Kotlin的Lambda表达式

一、什么是Lambda表达式

就是匿名函数

写法：{[参数列表] -> [函数体，最后一行是返回值]}

**举例：**val sum = {a: Int, b: Int -> a+b}

二、Lambda 的类型

（）-> Unit

无参，返回值为Unit

(Int) -> Int

传人整型，返回一个整型

（String,(String) -> String）->Boolean

传入字符串、Lambda表达式，返回Boolean

三、Lambda表达式的调用

用（）调用

等价于invoke()

举例：

定义一个：val sum = {a: Int,b: Int -> a+b}

调用一下：sum(2,3)

sum.invoke(2,3)

四、首先来写一个例子

//Lambda表达式,传人两个（Int,Int）-> Int
 val sum = {arg1: Int, arg2: Int ->
    //println("$arg1 + $arg2 = ${arg1 + arg2}")
    arg1 + arg2
}           

五、找到kotlin的Functions源码

因为是从0开始的，可以看到最多参数只能有23个

package kotlin.jvm.functions

/** A function that takes 0 arguments. */
public interface Function0<out R> : Function<R> {
    /** Invokes the function. */
    public operator fun invoke(): R
}
/** A function that takes 1 argument. */
public interface Function1<in P1, out R> : Function<R> {
    /** Invokes the function with the specified argument. */
    public operator fun invoke(p1: P1): R
}
/** A function that takes 2 arguments. */
public interface Function2<in P1, in P2, out R> : Function<R> {
    /** Invokes the function with the specified arguments. */
    public operator fun invoke(p1: P1, p2: P2): R
}
......
/** A function that takes 21 arguments. */
public interface Function21<in P1, in P2, in P3, in P4, in P5, in P6, in P7, in P8, in P9, in P10, in P11, in P12, in P13, in P14, in P15, in P16, in P17, in P18, in P19, in P20, in P21, out R> : Function<R> {
    /** Invokes the function with the specified arguments. */
    public operator fun invoke(p1: P1, p2: P2, p3: P3, p4: P4, p5: P5, p6: P6, p7: P7, p8: P8, p9: P9, p10: P10, p11: P11, p12: P12, p13: P13, p14: P14, p15: P15, p16: P16, p17: P17, p18: P18, p19: P19, p20: P20, p21: P21): R
}
/** A function that takes 22 arguments. */
public interface Function22<in P1, in P2, in P3, in P4, in P5, in P6, in P7, in P8, in P9, in P10, in P11, in P12, in P13, in P14, in P15, in P16, in P17, in P18, in P19, in P20, in P21, in P22, out R> : Function<R> {
    /** Invokes the function with the specified arguments. */
    public operator fun invoke(p1: P1, p2: P2, p3: P3, p4: P4, p5: P5, p6: P6, p7: P7, p8: P8, p9: P9, p10: P10, p11: P11, p12: P12, p13: P13, p14: P14, p15: P15, p16: P16, p17: P17, p18: P18, p19: P19, p20: P20, p21: P21, p22: P22): R
}           

六、源码位置