Kotlin语言编程技巧集

时间:2021-11-02 15:48:52

空语句

Kotlin 语言中的空语句有

  • {}
  • Unit
when (x) {
1 -> ...
2 -> ...
else -> {}
// else -> Unit
}

When 表达式

使用不带判断条件的 when 表达式来改写多路分支

val v = if (x < y) 1 else if (x == y) 2 else 3

val v = when {
x < y -> 1
x == y -> 2
else -> 3
}

使用带判断条件的 when 表达式来模拟模式匹配

val v = if (x == 1) 1 else if (x == 2) 3 else 5

val v = when (x) {
1 -> 1
2 -> 3
else -> 5
}

?. 与 ?:

// n的值为a,b,c,4当中第一个不是null的数
val n = a ?: b ?: c ?: 4
a b c n
1 / / 1
null 2 / 2
null null 3 3
null null null 4
// n的值为a.b.c,条件是a,a.b,a.b.c都不是null。否则n的值为4。
val n = a?.b?.c ?: 4
a a.b a.b.c n
null / / 4
!= null null / 4
!= null != null null 4
!= null != null 3 3

使用解构声明来声明两个带值的变量

var (a, b) = listOf(1, 2) // a == 1, b == 2
var (a, b) = Pair(1, 2) // a == 1, b == 2
var (a, b) = 1 to 2 // a == 1, b == 2

let

the tldr; on Kotlin’s let, apply, also, with and run functions

// Calls the specified function [block] with `this` value as its argument and returns its result.
public inline fun <T, R> T.let(block: (T) -> R): R = block(this)

调用代码块,代码块中调用方 this 为参数 it,返回代码块的结果。

// using 'let' to convert from one type to another
val answerToUniverse = strBuilder.let {
it.append("Douglas Adams was right after all")
it.append("Life, the Universe and Everything")
42
}
// using 'let' to only print when str is not null
str?.let { print(it) }

apply

// Calls the specified function [block] with `this` value as its receiver and returns `this` value.
public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }

调用代码块,代码块中调用方 this 为隐式调用方 receiver,返回调用方 this。

// old way of building an object
val andre = Person()
andre.name = "andre"
andre.company = "Viacom"
andre.hobby = "losing in ping pong"
// after applying 'apply' (pun very much intended)
val andre = Person().apply {
name = "Andre"
company = "Viacom"
hobby = "losing in ping pong"
}

also

// Calls the specified function [block] with `this` value as its argument and returns `this` value.
public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }

调用代码块,代码块中调用方 this 为参数 it,返回调用方 this 。

// transforming data from api with intermediary variable
val rawData = api.getData()
Log.debug(rawData)
rawData.map { /** other stuff */ }
// use 'also' to stay in the method chains
api.getData()
.also { Log.debug(it) }
.map { /** other stuff */ }

with

// Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
public inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()

调用代码块,代码块中指定参数为隐式调用方 receiver,返回代码块的结果。

// Every Android Developer ever after Wednesday May 17th 2017
messageBoard.init(“https://url.com”)
messageBoard.login(token)
messageBoard.post("Kotlin’s a way of life bro") // using 'with' to avoid repetitive references to identifier
with(messageBoard) {
init(“https://url.com”)
login(token)
post(“Kotlin’s a way of life bro")
}

run

// Calls the specified function [block] with `this` value as its receiver and returns its result.
public inline fun <T, R> T.run(block: T.() -> R): R = block()

调用代码块,代码块中调用方 this 为隐式调用方 receiver,返回代码块的结果。

// GoT developers after season 7
aegonTargaryen = jonSnow.run {
makeKingOfTheNorth()
swearsFealtyTo(daenerysTargaryen)
realIdentityRevealed(“Aegon Targaryen”)
}

let, apply, also, with & run

代码块/函数 let apply also with run
参数或调用方 this 为隐式调用方 receiver
调用方 this 为参数 it
返回调用方 this
返回代码块的结果

takeIf / takeUnless

difference between kotlin also, apply, let, use, takeIf and takeUnless in Kotlin

// Returns this value if it satisfies the given predicate or null, if it doesn't.
inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? = if (predicate(this)) this else null
// Returns this value if it does not satisfy the given predicate or null, if it does.
inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predicate(this)) this else null

takeIf:调用方 this 如果符合某个条件则返回调用方 this,否则返回 null。

takeUnless:调用方 this 如果不符合某个条件则返回调用方 this,否则返回 null。

println(myVar.takeIf { it is Person } ?: "Not a person!")
println(myVar.takeUnless { it is Person } ?: "It's a person!")