这几个函数非常相似，有细微区别，下面举例说明。
1、先新建一个数据类
data class User(var name : String, var age:Int) {
fun say(msg: String?): String? = msg
}
2、在main方法中测试下
with 操作当前对象，可返回另外一个值
fun main(args: Array) {
val user = User("wj", 25)
val withResult = with(user) {
name = "niuniuniu"
age = 11
"other value"
}
println(withResult ) //结果：other value
}
apply 操作当前对象，返回当前对象
fun main(args: Array) {
val user = User("wj", 25)
val applyResult = user.apply {
name ="haha"
age =11
}
println(applyResult) //结果：User(name=haha, age=11)
}
run 操作当前对象，可以返回另外一个值
fun main(args: Array) {
val user = User("wj", 25)
val runResult = user.run {
name ="run name"
age =666
//调用say方法，返回say方法的值
say("say run")
}
println(runResult) //结果：say run
}
let 当前对象作为参数，返回另外一个对象
fun main(args: Array) {
val user = User("wj", 25)
val letResult = user.let {
it.name
}
println(letResult ) //结果：wj
}
also 当前对象作为参数，返回当前对象
fun main(args: Array) {
val user = User("wj", 25)
val alsoResult = user.also {
println("内部打印:$it")
it.name ="new name"
}
println(alsoResult)
//结果：
内部打印:User(name=wj, age=25)
User(name=new name, age=25)
}
