混淆理解 lambda 和接收器

Question

Kotlin 版本 1.2.50

我一直在关注 YouTube 上本教程的一些示例 https://www.youtube.com/watch?v=gPH9XnvpoXE。并且有几件事我已经明白了，但还是有些迷茫。我在下面的代码中留下了我不确定发生了什么的评论。

fun main(args: Array<String>) {

    val javaClient = createClient {
        firstName = "joe"
        lastName = "bloggs"

        twitter {
            handle = "@joebloggs"
        }
    }

    println(javaClient.toConsole)
}

/* Are we passing in a lambda and receiver. What will the receiver be */
private fun JavaClientBuilder.twitter(suppler: JavaTwitterBuilder.() -> Unit) {
    /* We call JavaTwitterBuilder().apply(..) Will apply return the newly created object? Not sure why we have to pass the suppler in the apply */
    twitter = JavaTwitterBuilder().apply(suppler).build()
}

/* Are we passing in a lambda and receiver that return nothing */
private fun createClient(suppler: JavaClientBuilder.() -> Unit): JavaClient {
    val javaClientBuilder = JavaClientBuilder()

    /* confusion: Not sure about this, as we are calling suppler. Just wondering is the suppler the the JavaClientBuilder that was called in the above javaClient {} lambda */
    javaClientBuilder.suppler()

    return javaClientBuilder.build()
}

/* I understand this, an extension function that formats and returns the string from the JavaClient object it was called on */
private val JavaClient.toConsole: String
    get() =
        "Created client is: ${twitter.handle} ${company.name}"

Answer 1

/* Are we passing in a lambda and receiver. What will the receiver be */

private fun JavaClientBuilder.twitter(suppler: JavaTwitterBuilder.() -> Unit)

我们确实在这个函数中有一个接收器，它是 JavaClientBuilder 的实例，这个函数将被调用。

/* We call JavaTwitterBuilder().apply(..) Will apply return the newly created object? Not sure why we have to pass the suppler in the apply */

twitter = JavaTwitterBuilder().apply(suppler).build()

要了解 apply() 的工作原理，请查看其源代码（简化版）：

public inline fun <T> T.apply(block: T.() -> Unit): T {
    block()
    return this
}

它是一个扩展函数，在 T 类型的接收器上声明并返回一个 T 的实例，它接收一个块 - 另一个扩展函数 T 类型的接收器返回Unit。它通常用于替换 Builder 模式 - 将自定义初始化逻辑应用于对象。在您的例子中，suppler 是一个包含 JavaTwitterBuilder 实例的初始化逻辑的块。函数代码创建实例并使用 apply() 和 suppler 中的逻辑来初始化该实例。

/* Are we passing in a lambda and receiver that return nothing */

private fun createClient(suppler: JavaClientBuilder.() -> Unit): JavaClient

在这种情况下，createClient() 没有接收器，它是一个顶级函数。

/* confusion: Not sure about this, as we are calling suppler. Just wondering is the suppler the the JavaClientBuilder that was called in the above javaClient {} lambda */

javaClientBuilder.suppler()

suppler 是一个 lambda，JavaClientBuilder 是接收者类型，这允许我们在新创建的 JavaClientBuilder.

实例上调用它

/* I understand this, an extension function that formats and returns the string from the JavaClient object it was called on */

private val JavaClient.toConsole: String get() = "Created client is: ${twitter.handle} ${company.name}"

对了！只是一个小的更正，它是一个 扩展属性。属性可以有自定义 getters 和设置器。这个属性定义了一个自定义的 getter，所以实际上每当这个属性被访问时它都会产生一个字符串，其格式由 getter 代码描述。

Answer 2

希望以下示例有助于理解具有接收器类型的 lambda：

data class Person(val name: String)

fun getPrefixSafely(
    prefixLength: Int,
    person: Person?,
    getPrefix: Person.(Int) -> String): String
{
    if (person?.name?.length ?: 0 < prefixLength) return ""
    return person?.getPrefix(prefixLength).orEmpty()
}

// Here is how getPrefixSafely can be called
getPrefixSafely(
    prefixLength = 2,
    person = Person("name"),
    getPrefix = { x -> this.name.take(x) }
)

PS: 这些具有接收者类型的函数文字类似于扩展函数 IMO。

混淆理解 lambda 和接收器

Confusion with understanding lambda and receivers

lambda

kotlin

extension-function