【Kotlin】Kotlin笔记12-标准函数(run,with,apply)

Kotlin笔记12-标准函数和静态方法-run,with,apply

7. 标准函数和静态方法

• 标准函数with,run,apply

1. with

with: 参数一:任意类型的对象,参数二:Lambda表达式

​​标准格式:​​

val result = with(obj) {
  //这里是obj的上下文
  "value" //with函数的返回值
}

​​Example:​​

fun eatFruit() {
    val list = listOf("Apple", "Banana", "Orange", "Pear")
    val builder = StringBuilder()
    builder.append("Start eating Fruits! ")
    for ((index, fruit) in list.withIndex()) {
        builder.append(fruit).let {
            if (index != list.size - 1) it.append(" ")
        }
    }
    builder.append(". Ate all fruits.")
    val result = builder.toString()
    println(result)
}

​​采用with:​​

//with优化
fun eatFruit1(){
    val list= listOf("Apple","Banana","Orange","Pear")
    val result= with(StringBuilder()){
        append("Start eating fruits. ")
        for (fruit in list){
            append(fruit).append(" ")
        }
        append("Ate all fruits.")
        toString()
    }
    println(result)
}

2. run

run: 接受一个Lambda参数

​​标准格式:​​

val result = obj.run {
  // 这里是obj的上下文
  "value" // run函数的返回值
}

​​采用run:​​

fun eatFruit2(){
    val list= listOf("Apple","Banana","Orange","Pear")
    val result= StringBuilder().run {
        append("Start earting fruits. ")
        for (fruit in list){
            append(fruit).append(" ")
        }
        append("Ate all fruits.")
        toString()
    }
    println(result)
}

3. apply

apply: 接受一个Lambda参数

​​标准格式:​​

val result = obj.apply {
  // 这里是obj的上下文
}
// result == obj

​​采用apply:​​

fun eatFruit3(){
    val list= listOf("Apple","Banana","Orange","Pear")
    val result=StringBuilder().apply {
        append("Start eating fruits. ")
        for (fruit in list){
            append(fruit).append(" ")
        }
        append("Ate all fruits.")
    }
    println(result)
}

实用场景 - Intent
​​​Example:​​

val intent = Intent(context,SecondActivitu::class.java)
intent.putExtra("param1", "data1")
intent.putExtra("param2", "data2")
context.startActivity(intent)

​​apply优化:​​

val intent(context,SecondActivity::class.java).apply {
  putExtra("param1","data1")
  putExtra("param2","data2")
}
context.startActivity(intent)