Kotlin扩展方法超详细介绍

class Jump {
 fun test() {
 println("jump test")
 //在被扩展的类中使用
 doubleJump(1f)
 }
}
fun Jump.doubleJump(howLong: Float): Boolean {
 println("jump:$howLong")
 println("jump:$howLong")
 return true
}
Jump().doubleJump(2f)
//在被扩展类的外部使用
Jump().test()

KotlinExtensionKt.doubleJump(new Jump(), 2.0f);

fun main() {
 val test = mutableListOf(1, 2, 3)
 test.swap(1, 2)
 println(test)
}
fun MutableList<Int>.swap(index1: Int, index2: Int) {
 val tmp = this[index1]
 this[index1] = this[index2]
this[index2] = tmp
}

public final class KotlinExtensionKt {
 public static final void main() {
 List test = CollectionsKt.mutableListOf(new Integer[]{1, 2, 3});
 swap(test, 1, 2);
 boolean var1 = false;
 System.out.println(test);
 }
 // $FF: synthetic method
 public static void main(String[] var0) {
 main();
 }
 public static final void swap(@NotNull List $this$swap, int index1,
int index2) {
 Intrinsics.checkParameterIsNotNull($this$swap, "$this$swap");
 int tmp = ((Number)$this$swap.get(index1)).intValue();
 $this$swap.set(index1, $this$swap.get(index2));
 $this$swap.set(index2, tmp);
 }
}

//泛型化扩展函数
fun <T> MutableList<T>.swap1(index1: Int, index2: Int) {
 val tmp = this[index1]
 this[index1] = this[index2]
 this[index2] = tmp
}
val test2 = mutableListOf("Android Q", "Android N", "Android M")
test2.swap1(0,1)
println(test2)

//为 String 添加一个 lastChar 属性，用于获取字符串的最后一个字符
val String.lastChar: Char get() = this.get(this.length - 1)
///为 List 添加一个 last 属性用于获取列表的最后一个元素，this 可以省略
val <T>List<T>.last: T get() = get(size - 1)
val listString = listOf("Android Q", "Android N", "Android M")
println("listString.last${listString.last}")

class Jump {
 compaNIOn object {}
}
fun Jump.Companion.print(str: String) {
 println(str)
}
Jump.print("伴生对象的扩展")

fun <T, R> T.let(f: (T) -> R): R = f(this)

fun testLet(str: String?) {
 //限制 str2 的作用域
 str.let {
 val str2 = "let 扩展"
 println(it + str2)
 }
// println(str2)//报错
 //避免为 null 的操作
 str?.let {
 println(it.length)
 }
}

fun <T, R> T.run(f: T.() -> R): R = f()

data class Room(val address: String, val price: Float, val size: Float)
fun testRun(room: Room) {
 room.run {
 println("Room:$address,$price,$size")
 }
}

fun <T> T.apply(f: T.() -> Unit): T { f(); return this }

fun testApply() {
 ArrayList<String>().apply {
 add("testApply")
 add("testApply")
 add("testApply")
 println("$this")
 }.let { println(it) }
}

//为 Activity 添加 find 扩展方法，用于通过资源 id 获取控件
fun <T : View> Activity.find(@IdRes id: Int): T {
 return findViewById(id)
}
//为 Int 添加 onClick 扩展方法，用于为资源 id 对应的控件添加 onClick 监听
fun Int.onClick(activity: Activity, click: () -> Unit) {
 activity.find<View>(this).apply {
 setOnClickListener {
 click()
 }
 }
}

class MainActivity : AppCompatActivity() {
 override fun onCreate(savedInstanceState: Bundle?) {
 super.onCreate(savedInstanceState)
 setContentView(R.layout.activity_main)
 val textView = find<TextView>(R.id.test)
 R.id.test.onClick(this) {
 textView.text = "Kotlin 泛型"
 }
 }
}