Kotlin中的多变量let

Question

有没有办法在Kotlin中为多个可为空的变量连锁多个let？

fun example(first: String?, second: String?) {
    first?.let {
        second?.let {
            // Do something just if both are != null
        }
    }
}

fun example(first: String?, second: String?) {
    first?.let && second?.let { 
        // Do something just if both are != null
    }
}

Answer 1

如果感兴趣，这里有两个函数来解决这个问题。

inline fun <T: Any> guardLet(vararg elements: T?, closure: () -> Nothing): List<T> {
    return if (elements.all { it != null }) {
        elements.filterNotNull()
    } else {
        closure()
    }
}

inline fun <T: Any> ifLet(vararg elements: T?, closure: (List<T>) -> Unit) {
    if (elements.all { it != null }) {
        closure(elements.filterNotNull())
    }
}

用法：

// Will print
val (first, second, third) = guardLet("Hello", 3, Thing("Hello")) { return }
println(first)
println(second)
println(third)

// Will return
val (first, second, third) = guardLet("Hello", null, Thing("Hello")) { return }
println(first)
println(second)
println(third)

// Will print
ifLet("Hello", "A", 9) {
 (first, second, third) ->
 println(first)
 println(second)
 println(third)
}

// Won't print
ifLet("Hello", 9, null) {
 (first, second, third) ->
 println(first)
 println(second)
 println(third)
}