kotlin_0

Let

• 函數(shù)定義

  public inline fun <T, R> T.let(block: (T) -> R): R = block(this)
  

• 函數(shù)說明

  Calls the specified function block with this value as its argument and returns its result.
  調(diào)用某對(duì)象的let函數(shù)挥下，即將該對(duì)象作為let函數(shù)的參數(shù)撩嚼。在函數(shù)塊內(nèi)阐滩，可以通過it獲取該對(duì)象蒿偎。函數(shù)返回值為函數(shù)塊的最后一行或者指定的return結(jié)果急迂。

• 函數(shù)實(shí)例

  /**
   * 姓名拆分，通過空格拆分出姓和名
   */
  fun nameSplit(name: String): Boolean {
      name.let {
          if (it.indexOf(" ") == -1) return false
          println("Your first Name is " + it.substring(0, it.indexOf(" ")))
          println("Your last Name is " + it.substring(it.indexOf(" ")))
          return true
      }
  }
  

• 測(cè)試函數(shù)

  private KotlinUnit mUnit = new KotlinUnit();
  
  @Test
  public void testLet_0() throws Exception {
      assertTrue("testLet_0",mUnit.nameSplit("ted xiong"));
  }
  
  //輸出結(jié)果
  //Your first Name is ted
  //Your last Name is  xiong
  

Apply

• 函數(shù)定義

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

• 函數(shù)說明

  Calls the specified function [block] with this value as its receiver and returns this value.
  調(diào)用某對(duì)象的apply函數(shù)饺饭，在函數(shù)塊內(nèi)惯悠，可以任意調(diào)用該對(duì)象的任意方法卤材，最后返回自身。

• 函數(shù)實(shí)例

  /**
   * 在一個(gè)空的數(shù)組里塞入日期和時(shí)間的字符串
   */
  fun getDateStr(space: ArrayList<String>): Boolean {
      space.apply {
          add(SimpleDateFormat("yyyy-MM-dd", Locale.CHINA).format(Date()))
          add(SimpleDateFormat.getTimeInstance().format(Date()))
      }.let {
          println("The date info is " + it[0])
          println("The time info is " + it[1])
      }
      return true
  }
  

• 測(cè)試函數(shù)

  private KotlinUnit mUnit = new KotlinUnit();
  
  @Test
  public void testApply_0() throws Exception {
      ArrayList<String> space = new ArrayList<>();
      assertTrue("testApply_0", mUnit.getDateStr(space));
  }
  
  //輸出結(jié)果
  //The date info is 2017-08-16
  //The time info is 16:38:47
  

With

• 函數(shù)定義

  public inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()
  

• 函數(shù)說明

  Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
  With跟其他函數(shù)不一樣峦失，并不是變量的擴(kuò)展扇丛，而是單獨(dú)的一個(gè)函數(shù)。將一個(gè)對(duì)象作為參數(shù)傳遞給With函數(shù)尉辑，在函數(shù)塊內(nèi)可以任意調(diào)用該對(duì)象的任意方法晕拆，并且可以通過this獲取到該對(duì)象，差不多算是let和apply的混合材蹬。

• 函數(shù)實(shí)例

  /**
   * 獲取到編譯環(huán)境相關(guān)的系統(tǒng)信息
   */
  fun getCompileInfo(map: HashMap<String, String>): Boolean {
      return with(map) {
          put("Java環(huán)境版本", System.getProperties().getProperty("java.version"))
          put("Java環(huán)境供應(yīng)商", System.getProperties().getProperty("java.vendor"))
          put("操作系統(tǒng)名稱", System.getProperties().getProperty("os.name"))
          put("操作系統(tǒng)架構(gòu)", System.getProperties().getProperty("os.arch"))
          put("操作系統(tǒng)版本", System.getProperties().getProperty("os.version"))
          println(this)
          true
      }
  }    
  

• 測(cè)試函數(shù)

  private KotlinUnit mUnit = new KotlinUnit();
  
  @Test
  public void testWith_0() throws Exception {
      HashMap<String,String> map = new HashMap<>();
      assertTrue("testWith_0", mUnit.getCompileInfo(map));
  }
  
  //輸出結(jié)果
  //{操作系統(tǒng)名稱=Mac OS X, Java環(huán)境供應(yīng)商=JetBrains s.r.o, 操作系統(tǒng)架構(gòu)=x86_64, Java環(huán)境版本=1.8.0_112-release, 操作系統(tǒng)版本=10.12.6}
  

Run

• 函數(shù)定義

  public inline fun <T, R> T.run(block: T.() -> R): R = block()
  

• 函數(shù)說明

  Calls the specified function [block] with this value as its receiver and returns its result.
  Run 跟Apply近似实幕，唯一不同的是Apply返回的是自身對(duì)象，而Run返回的是最后一行

• 函數(shù)實(shí)例

  /**
   * 接收一個(gè)姓名堤器，打印三句話
   */
  fun sayHiTo(name: String): Boolean {
      println(name.run {
          println("Hi," + name)
          println("Nice to meet you," + this)
          "The run has over!!!!"
      })
      return true
  }   
  

• 測(cè)試函數(shù)

  private KotlinUnit mUnit = new KotlinUnit();
  
  @Test
  public void testRun_0() throws Exception {
      assertTrue("testRun_0", mUnit.sayHiTo("xiongwei"));
  }
  
  //輸出結(jié)果
  //Hi,xiongwei
  //Nice to meet you,xiongwei
  //The run has over!!!!
  

Also

• 函數(shù)定義

  @SinceKotlin("1.1")
  

public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }
```

• 函數(shù)說明

  Calls the specified function [block] with this value as its argument and returns this value.
  Also跟Apply很接近昆庇，唯一一點(diǎn)不同是在函數(shù)塊內(nèi)Also可以使用it來指向?qū)ο蟊颈旧恚鳤pply不行闸溃。

• 函數(shù)實(shí)例

  /**
   * 計(jì)算圓周長(zhǎng)
   */
  fun getCircumference(dia: Int): Boolean {
      println("The dia is " +
              dia.also {
                  println("The Circumference is " + Math.PI * it)
              }
      )
      return true
  }
  

• 測(cè)試函數(shù)

  private KotlinUnit mUnit = new KotlinUnit();
  
  @Test
  public void testAlso_0() throws Exception {
      assertTrue("testAlso_0", mUnit.getCircumference(10));
  }
  
  //輸出結(jié)果
  //The Circumference is 31.41592653589793
  //The dia is 10
  

總結(jié)

參考文章：
https://medium.com/@tpolansk/the-difference-between-kotlins-functions-let-apply-with-run-and-else-ca51a4c696b8
項(xiàng)目源碼：https://git.tedxiong.com/TedXiong/KotlinSample