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

超、凢脫俗 2022-11-16 10:52 156阅读 0赞

### 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

<table> 
 <thead> 
  <tr> 
   <th>序号</th> 
   <th>Tips</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td>Lambda表达式的最后一行作为返回值返回</td> 
  </tr> 
 </tbody> 
</table>

`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)
    }

--------------------

1.  **run**

> **run:** 接受一个Lambda参数

<table> 
 <thead> 
  <tr> 
   <th>1</th> 
   <th>Tips</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td>run函数不能直接调用,调用某个对象的run函数</td> 
  </tr> 
 </tbody> 
</table>

`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)
    }

--------------------

1.  **apply**

> **apply:** 接受一个Lambda参数

<table> 
 <thead> 
  <tr> 
   <th>序号</th> 
   <th>Tips</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td>在某个对象上调用,在表达式中提供调用对象的上下文</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>apply无法指定返回值,直接返回调用对象本身</td> 
  </tr> 
 </tbody> 
</table>

`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)