2024五月發(fā)布了Kotlin 2.0，包含了一些新特性比如Enhanced Type Inference、Multiple Context Receivers、Better Coroutines等，本文研究下這幾個(gè)新特性。

1 Improved Type Inference

在Kotlin 1.x版本中，有下面的類型推導(dǎo)寫法：

val listOfPairs = listOf(Pair(1, "One"), Pair(2, "Two"))
val mapFromList = listOfPairs.associate { pair: Pair<Int, String> ->
    pair.first to pair.second
}

在Kotlin 2.x版本，就可以使用更好的方式：

val listOfPairs = listOf(1 to "One", 2 to "Two")
val mapFromList = listOfPairs.associate { it.first to it.second }

2 Multiple Context Receivers

在Kotlin 2.x版本，支持了傳入多個(gè)context-receivers參數(shù)，首先我們需要了解下什么是context-receivers。

2.1 了解 context-receivers

比如我們要給String寫個(gè)彈Toast的擴(kuò)展函數(shù)，一般我們會這樣寫：

fun String.toast1(activity: Activity, duration: Int = Toast.LENGTH_LONG) {
    Toast.makeText(activity, this, duration).show()
}

借助context-receivers，我們對上面的函數(shù)進(jìn)行改造，使之更加簡潔易用：
第一步， 在app/build.gradle.kts添加配置：

kotlinOptions {
    jvmTarget = "1.8"
    freeCompilerArgs += arrayOf("-Xcontext-receivers")
}

如果不指定-Xcontext-receivers，會有錯(cuò)誤提示：The feature "context receivers" is experimental and should be enabled explicitly。

第二步，新的函數(shù)實(shí)現(xiàn)：

context(Activity)
fun String.toast2(duration: Int = Toast.LENGTH_LONG) {
    Toast.makeText(this@Activity, this, duration).show()
}

這樣，我們可以直接在一個(gè)Activity的實(shí)例中調(diào)用"hola amigo".toast2(5000)，而不必再傳Activity參數(shù)了，是不是比第一個(gè)實(shí)現(xiàn)方式有那么一點(diǎn)簡潔呢？
context-receivers也挺簡單的，下面介紹下Multiple Context Receivers。顧名思義，就是支持傳入多個(gè)參數(shù)?？梢钥聪旅娴拇a：

class Database {
    fun queryData(): String = "Data from database"
}

class Network {
    fun fetchData(): String = "Data from network"
}

context(Database, Network)
fun getData(): String {
    val dbData = queryData()
    val networkData = fetchData()
    return "$dbData - $networkData"
}

fun testWith() {
    val database = Database()
    val network = Network()

    with(database) {
        with(network) {
            println(getData())
        }
    }
}

可以看到，getData函數(shù)支持傳兩個(gè)context-receiver參數(shù)，分別是Database和Network。

3 Improved Coroutines

關(guān)于協(xié)程優(yōu)化，我們貼出兩個(gè)功能一模一樣的函數(shù)代碼，只是實(shí)現(xiàn)方式有點(diǎn)小diff，一段是Kotlin1.x的實(shí)現(xiàn)kotlin1_x()，一段是Kotlin2.x的實(shí)現(xiàn)kotlin2_x()。

suspend fun preBlockTest(): Int {
    StarkLog.i("zzh", "abtestclick preblock start 0000")
    delay(300)
    StarkLog.i("zzh", "abtestclick preblock end 1111")
    return -1
}
suspend fun networkRequest(): Int {
    StarkLog.i("zzh", "abtestclick network start 0000")
    delay(300)
    StarkLog.i("zzh", "abtestclick network end 1111")
    return 1
}

suspend fun databaseQuery(): String {
    StarkLog.i("zzh", "abtestclick database start 0000")
    delay(200)
    StarkLog.i("zzh", "abtestclick database end 1111")
    return "succ"
}

fun kotlin1_x() {
    StarkLog.i("zzh", "abtestclick start 0000")
    runBlocking {
        var d1: Deferred<Int> = async { preBlockTest() }
        d1.invokeOnCompletion { handler ->
            run {
                StarkLog.i("zzh", "abtestclick preblock complete")
            }
        }
        var d1ret = d1.await() // 這里的await會阻塞下面兩個(gè)協(xié)程的執(zhí)行，否則不會阻塞，
        StarkLog.i("zzh", "abtestclick preblock await d1ret=${d1ret}")
        // kotlin 1.x
        val deferredResults = listOf(
            async { networkRequest() },
            async { databaseQuery() }
        )
        val results = deferredResults.awaitAll()
        StarkLog.i("zzh", "abtestclick Results: $results")
    }
    StarkLog.i("zzh", "abtestclick end 1111")
}
fun kotlin2_x() {
    StarkLog.i("zzh", "abtestclick start 0000")
    runBlocking {
        var d1: Deferred<Int> = async { preBlockTest() }
        d1.invokeOnCompletion { handler ->
            run {
                StarkLog.i("zzh", "abtestclick preblock complete")
            }
        }
        var d1ret = d1.await() // 這里的await會阻塞下面兩個(gè)協(xié)程的執(zhí)行，否則不會阻塞，
        StarkLog.i("zzh", "abtestclick preblock await d1ret=${d1ret}")
        // kotlin 2.x
        val results2 = awaitAll(
            async { networkRequest() },
            async { databaseQuery() }
        )
        println("Results: $results2")
    }
    StarkLog.i("zzh", "abtestclick end 1111")
}

參考：

Using Kotlin 2.0 for Android Development

Context Receivers