Skip to content

Latest commit

 

History

History
223 lines (163 loc) · 6.11 KB

kotlin.md

File metadata and controls

223 lines (163 loc) · 6.11 KB

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() })
  • Jake Wharton 加持 (誤

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/ ，剛開始看到是覺得確實很敏捷，但是對於成熟度抱著遲疑得態度。

在這之後，筆者是在 2015 年一月份 Jake Wharton 在 G+ 發表了一篇貼文之後，確實很多人跟筆者一樣，較為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~)

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(Listener { 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

public var context: Context? = null
  get
  set (value) {
    $context = value
  }

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

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'

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

捨棄 xml 直接用 kotlin 語言來配置 UI。 立意良好。但是來誰作一下視覺化預覽阿？！

Functional constructs and patterns - funKtionale

2015/3 mid

1

val sum2ints = { x: Int, y: Int -> x + y }
val curried: (Int) -> (Int) -> Int = sum2ints.curried()

assertEquals(curried(2)(4), 6)

val add5 = curried(3)
assertEquals(add5(5), 8)
val format = { prefix: String, x: String, postfix: String ->
    "${prefix}${x}${postfix}"
}

val prefixAndBang = format(p3 = "!")

val hello = prefixAndBang(p1 = "Hello, ")

println(hello("world"))

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