回context對(duì)象這種情況下可以作為一個(gè)鏈?zhǔn)秸{(diào)用使用.
當(dāng)然也可以不去操作context對(duì)象井仰，用來輸出一些信息然后返回context對(duì)象
fun getRandomInt(): Int {
return Random.nextInt(100).also {
writeToLog("getRandomInt() generated value $it")
}
}
val i = getRandomInt()
另外一種情況就是let,run和with返回的是lambda執(zhí)行的結(jié)果埋嵌，如下
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.")
每個(gè)作用域函數(shù)的區(qū)別
每個(gè)作用域函數(shù)實(shí)際上在使用上是可以互換的，主要是看如何使用起來更加方便易讀.
let
1,let用來在一個(gè)調(diào)用鏈返回后調(diào)用一個(gè)或者更多函數(shù)俱恶，舉個(gè)例子
val numbers = mutableListOf("one", "two", "three", "four", "five")
val resultList = numbers.map { it.length }.filter { it > 3 }
println(resultList)
如果使用write你可以重寫成
val numbers = mutableListOf("one", "two", "three", "four", "five")
numbers.map { it.length }.filter { it > 3 }.let {
println(it)
// and more function calls if needed
}
2,let用來執(zhí)行一個(gè)非空對(duì)象的代碼塊.
val str: String? = "Hello"
//processNonNullString(str) // compilation error: str can be null
val length = str?.let {
println("let() called on $it")
processNonNullString(it) // OK: 'it' is not null inside '?.let { }'
it.length
}
3,另外一個(gè)場景是let引進(jìn)一個(gè)本地變量去提高代碼的可讀性.
val numbers = listOf("one", "two", "three", "four")
val modifiedFirstItem = numbers.first().let { firstItem ->
println("The first item of the list is '$firstItem'")
if (firstItem.length >= 5) firstItem else "!" + firstItem + "!"
}.toUpperCase()
println("First item after modifications: '$modifiedFirstItem'")
with
With通常需要穿一個(gè)context對(duì)象作為參數(shù)雹嗦，在函數(shù)內(nèi)部使用this作為context對(duì)象調(diào)用，返回值是lambda的執(zhí)行結(jié)果.
1, 通常我們用它做一些伴生操作合是，官方文檔認(rèn)為它 “with this object, do the following.” 自行體會(huì)
val numbers = mutableListOf("one", "two", "three")
with(numbers) {
println("'with' is called with argument $this")
println("It contains $size elements")
}
2了罪，另外可以用來計(jì)算一個(gè)結(jié)果后返回值
val numbers = mutableListOf("one", "two", "three")
val firstAndLast = with(numbers) {
"The first element is ${first()}," +
" the last element is ${last()}"
}
println(firstAndLast)
run
run屬于context對(duì)象是this, 返回結(jié)果是lambda執(zhí)行的結(jié)果.
1,通常用來執(zhí)行一些對(duì)象初始化和一些操作后返回結(jié)果, 它可以和let進(jìn)行互換
val service = MultiportService("https://example.kotlinlang.org", 80)
val result = service.run {
port = 8080
query(prepareRequest() + " to port $port")
}
// the same code written with let() function:
val letResult = service.let {
it.port = 8080
it.query(it.prepareRequest() + " to port ${it.port}")
}
2, run還有另外一個(gè)特點(diǎn)，它可以不作為對(duì)象的擴(kuò)展函數(shù)聪全，直接調(diào)用執(zhí)行一個(gè)代碼塊.
val hexNumberRegex = run {
val digits = "0-9"
val hexDigits = "A-Fa-f"
val sign = "+-"
Regex("[$sign]?[$digits$hexDigits]+")
}
for (match in hexNumberRegex.findAll("+1234 -FFFF not-a-number")) {
println(match.value)
}
apply
apply的context是this, 返回對(duì)象的對(duì)象自己.
使用apply通常是對(duì)象的屬性進(jìn)行賦值操作,然后返回它自己泊藕，進(jìn)行一些鏈?zhǔn)讲僮?/p>
val adam = Person("Adam").apply {
age = 32
city = "London"
}
println(adam)
also
Context對(duì)象是it, 返回值是對(duì)象本身
通常用來做些輸出日志，打印調(diào)試信息的操作难礼，可以理解成and also do the following.
val numbers = mutableListOf("one", "two", "three")
numbers
.also { println("The list elements before adding new one: $it") }
.add("four")
對(duì)于上面的描述娃圆，官方文檔給出了一張表格
