JuneLeo's Blog
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Kotlin学习2

发表于 更新于

依赖

core-ktx

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+--- androidx.core:core-ktx:1.7.0
|    +--- org.jetbrains.kotlin:kotlin-stdlib:1.5.31
|    |    +--- org.jetbrains:annotations:13.0
|    |    \--- org.jetbrains.kotlin:kotlin-stdlib-common:1.5.31
|    +--- androidx.annotation:annotation:1.1.0 -> 1.2.0
|    \--- androidx.core:core:1.7.0
|         +--- androidx.annotation:annotation:1.2.0
|         +--- androidx.annotation:annotation-experimental:1.1.0
|         +--- androidx.lifecycle:lifecycle-runtime:2.3.1
|         |    +--- androidx.lifecycle:lifecycle-common:2.3.1
|         |    |    \--- androidx.annotation:annotation:1.1.0 -> 1.2.0
|         |    +--- androidx.arch.core:core-common:2.1.0
|         |    |    \--- androidx.annotation:annotation:1.1.0 -> 1.2.0
|         |    \--- androidx.annotation:annotation:1.1.0 -> 1.2.0
|         \--- androidx.versionedparcelable:versionedparcelable:1.1.1
|              +--- androidx.annotation:annotation:1.1.0 -> 1.2.0
|              \--- androidx.collection:collection:1.0.0
|                   \--- androidx.annotation:annotation:1.0.0 -> 1.2.0

kotlin-stdlib

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+--- org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.7.20
|    +--- org.jetbrains.kotlin:kotlin-stdlib:1.7.20
|    |    +--- org.jetbrains.kotlin:kotlin-stdlib-common:1.7.20
|    |    \--- org.jetbrains:annotations:13.0
|    \--- org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.7.20
|         \--- org.jetbrains.kotlin:kotlin-stdlib:1.7.20 (*)

从上述看kotlin-stdlib是关键的kotlin库

基础

解构

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fun main(args: Array<String>) {
    val person = Person("songpengfei", 31)
    val (name,age) = person
    println(name)
    println(age)
}

class Person(val name: String, val age: Int)

operator fun Person.component1() = name

operator fun Person.component2() = age

密封类

  • 每个枚举常量只存在一个实例，而密封类的一个子类可以有可包含状态的多个实例。

  • 有限几种类型。使用when时，不需要有else

    内联类

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    inline class Name(val s: String) {
        val length: Int
            get() = s.length

        fun greet() {
            println("Hello, $s")
        }
    }
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    public final class Name {


      @Lkotlin/jvm/JvmInline;()

      @Lkotlin/Metadata;(mv={1, 7, 1}, k=1, xi=48, d1={"\u0000*\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0000\n\u0002\u0010\u000e\n\u0002\u0008\u0003\n\u0002\u0010\u0008\n\u0002\u0008\u0005\n\u0002\u0010\u000b\n\u0002\u0008\u0004\n\u0002\u0010\u0002\n\u0002\u0008\u0007\u0008\u0087@\u0018\u00002\u00020\u0001B\u0012\u0012\u0006\u0010\u0002\u001a\u00020\u0003\u00f8\u0001\u0000\u00a2\u0006\u0004\u0008\u0004\u0010\u0005J\u001a\u0010\u000c\u001a\u00020\r2\u0008\u0010\u000e\u001a\u0004\u0018\u00010\u0001H\u00d6\u0003\u00a2\u0006\u0004\u0008\u000f\u0010\u0010J\r\u0010\u0011\u001a\u00020\u0012\u00a2\u0006\u0004\u0008\u0013\u0010\u0014J\u0010\u0010\u0015\u001a\u00020\u0007H\u00d6\u0001\u00a2\u0006\u0004\u0008\u0016\u0010\u0009J\u0010\u0010\u0017\u001a\u00020\u0003H\u00d6\u0001\u00a2\u0006\u0004\u0008\u0018\u0010\u0005R\u0011\u0010\u0006\u001a\u00020\u00078F\u00a2\u0006\u0006\u001a\u0004\u0008\u0008\u0010\u0009R\u0011\u0010\u0002\u001a\u00020\u0003\u00a2\u0006\u0008\n\u0000\u001a\u0004\u0008\n\u0010\u000b\u0088\u0001\u0002\u00f8\u0001\u0000\u0082\u0002\u0004\n\u0002\u0008\u0019\u00a8\u0006\u0019"}, d2={"LName;", "", "s", "", "constructor-impl", "(Ljava/lang/String;)Ljava/lang/String;", "length", "", "getLength-impl", "(Ljava/lang/String;)I", "getS", "()Ljava/lang/String;", "equals", "", "other", "equals-impl", "(Ljava/lang/String;Ljava/lang/Object;)Z", "greet", "", "greet-impl", "(Ljava/lang/String;)V", "hashCode", "hashCode-impl", "toString", "toString-impl", "kotlin-demo"})

      // access flags 0x12
      private final Ljava/lang/String; s

      @groovyx.ast.bytecode.Bytecode
      public final String getS() {
        aload 0
        getfield 'Name.s','Ljava/lang/String;'
        areturn
      }

      @groovyx.ast.bytecode.Bytecode
      public final static int getLength-impl(String a) {
        aload 0
        invokevirtual 'java/lang/String.length','()I'
        ireturn
      }

      @groovyx.ast.bytecode.Bytecode
      public final static void greet-impl(String a) {
        _new 'java/lang/StringBuilder'
        dup
        invokespecial 'java/lang/StringBuilder.<init>','()V'
        ldc "Hello, "
        invokevirtual 'java/lang/StringBuilder.append','(Ljava/lang/String;)Ljava/lang/StringBuilder;'
        aload 0
        invokevirtual 'java/lang/StringBuilder.append','(Ljava/lang/String;)Ljava/lang/StringBuilder;'
        invokevirtual 'java/lang/StringBuilder.toString','()Ljava/lang/String;'
        getstatic 'java/lang/System.out','Ljava/io/PrintStream;'
        swap
        invokevirtual 'java/io/PrintStream.println','(Ljava/lang/Object;)V'
        return
      }

      @groovyx.ast.bytecode.Bytecode
      public static String toString-impl(String a) {
        _new 'java/lang/StringBuilder'
        dup
        invokespecial 'java/lang/StringBuilder.<init>','()V'
        ldc "Name(s="
        invokevirtual 'java/lang/StringBuilder.append','(Ljava/lang/String;)Ljava/lang/StringBuilder;'
        aload 0
        invokevirtual 'java/lang/StringBuilder.append','(Ljava/lang/String;)Ljava/lang/StringBuilder;'
        bipush 41
        invokevirtual 'java/lang/StringBuilder.append','(C)Ljava/lang/StringBuilder;'
        invokevirtual 'java/lang/StringBuilder.toString','()Ljava/lang/String;'
        areturn
      }

      @groovyx.ast.bytecode.Bytecode
      public String toString() {
        aload 0
        getfield 'Name.s','Ljava/lang/String;'
        invokestatic 'Name.toString-impl','(Ljava/lang/String;)Ljava/lang/String;'
        areturn
      }

      @groovyx.ast.bytecode.Bytecode
      public static int hashCode-impl(String a) {
        aload 0
        invokevirtual 'java/lang/String.hashCode','()I'
        ireturn
      }

      @groovyx.ast.bytecode.Bytecode
      public int hashCode() {
        aload 0
        getfield 'Name.s','Ljava/lang/String;'
        invokestatic 'Name.hashCode-impl','(Ljava/lang/String;)I'
        ireturn
      }

      @groovyx.ast.bytecode.Bytecode
      public static boolean equals-impl(String a,Object b) {
        aload 1
        _instanceof 'Name'
        ifne l0
        iconst_0
        ireturn
       l0
        aload 1
        checkcast 'Name'
        invokevirtual 'Name.unbox-impl','()Ljava/lang/String;'
        aload 0
        swap
        invokestatic 'kotlin/jvm/internal/Intrinsics.areEqual','(Ljava/lang/Object;Ljava/lang/Object;)Z'
        ifne l1
        iconst_0
        ireturn
       l1
        iconst_1
        ireturn
      }

      @groovyx.ast.bytecode.Bytecode
      public boolean equals(Object a) {
        aload 0
        getfield 'Name.s','Ljava/lang/String;'
        aload 1
        invokestatic 'Name.equals-impl','(Ljava/lang/String;Ljava/lang/Object;)Z'
        ireturn
      }

      @groovyx.ast.bytecode.Bytecode
      private void <init>(String a) {
        aload 0
        invokespecial 'java/lang/Object.<init>','()V'
        aload 0
        aload 1
        putfield 'Name.s','Ljava/lang/String;'
        return
      }

      @groovyx.ast.bytecode.Bytecode
      public static String constructor-impl(String a) {
        // annotable parameter count: 1 (invisible)
        aload 0
        ldc "s"
        invokestatic 'kotlin/jvm/internal/Intrinsics.checkNotNullParameter','(Ljava/lang/Object;Ljava/lang/String;)V'
        aload 0
        areturn
      }

      @groovyx.ast.bytecode.Bytecode
      public final static Name box-impl(String a) {
        _new 'Name'
        dup
        aload 0
        invokespecial 'Name.<init>','(Ljava/lang/String;)V'
        areturn
      }

      @groovyx.ast.bytecode.Bytecode
      public final String unbox-impl() {
        aload 0
        getfield 'Name.s','Ljava/lang/String;'
        areturn
      }

      @groovyx.ast.bytecode.Bytecode
      public final static boolean equals-impl0(String a,String b) {
        aload 0
        aload 1
        invokestatic 'kotlin/jvm/internal/Intrinsics.areEqual','(Ljava/lang/Object;Ljava/lang/Object;)Z'
        ireturn
      }
    }

    中缀函数

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    infix fun Int.shl(x: Int): Int { …… }

    // 用中缀表示法调用该函数
    1 shl 2

    // 等同于这样
    1.shl(2)

    中缀函数调用的优先级低于算术操作符、类型转换以及 rangeTo 操作符。

    尾递归函数

    作用域函数

  • let

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    fun main() {
        val person = Person("songpengfei",32,"beijing").let { 
            it.age = 21
            it  // 需要返回it
        }
        println(person.age) // let闭包中上下文是it
    }

  • run

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    fun main() {
        val person = Person("songpengfei", 32, "beijing").run {
            age = 21
            this // 需要返回it
        }
        println(person.age) // let闭包中上下文是this
    }

    fun main() {
        val person = run {
            val person = Person("songpengfei", 32, "beijing")
            person.age = 21
            person
        }
        println(person.age)
    }

  • apply

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    fun main() {
        val person = Person("songpengfei", 32, "beijing").apply {
            age = 21
        }
        println(person.age) // 同run
    }

  • also

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    fun main() {
        val person = Person("songpengfei", 32, "beijing").also {
            it.age = 21
        }
        println(person.age) // 同let
    }

  • with

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    fun main() {
        val p = with(Person("songpengfei", 32, "beijing")) {
            age = 21
            this
        }
        println(p)
    }

  • takeIf

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    fun main() {
        val person = Person("songpengfei", 32, "beijing").takeIf {
            it.age != 32
        }
        println(person)
    }

  • takeUnless

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    fun main() {
        val person = Person("songpengfei", 32, "beijing").takeUnless {
            it.age == 32
        }
        println(person)
    }

    解构声明

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    data class Person(var name: String, var age: Int, var city: String)
    fun main() {
        val (name,age) = Person("song",32,"beijing")
        println(name)
        println(age)
    }

    image.png
    data类 会生成 componentN函数

    This 表达式

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    class A { // 隐式标签 @A
        inner class B { // 隐式标签 @B
            fun Int.foo() { // 隐式标签 @foo
                val a = this@A // A 的 this
                val b = this@B // B 的 this
                val c = this // foo() 的接收者，一个 Int
                val c1 = this@foo // foo() 的接收者，一个 Int
                val funLit = lambda@ fun String.() {
                    val d = this // funLit 的接收者
                }
                val funLit2 = { s: String ->
                    // foo() 的接收者，因为它包含的 lambda 表达式
                    // 没有任何接收者
                    val d1 = this
                }
            }
        }
    }

    注解

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    annotation class Ann
    class Example(
        @field:Ann val foo: Int,    // 标注 Java 字段
        @get:Ann val bar: Int,      // 标注 Java getter
        @param:Ann val quux: Int
    )

    反射

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    implementation("org.jetbrains.kotlin:kotlin-reflect:1.7.20")

    KClasses 中定义了拓展反射方法

协程

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implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.1")
  • runBlocking 方法会_阻塞_当前线程来等待
  • coroutineScope 只是挂起，会释放底层线程用于其他用途
  • runBlocking嵌套runBlocking ，被嵌套的runBlocking会让外部runBlocking阻塞
  • Dispatchers.Unconfined 不会受到限制，直接执行，如果块内部调用了delay，阻塞完成后会切换线程

委托属性

  • 延迟属性（lazy properties）: 其值只在首次访问时计算；
  • 可观察属性（observable properties）: 监听器会收到有关此属性变更的通知；
  • 把多个属性储存在一个映射（map）中，而不是每个存在单独的字段中。