kotlin apply 和 also 的差别

/**
 * Calls the specified function [block] with `this` value as its receiver and returns `this` value.
 *
 * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#apply).
 */
@kotlin.internal.InlineOnly
public inline fun <T> T.apply(block: T.() -> Unit): T {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    block()
    return this
}

/**
 * Calls the specified function [block] with `this` value as its argument and returns `this` value.
 *
 * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#also).
 */
@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun <T> T.also(block: (T) -> Unit): T {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    block(this)
    return this
}

also 和 apply的差别主要存在于，lambda表达式内context表示方式，also是通过传入 的参数（it）来表示的，而apply是通过this来表示的。

also 可用于不更改对象的其他操作，例如记录或打印调试信息。通常，您可以在不破坏程序逻辑的情况下从调用链中删除也是调用。

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

apply主要对接收器对象的成员进行操作。 apply的常见情况是对象配置。此类调用可以读作“将以下赋值应用于对象"

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