Google推送的31天快速了解Kotlin，这篇就是最后一个了。但我还会继续从平时的学习中分享一些有用的学习经历

1. Android KTX提供了bundle creator，可以让我们在创建bundle的时候更加简洁，不用再调用'putString', 'putInt'等方法，调用一次就返回一个新bundle

   val bundle = bundleOf(
        "KEY_INT" to 1, 
        "KEY_LONG" to 2L,
        "KEY_BOOLEAN" to true,
        "KEY_NULL" to null,
        "KEY_ARRAY" to arrayOf(1, 2)
    )
   

   android/android-ktxandroid-ktx - A set of Kotlin extensions for Android app development.github.com

2. 当我们最后一个参数是回调的时候，你可以通过Lambda来cleanup callbacks，Callable以及Runnable。比如，Android KTX提供了‘postDelayed’ with Lambda。

   // Android KTX API
   fun Handler.postDelayed(delay: Int /* ... */, action: ()-> Unit)
   
   // Call it like this - no need for a Runnable in your code
   handler.postDelayed(50) {
    // pass a lambda to postDelayed
   }
   

   android/android-ktxandroid-ktx - A set of Kotlin extensions for Android app development.github.com

3. Kotlin KTX给一些最常用的spans添加了extension，这样可以让其相关的API更加使用起来更简单。

   var string = buildSpannedString {
       append("no styling text")
       bold {
           append("bold")
           italic {
               append("bold and italic")
           }
       }
       inSpans(RelativeSizeSpan(2f), QuoteSpan()) {
           append("double sized quote text")
       }
   }
   

   android/android-ktxandroid-ktx - A set of Kotlin extensions for Android app development.github.com

4. Kotlin提供了Parcelize可以让你实现Parcelable的时候更加的简洁

   @Parcelize
   data class User(val name: String, val occupation: Work): Parcelable
   
   // build.gradle
   androidExtensions {
       // Enable experimental Kotlin features in gradle to enable Parcelize
       experimental = true
   }
   

   Kotlin/KEEPKotlin Evolution and Enhancement Process. Contribute to KEEP development by creating an…github.com

5. 通过使用default的参数列表来扩展现有函数API。Android KTX可以让你设置任意的一个参数。

   view.updatePadding(left = newPadding)
   view.updatePadding(top = newPadding)
   view.updatePadding(right = newPadding)
   view.updatePadding(bottom = newPadding)
   
   view.updatePadding(top = newPadding, bottom = newPadding)
   

   android/android-ktxandroid-ktx - A set of Kotlin extensions for Android app development.github.com

6. 'let', 'apply', 'with'和‘run’都有一个receiver(this), 可能有一个argument(it)以及可能有一个返回值，下面来看一下它们的区别：

   val string = "a"
   val result = string.run {
       // this = "a"
       // it = not available
       1 // Block return value
       // result = 1
   }
   

   val string = "a"
   val result = string.let {
       // this = this@MyClass
       // it = "a"
       2 // Block return value
       // result = 2
   }
   

   val string = "a"
   val result = with(string) {
       // this = "a"
       // it = not available
       3 // Block return value
       // result = 3
   }
   

   val string = "a"
   val result = string.apply {
       // this = "a"
       // it = not available
       4 // Block return value unused
       // result = "a"
   }
   

   Read More in Kotlin Core Funcitons