Kotlin

卡特琳

特點：

  • null-safety / Optional 最佳解決方案。 ?:
  • 語法簡潔 for ((key, value) in map)
  • AutoValue?
  • lambdas setOnClickListener { finish() }
  • 字串變數 "Hello, ${username}!"

對於 android 來說，Kotlin 開始知名的時候，大概可以追溯到 2014 年中旬登上 Android 開發週報的：http://blog.gouline.net/2014/08/31/kotlin-the-swift-of-android/ ，不過對於生態成熟度抱著遲疑得態度。

在 2015 年一月份 Jake Wharton 在 G+ 發表了一篇貼文 之後，較積極的看待這個語言。

除了這些特性之外，對於 android 來說，滿大的優勢在於 symbol size 以及 code size 相較於其他語言，十分羽量。(kotlin: 6k~, scala: 50k~, groovy: 30k~)

POJO

Before:

@AutoValue
public abstract class Money {
  public abstract String currency();
  public abstract int amount();

  public static Money of(String currency, int amount) {
    return new AutoValue_Money(currency, amount);
  }
}

After:

data class Money(val currency: String, val amount: Int)

Lambda

Before:

thing.setListener(new Listener() {
  @Override public void onThing() {
    System.out.println("Thing!");
  }
});

After:

t.setListener { println("Thing!") }

Multi-assignment for loop

Before:

for (Map.Entry<String, String> entry : map.entrySet()) {
  System.out.println(entry.getKey() + ": " + entry.getValue());
}

After:

for ((key, value) in map) {
  println(key + ": " + value)
}

簡便 getter 與 setter

After:

public var name: String? = null
  get // not override
  set (value) {
    $name = if (value != null) ? value.capitalize() : value
  }

name = "andrew";
assertThat(name).isEqualTo("Andrew");

Null Safety

Nullable (類 @Nullable) :

var a: String? = "bar"
// `a = null` is ok
// `a.length()` throws exception
// `a?.length()` is ok
// `a!!.length()` // throws NPE if a is null

NonNull (類 @NonNull):

var a: String = "bar"
// `a = null` throws NPE
// `a.length()` is ok

Elvis operator, before:

val l = if (a != null) a.length() else -1

After:

val l = a?.length() ?: -1

字串變數

Before:

String username = "Andrew Chen";
System.out.println("Hello, " + username + "!");

After

val username: String = "Andrew Chen"
System.out.println("Hello, ${username}!");

Immutable: val

  • var: variable
  • val: immutable variable, final variable

Delegated Property

Before:

Resources resources;

final Resources getResources() {
  if (resources == null) resources = context.getResources();
  return resources;
}

After:

val resources by lazy { context.getResources() }

Smart cast

Before:

void setText(View view) {
  if (view instanceof TextView) {
    ((TextView) view).setText("hello");
  }
}

After:

fun setText(view: View) {
  if (view is TextView) {
    view.setText("hello");
  }
}

switch case: when with smart cast

After:

fun setText(view: View) {
  when (view) {
      is TextView -> view.setText("hello");
  }
}

range

for (i in 1..10) println(i)

集合運算子

filterNotNull()

Before:

List<String> repos = Arrays.asList("RetroFacebook", "NotRetrofit", null, "RxParse");
List<String> reposNotNull = new ArrayList<>();

for (String repo : repos) {
    if (repo != null) reposNotNull.add(repo);
}

After:

val repos = listOf("RetroFacebook", "NotRetrofit", null, "RxParse");
repos.filterNotNull()

sort()

Before:

Collections.sort(repoNotNull, new Comparator<String>() {
    @Override public int compare(String l, String r) {
        return r.length() - l.length();
    }
});

After:

repos.sortedBy { it.length() }

toUpperCase()

repos.map { it.toUpperCase() }

any(其中)

其中一個項目成立，就回傳真。

語法：

list.any(() -> Boolean)

val list = listOf(1, 2, 3, 4, 5, 6)
// 任何一個數字能整除 2
assertTrue(list.any { it % 2 == 0 })
// 任何一個數字大於 10
assertFalse(list.any { it > 10 })

all(都)

其中一個項目不成立，就回傳假。(所有的項目都成立，就回傳真。)

val list = listOf(1, 2, 3, 4, 5, 6)
// 都小於 10
assertTrue(list.all { it < 10 })
// 都整除 2
assertFalse(list.all { it % 2 == 0 })

count(共有幾個)

val list = listOf(1, 2, 3, 4, 5, 6)
// 整除 2 的項目共有 3 個
assertEquals(3, list.count { it % 2 == 0 })

reduce

不解釋

val list = listOf(1, 2, 3, 4, 5, 6)
assertEquals(21, list.reduce { total, next -> total + next })

reduceRight (倒著 reduce)

不解釋

val list = listOf(1, 2, 3, 4, 5, 6)
assertEquals(21, list.reduceRight { total, next -> total + next })

fold (類似有初始值的 reduce)

val list = listOf(1, 2, 3, 4, 5, 6)
// 4 + 1+2+3+4+5+6 = 25
assertEquals(25, list.fold(4) { total, next -> total + next })

foldRight (同 fold ，只是倒著)

val list = listOf(1, 2, 3, 4, 5, 6)
// 4 + 6+5+4+3+2+1 = 25
assertEquals(25, list.foldRight(4) { total, next -> total + next })

forEach

不解釋

val list = listOf(1, 2, 3, 4, 5, 6)
list forEach { println(it) }

forEachIndexed

不解釋

val list = listOf(1, 2, 3, 4, 5, 6)
list forEachIndexed { index, value
       -> println("$index : $value") }

max

不解釋

val list = listOf(1, 2, 3, 4, 5, 6)
assertEquals(6, list.max())

maxBy

在算最大之前，先運算一次。

val list = listOf(1, 2, 3, 4, 5, 6)
// 所有數值負數後，最大的那個是 1
assertEquals(1, list.maxBy { -it })

min

不解釋

val list = listOf(1, 2, 3, 4, 5, 6)
assertEquals(1, list.min())

minBy

不解釋

val list = listOf(1, 2, 3, 4, 5, 6)
assertEquals(6, list.minBy { -it })

none (沒有一個)

val list = listOf(1, 2, 3, 4, 5, 6)
// 沒有一個整除 7
assertTrue(list.none { it % 7 == 0 })

sumBy

val list = listOf(1, 2, 3, 4, 5, 6)
// 2 餘數的總和
assertEquals(3, list.sumBy { it % 2 })

withIndices

Before:

int index = 0;
for (String item : list) {
    System.out.println(index + " : " + item);
    index++;
}

After:

for ((index, item) in list.withIndices()) {
    println("$index : $item")
}

導入方法

  • 可利用 Android Studio kotlin plugin 轉換程式碼 (轉完不一定可動，大多稍微改一下就好了)
  • buildscript.dependencies: classpath "org.jetbrains.kotlin:kotlin-gradle-plugin
  • apply plugin: 'kotlin-android'
  • dependencies: compile 'org.jetbrains.kotlin:kotlin-stdlib:0.12.200'

動手玩

git clone [email protected]:yongjhih/gradle-init.kt.git
cd gradle-init.kt
./gradlew run

主程式： src/main/kotlin/hello/HelloWorld.kt

另一種玩法 kotlin-cli：

wget https://github.com/JetBrains/kotlin/releases/download/build-1.0.0-beta-1103/kotlin-compiler-1.0.0-beta-1103.zip
unzip kotlin-compiler-1.0.0-beta-1103.zip

HelloWorld.kts:

println("Hello world!")

執行：

./kotlinc/bin/kotlinc-jvm -script HelloWorld.kts

FAQ

  • 如果多方繼承(class/interface) 時，super.XXX() 就會不清楚你要呼叫哪位 parent ，所以改成 super<>.XXX 即可。

RxKotlin

observable<String> { subscriber ->
        subscriber.onNext("H")
        subscriber.onNext("e")
        subscriber.onNext("l")
        subscriber.onNext("")
        subscriber.onNext("l")
        subscriber.onNext("o")
        subscriber.onCompleted()
    }.filter { it.isNotEmpty() }.
    fold (StringBuilder()) { sb, e -> sb.append(e) }.
    map { it.toString() }.
    subscribe { result ->
      a.received(result)
    }

    verify(a, times(1)).received("Hello")

Anko - kotlin template engine

#data-binding, #reactjs, #angularjs, #jax

捨棄 xml 直接用 kotlin 語言來配置 UI。

verticalLayout {
    val name = editText()
    button("Say Hello") {
        onClick { toast("Hello, ${name.text}!") }
    }
}
  • Android Studio 預覽插件：Anko Preview Plugin for idea

議題：

  • 重用性 - 如何重用 layout?
  • 獨立 layout
  • 如何引用獨立 layout
  • qualifier

擴充 Cursor

Before:

String firstName = cursor.getString(cursor.getColumnIndexOrThrow("first_name"));

After:

val firstName = cursor.getString("first_name");
fun Cursor.getString(columnName: String): String {
  return getString(getColumnIndexOrThrow(columnName))
}

避免 NullPointerException:

Before:

String firstName = null;
int firstNameColumnIndex = cursor.getColumnIndexOrThrow("first_name");
if (!cursor.isNull(firstNameColumnIndex)) {
  firstName = cursor.getString(firstNameColumnIndex);
}
firstNameView.setText(firstName != null ? firstName : "Andrew");

After:

val firstName = cursor.getStringOrNull("first_name")
firstNameView.setText(firstName ?: "Andrew")

fun Cursor.getStringOrNull(columnName: String): String? {
  val index = getColumnIndexOrThrow(columnName)
  return if (isNull(index)) null else getString(index)
}

findViewById?

Before:

@InjectView(R.id.first_name)
TextView firstNameTextView;

@Override
public void onCreate(...) {
  super.onCreate(...);
  setContentView(R.layout.activity_main);
  ButterKnife.inject(this);

  firstNameTextView.setText("Andrew");
}

After:

import kotlinx.android.synthetic.activity_main.first_name as firstNameTextView

firstNameTextView.setText("Andrew");

具名 SharedPreferences - 透過 Delegated ReadWriteProperty<in R, T> 取得 property 名稱

val user = User(context)
user.name = "Andrew Chen"
user.age = "18"

public class User : Preferences() {
  var name: String by StringPreference() // TODO by Preference<T>
  var age: Int by IntPreference() // TODO by Preference<T>
}

public class Preferences(prefs: SharedPreferences) : SharedPreferences by prefs {
  constructor(context: Context) : this(prefs = PreferenceManager.getDefaultSharedPreferences(context)) {
  }

  public inner class StringPreference : ReadWriteProperty<SharedPreferences, String>() {
    override fun getValue(prefs: SharedPreferences, property: KProperty<*>): String {
      return prefs.getString(property.name, null)
    }

    override fun setValue(prefs: SharedPreferences, property: KProperty<*>, value: String) {
      SharedPreferences.Editor editor = prefs.edit()
      editor.putString(property.name, value)
      editor.apply()
    }
  }

  public inner class IntPreference : ReadWriteProperty<SharedPreferences, Int>() {
    override fun getValue(prefs: SharedPreferences, property: KProperty<*>): Int {
      return prefs.get(property.name, null)
    }

    override fun setValue(prefs: SharedPreferences, property: KProperty<*>, value: Int) {
      SharedPreferences.Editor editor = prefs.edit()
      editor.putInt(property.name, value)
      editor.apply()
    }
  }
}

TODO:

User(context).edit {
  name = "Andrew Chen"
  age = "18"
}

public class Preferences(prefs: SharedPreferences) : SharedPreferences by prefs {
  // ...

  fun edit(func: <in R>.Editor.() -> Unit) { // FIXME
    val editor = edit()
    editor.func()
    editor.apply()
  }
}

擴充資料庫 Transaction

Before:

db.beginTransaction();
try {
  db.delete("users", "first_name = ?", new String[] { "Andrew" });
  db.setTransactionSuccessful();
} finally {
  db.endTransaction()
}

or

Databases.from(db).inTransaction(db -> {
  db.delete("users", "first_name = ?", new String[] { "Andrew" });
});

After:

db.inTransaction {
  delete("users", "first_name = ?", arrayOf("Andrew"))
}

// inflix, literal
inline fun SQLiteDatabase.inTransaction(func: SQLiteDatabase.() -> Unit) {
  beginTransaction()
  try {
    func()
    setTransactionSuccessful()
  } finally {
    endTransaction()
  }
}

擴充 SharedPreferences

Before:

SharedPreferences.Editor editor = preferences.edit();

editor.putString("first_name", "Andrew");
editor.putString("last_name", "Chen");
editor.remove("age");

editor.apply();

or

SharedPreferencesUtils.from(preferences).apply(editor -> {
  editor.putString("first_name", "Andrew");
  editor.putString("last_name", "Chen");
  editor.remove("age");
});

After:

preferences.edit {
  putString("first_name", "Andrew")
  putString("last_name", "Chen")
  remove("age")
}

inline fun SharedPreferences.edit(func: SharedPreferences.Editor.() -> Unit) {
  val editor = edit()
  editor.func()
  editor.apply()
}

擴充 Notification Builder

Before:

Notification notification = new NotificationCompat.Builder(context)
  .setContentTitle("Hello")
  .setSubText("World")
  .build();

After:

val notification = Notification.build(context) {
  setContentTitle("Hello")
  setSubText("World")
}

inline fun Notification.build(context: Context, func: NotificationCompat.Builder.() -> Unit): Notification {
  val builder = NotificationCompat.Builder(context)
  builder.func()
  return builder.build()
}

kovenant - Promise

async {
    download(url)
} then {
    file -> "file: $file"
} success {
    println(it)
}

async { "world" } and async { "Hello" } success {
    println("${it.second} ${it.first}!")
}

successUi/failUi

async {
    download(url)
} then {
    file -> "file: $file"
} successUi {
    toast(it)
} failUi {
    toast("fail")
}

promiseOnUi

promiseOnUi {
    textView.getText();
} then {
    download(it);
} successUi {
    toast(it)
}

名稱由來： Covenant(公約) -> Kovenant (The Kovenanat Band)

安裝

build.gradle:

    compile 'nl.komponents.kovenant:kovenant-core:3.0.0-beta.3'
    compile 'nl.komponents.kovenant:kovenant-android:3.0.0-beta.3'
    compile 'nl.komponents.kovenant:kovenant-combine:3.0.0-beta.3'
    compile 'nl.komponents.kovenant:kovenant-jvm:3.0.0-beta.3'
    compile 'nl.komponents.kovenant:kovenant-functional:3.0.0-beta.3'

injekt - DI

funKtionale - functional (Deprecated)

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

assertEquals(curriedSum(2)(4), 6)

val add3 = curriedSum(3)
assertEquals(add3(5), 8)
  • ref: 2015/3 mid 1

Closure

Before:

add(1) { 2 }

fun add(x: Int, func: () -> Int): Int {
    return x + func()
}

After:

add(1)(2)

fun add(x: Int): (Int) -> Int {
    return { y -> x + y }
}

Fuel - networking

"http://github.com/yongjhih/".httpGet().responseString { request, response, either ->
    //do something with response
    when (either) {
        is Left -> // left means failure
        is Right -> // right means success
    }
}

對照表

(origin from Using Project Kotlin for Android)

Library Jar Size Dex Size Method Count Field Count
kotlin-runtime-0.10.195 354 KB 282 KB 1071 391
kotlin-stdlib-0.10.195 541 KB 835 KB 5508 458
Library Jar Size Dex Size Method Count Field Count
rxjava-1.0.4 678 KB 513 KB 3557 1668
support-v4-21.0.3 745 KB 688 KB 6721 1886
play-services-base-6.5.87 773 KB 994 KB 5212 2252
okio-1.2.0 54 KB 55 KB 508 76
okhttp-2.2.0 304 KB 279 KB 1957 882
retrofit-1.9.0 119 KB 93 KB 766 228
picasso-2.4.0 112 KB 97 KB 805 342
dagger-1.2.2 59 KB 54 KB 400 119
butterknife-6.0.0 48 KB 50 KB 307 73
wire-runtime-1.6.1 71 KB 71 KB 471 147
gson-2.3.1 206 KB 170 KB 1231 390
Total 2963 KB 2894 KB 21935 8063
Library Jar Size Dex Size Method Count Field Count
scala-library-2.11.5 5.3 MB 4.9 MB 50801 5820
groovy-2.4.0-grooid 4.5 MB 4.5 MB 29636 8069
guava-18.0 2.2 MB 1.8 MB 14833 3343

See Also

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