Android开发必备知识 为什么说Kotlin值得一试

data class Mondai(var index: Int = 0,
     var title: String = "",
     val ans: ArrayList<String> = ArrayList(),
     var correct: Int = 0,
     var comment: String = "",
     var color: String = "",
     private var lives: Int = 50)

apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
ext.anko_version = '0.8.2'
ext.kotlin_version = '1.0.0'
……

dependencies{
……
 compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
 compile "org.jetbrains.anko:anko-sdk15:$anko_version"
 compile "org.jetbrains.anko:anko-support-v4:$anko_version"
 compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
……
}

buildscript {
 repositories {
  jcenter()
 }
 dependencies {
  classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
 }
}
……

//实际上就是创建一个ArrayList
 val list = arrayListOf(1,2,3,4)
 list.add(5)
 list.remove(3)
 for(item in list){
  println(item)
 }

 val list = arrayListOf(1, 2, 3, 4, 5)
 
 //doubleList = [2,4,6,8,10]
 val doubleList = list.map { 
  it * 2
 }
 
 //oddList = [1,3,5]
 val oddList = list.filter{
  it % 2 == 1
 }
 
 //将list挨个打印出来
 list.forEach { 
  println(it)
 }

public class JavaClass {
 private int anInt = 0;

 public int getAnInt() {
  return anInt;
 }

 public void setAnInt(int anInt) {
  this.anInt = anInt;
 }
}

 val javaClass = JavaClass()
 javaClass.anInt = 5
 print(javaClass.anInt)

view.background = ...
textView.text = ...

val anInt: Int = null // 错误
val anotherInt: Int? = null // 正确

val nullable: Int? = 0
val nonNullable: Int = 2
nullable.toFloat() // 编译错误
nullable?.toFloat() // 如果null，什么都不做，否则调用toFloat
nullable!!.toFloat() // 强制转换为非空对象，并调用toFloat；如果nullable为null，抛空指针异常
nonNullable.toFloat() // 正确

public class DemoActivity extends Activity{
 
 private TextView aTextView;
 
 public void onCreate(Bundle savedInstanceState){
  super.OnCreate(savedInstanceState);
  setContentView(R.layout.main);
  
  aTextView = (TextView) findViewById(R.id.a_textview);
  aTextView.setText("Hello");
  aTextView.setTextSize(20);
  ...
 }
}

class DemoActivity : Activity(){
 private var aTextView: TextView? = null
 
 override fun onCreate(savedInstanceState: Bundle?){
  super.onCreate(savedInstanceState)
  setContentView(R.layout.main)
  //当然有更好用的方式，暂且先这么写
  aTextView = findViewById(R.id.a_textview) as TextView
  aTextView!!.text = "Hello"
  aTextView!!.textSize = 20
  ...
 }
}

class DemoActivity : Activity(){
 private var aTextView: TextView // 编译错误，必须初始化！！！
 
 ...
}

class DemoActivity : Activity(){
 private val aTextView: TextView by lazy{
  findViewById(R.id.a_textview) as TextView
 }
 
 override fun onCreate(savedInstanceState: Bundle?){
  super.onCreate(savedInstanceState)
  setContentView(R.layout.main)
  aTextView.text = "Hello"
  aTextView.textSize = 20
  ...
 }
}

private val aTextView: TextView by lazy(LazyThreadSafetyMode.NONE){
  findViewById(R.id.a_textview) as TextView
}

class Demo {
 lateinit var anJsonObject: JsonObject
 
 fun initDemo(){
  anJsonObject = JsonObject("{...}")
 }
 
}

public interface ReadWriteProperty<in R, T> {
 /**
  * Returns the value of the property for the given object.
  * @param thisRef the object for which the value is requested.
  * @param property the metadata for the property.
  * @return the property value.
  */
 public operator fun getValue(thisRef: R, property: KProperty<*>): T

 /**
  * Sets the value of the property for the given object.
  * @param thisRef the object for which the value is requested.
  * @param property the metadata for the property.
  * @param value the value to set.
  */
 public operator fun setValue(thisRef: R, property: KProperty<*>, value: T)
}

class Preference<T>(val context: Context, val name: String, val default: T) : ReadWriteProperty<Any?, T> {

 val prefs by lazy { context.getSharedPreferences("default", Context.MODE_PRIVATE) }

 override fun getValue(thisRef: Any?, property: KProperty<*>): T {
  return findPreference(name, default)
 }

 override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
  putPreference(name, value)
 }

 private fun <U> findPreference(name: String, default: U): U = with(prefs) {
  val res: Any = when (default) {
   is Long -> getLong(name, default)
   is String -> getString(name, default)
   is Int -> getInt(name, default)
   is Boolean -> getBoolean(name, default)
   is Float -> getFloat(name, default)
   else -> throw IllegalArgumentException("This type can be saved into Preferences")
  }

  res as U
 }

 private fun <U> putPreference(name: String, value: U) = with(prefs.edit()) {
  when (value) {
   is Long -> putLong(name, value)
   is String -> putString(name, value)
   is Int -> putInt(name, value)
   is Boolean -> putBoolean(name, value)
   is Float -> putFloat(name, value)
   else -> throw IllegalArgumentException("This type can be saved into Preferences")
  }.apply()
 }
}

class WhateverActivity : Activity(){
 var aInt: Int by Preference(this, "aInt", 0)
 
 fun whatever(){
  println(aInt)//会从SharedPreference取这个数据
  aInt = 9 //会将这个数据写入SharedPreference
 }
}

data class Coordinate(val x: Double, val y: Double)

val Coordinate.theta: Double
 get() {
  return Math.atan(y/x)
 }

fun Coordinate.R():Double{
 return Math.hypot(x, y)
}

fun main(args: Array<String>) {
 val coord = Coordinate(3.0,4.0)
 println(coord.theta)
 println(coord.R())
}

package com.benny.utils

import android.util.Log

inline fun <reified T> T.debug(log: Any){
 Log.d(T::class.simpleName, log.toString())
}

 public static void debug(Class<?> clazz, Object log){
  Log.d(clazz.getSimpleName(), log.toString());
 }

public static <T> void debug(Object log){
 Log.d(T.getSimpleName(), log.toString());//错误，T是泛型参数，无法直接使用
}

executor.submit(new Runnable(){
 @Override
 public void run(){
  //todo
 }
});

executor.submit({
 //todo
})

textView.setOnClickListener(new OnClickListener(){
 @Override
 public void onClick(View view){
  //todo
 }
});

handle.post(new Runnable(){
 @Override
 public void run(){
  //todo
 }
});

textView.setOnClickListener{ /*todo*/ }
handler.post{ /*todo*/ }

public inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()
public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }

fun getStringBuilder: StringBuilder{
 return StringBuilder().apply{
  append("whatever")
 }
}

  Observable.create<ArrayList<Dummy>> {
   it.onStart()
   try {
    it.onNext(dummyObjs)
   } catch(e: Exception) {
    it.onError(e)
   } finally {
    it.onCompleted()
   }
  }.subscribe(object : Subscriber<ArrayList<Dummy>>() {
   override fun onCompleted() {

   }

   override fun onNext(t: ArrayList<Dummy>?) {

   }

   override fun onError(e: Throwable?) {

   }

  })

object HelloScala{
 // do something
}

val x = 7
when (x) {
 in 1..10 -> print("x is in the range")
 in validNumbers -> print("x is valid")
 !in 10..20 -> print("x is outside the range")
 else -> print("none of the above")
}

when {
 x.isOdd() -> print("x is odd")
 x.isEven() -> print("x is even")
 else -> print("x is funny")
}

object Hello {
 def main(args: Array[String]) {
 easyMatch((1, 3))
 easyMatch(Array(1,3,4))
 easyMatch(Bean(3.0, 4.0))
 }

 def easyMatch(value : Any) = value match {
  case int :Int => {
  println("This is an Int.")
  }
  case (a, b) =>{
  println(s"a tuple with : $a , $b")
  }
  case Bean(x, y) => {
  println(s"$x, $y")
 }
  case whatever => println(whatever)
 }
}

case class Bean(val x: Double, val y: Double)

<project version="4">
 <component name="Encoding">
 <file url="PROJECT" charset="UTF-8" />
 </component>
</project>

class Project {
 var version: String? = null
  get() =
  if (field == null) ""
  else {
   " version=\"${field}\""
  }

 lateinit private var component: Component

 fun component(op: Component.() -> Unit) {
  component = Component().apply {
   op()
  }
 }

 override fun toString(): String {
  return "<project${version}>${component}<project>"
 }


}

fun project(op: Project.() -> Unit): Project {
 return Project().apply {
  op()
 }
}

class Component {
 var name: String? = null
  get() =
  if (field == null) ""
  else {
   " name=\"${field}\""
  }


 lateinit private var file: File

 fun file(op: File.() -> Unit) {
  file = File().apply {
   op()
  }
 }

 override fun toString(): String {
  return "<component${name}>${file}<component>"
 }
}

class File {
 var url: String? = null
  get() =
  if (field == null) ""
  else {
   " url=\"${field}\""
  }

 var charset: String? = null
  get() =
  if (field == null) ""
  else {
   " charset=\"${field}\""
  }

 override fun toString(): String {
  return "<file${url}${charset}/>"
 }
}

fun main(args: Array<String>) {
 val xml = project {
  version = "4"
  component {
   name = "Encoding"
   file {
    url = "PROJECT"
    charset = "UTF-8"
   }
  }
 }

 println(xml)
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/open_bj"
    android:orientation="vertical">

  <TextView
   android:id="@+id/textView"  
   android:text="Hello"
   android:textSize="50sp"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"/>

  <Button
   android:id="@+id/start"
   android:clickable="false"
   android:layout_gravity="center_horizontal"
   android:background="@drawable/start_selector"
   android:textSize="50sp"
   android:layout_marginTop="20dp"
   android:layout_marginBottom="200dp"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"/>

</RelativeLayout>

package com.benny

……
import kotlinx.android.synthetic.main.load_activity.*
import org.jetbrains.anko.onClick
import org.jetbrains.anko.startActivity
import org.jetbrains.anko.toast
……

class LoadActivity : Activity() {
 override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  setContentView(R.layout.main)

  start.onClick {
   toast("开始")
   startActivity<AnotherActivity>()
  }
  textView.text = "你好"
    
 }
}

handler.post(new Runnable(){
 @Override
 public void run(){
  //todo 
 }
});

MainActivity.this.runOnUiThread(
 public void run(){
  //todo
 }
});

async() {
 //do something asynchronously
 uiThread {
  //do something on UI thread
 }
}

linearLayout {
 button("Login") {
  textSize = 26f
 }.lparams(width = wrapContent) {
  horizontalMargin = dip(5)
  topMargin = dip(10)
 }
}

 override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)

  setContentView(linearLayout {
   button("This is a button") {
    onClick {
     toast("clicked!")
    }
   }.lparams {
    width = matchParent
    verticalMargin = dip(5)
   }
  })
 }

val int: Int = 0xffffffff // error
val anotherInt: Int = 0xffffffff.toInt() // correct