Kotlin难点

    fun a(funParam: (Int) -> String): String {
        return funParam(1)
    }

    fun b(param: Int): String {
        return param.toString()
    }

a(::b)
var d = ::b
b(1) // 调用函数
d(1) // 实际上会调用 d.invoke(1)
(::b)(1) // 用对象 :: b 后面加上括号来实现 b() 的等价操作, 实际上会调用 (::b).invoke(1)
b.invoke(1) // 报错

fun isOdd(x: Int) = x % 2 != 0

val numbers = listOf(1, 2, 3)
println(numbers.filter(::isOdd))

    val args: Array<String> = arrayOf("1", "2")
  args.filter(String::isNotEmpty) 
  
  class PdfPrinter {
        fun println(any: Any) {
            kotlin.io.println(any)  //重名了可以用包名调用
        }
    }
      val pdfPrinter = PdfPrinter()
      args.forEach(pdfPrinter::println)

val c = MyClass::class

data class MediaItem(val title: String, val url: String)

var items= mutableListOf<MediaItem>()
items
    .sortedBy { it.title }
    .map { it.url }
    .forEach { print(it) }

items
    .sortedBy(MediaItem::title)
    .map(MediaItem::url)
    .forEach(::println)

fun b(param: Int): String {
    return param.toString()
}

a(fun b(param: Int): String {
  return param.toString()
});

val d = fun b(param: Int): String {
  return param.toString()
}

//名字没意义，省略
a(fun(param: Int): String {
  return param.toString()
});
val d = fun(param: Int): String {
  return param.toString()
}

public interface OnClickListener {
  void onClick(View v);
}
public void setOnClickListener(OnClickListener listener) {
  this.listener = listener;
}

view.setOnClickListener(new OnClickListener() {
  @Override
  void onClick(View v) {
    switchToNextPage();
  }
});

fun setOnClickListener(onClick: (View) -> Unit) {
  this.onClick = onClick
}
view.setOnClickListener(fun(v: View): Unit) {
  switchToNextPage()
})

view.setOnClickListener({ v: View ->
  switchToNextPage()
})

    view.setOnClickListener({ v: View ->
        switchToNextPage()
    })

//如果 Lambda 是函数的最后一个参数，你可以把 Lambda 写在括号的外面：
    view.setOnClickListener() { v: View ->
        switchToNextPage()
    }
//而如果 Lambda 是函数唯一的参数，你还可以直接把括号去了：
    view.setOnClickListener { v: View ->
        switchToNextPage()
    }
//另外，如果这个 Lambda 是单参数的，它的这个参数也省略掉不写：
//根据上下文推导，根据最后一行代码来推断出返回值类型
    view.setOnClickListener {
        switchToNextPage()
    }

val sum: (Int, Int) -> Int = { x: Int, y: Int -> x + y }
val sum = { x: Int, y: Int -> x + y }

val items = listOf(1, 2, 3, 4, 5)

// Lambdas 表达式是花括号括起来的代码块。
items.fold(0, { 
    // 如果一个 lambda 表达式有参数，前面是参数，后跟“->”
    acc: Int, i: Int -> 
    print("acc = $acc, i = $i, ") 
    val result = acc + i
    println("result = $result")
    // lambda 表达式中的最后一个表达式是返回值：
    result
})

// lambda 表达式的参数类型是可选的，如果能够推断出来的话：
val joinedToString = items.fold("Elements:", { acc, i -> acc + " " + i })

acc = 0, i = 1, result = 1
acc = 1, i = 2, result = 3
acc = 3, i = 3, result = 6
acc = 6, i = 4, result = 10
acc = 10, i = 5, result = 15
joinedToString = Elements: 1 2 3 4 5

var onVideoStartCallBack: (() -> Unit)? = null

onVideoStartCallBack?.invoke()

videioView.onVideoStartCallBack = {

}

object UploaderListHelper {

    fun startTaskUpload(activity: Activity, startCallBack: ((Int) -> Unit)?) {
        startCallBack.invoke(position)
    }
}

UploaderListHelper.startTaskUpload(activity) {
    refreshProgress(it)
}

 val person = findPerson();
        //person是可null的，所以需要？
        println(person?.age)
        println(person?.name)
        //上面太麻烦，findPerson加了？,所以后面不需要了,减少的判空操作。let可以安全调用
        findPerson()?.let { person ->
            person.work()
            println(person.age)
        }
        //还可以更简洁，person也不用写
        findPerson()?.apply {
            work()
            println(age)
        }

val adam = Person("Adam").apply {
    age = 32
    city = "London"        
}
println(adam)

val numbers = mutableListOf("one", "two", "three")
numbers
    .also { println("The list elements before adding new one: $it") }
    .add("four")

val numbers = mutableListOf("one", "two", "three")
with(numbers) {
    println("'with' is called with argument $this")
    println("It contains $size elements")
}

val service = MultiportService("http://example.kotlinlang.org", 80)

val result = service.run {
    port = 8080
    query(prepareRequest() + " to port $port")
}

// 同样的代码如果用 let() 函数来写:
val letResult = service.let {
    it.port = 8080
    it.query(it.prepareRequest() + " to port ${it.port}")
}