kotlin语法糖以及原理

kotlin by关键字实现委托属性与委托类

委托类

// 创建接口interface Base {   
    fun print()}


// 实现此接口的被委托的类class BaseImpl(val x: Int) : Base {
    override fun print() { print(x) }}


// 通过关键字 by 建立委托类class Derived(b: Base) : Base by b


fun main(args: Array) {
    val b = BaseImpl(10)
    Derived(b).print() // 输出 10}

class CountingSet3(
    val innerSet: MutableSet = HashSet()
) : MutableSet by innerSet {

需要注意的是：

• CountingSet3 必须实现一个接口，而不能继承于一个类；
• innerSet 的类型必须是 CountingSet3 所实现接口的子类型；
• 可以直接在 by 创建委托对象，如下所示： 但是，这样的话，在 CountingSet4 类中无法获取到委托对象的引用了。

class CountingSet4(
) : MutableSet by HashSet() {
}

class CountingSet3(
    val innerSet: MutableSet = HashSet()
) : MutableSet by innerSet {
    var objectAdded = 0
    override fun add(element: T): Boolean {
        objectAdded++
        return innerSet.add(element)
    }


    override fun addAll(elements: Collection): Boolean {
        objectAdded += elements.size
        return innerSet.addAll(elements)
    }
}


fun main() {
    val cset = CountingSet3()
    cset.add(1)
    cset.addAll(listOf(2, 2, 3))
    println("${cset.objectAdded} objects were added, ${cset.size} remain")
}

其对应class反编译代码为

public final class CountingSet3 implements Set, KMutableSet {
   private int objectAdded;
   @NotNull
   private final Set innerSet;


   public final int getObjectAdded() {
      return this.objectAdded;
   }


   public final void setObjectAdded(int var1) {
      this.objectAdded = var1;
   }


   public boolean add(Object element) {
      int var10001 = this.objectAdded++;
      return this.innerSet.add(element);
   }


   public boolean addAll(@NotNull Collection elements) {
      Intrinsics.checkNotNullParameter(elements, "elements");
      this.objectAdded += elements.size();
      return this.innerSet.addAll(elements);
   }


   @NotNull
   public final Set getInnerSet() {
      return this.innerSet;
   }


   public CountingSet3(@NotNull Set innerSet) {
      Intrinsics.checkNotNullParameter(innerSet, "innerSet");
      super();
      this.innerSet = innerSet;
   }


   // $FF: synthetic method
   public CountingSet3(Set var1, int var2, DefaultConstructorMarker var3) {
      if ((var2 & 1) != 0) {
         var1 = (Set)(new HashSet());
      }


      this(var1);
   }


   public CountingSet3() {
      this((Set)null, 1, (DefaultConstructorMarker)null);
   }


   public int getSize() {
      return this.innerSet.size();
   }


   // $FF: bridge method
   public final int size() {
      return this.getSize();
   }


   public void clear() {
      this.innerSet.clear();
   }


   public boolean contains(Object element) {
      return this.innerSet.contains(element);
   }


   public boolean containsAll(@NotNull Collection elements) {
      Intrinsics.checkNotNullParameter(elements, "elements");
      return this.innerSet.containsAll(elements);
   }


   public boolean isEmpty() {
      return this.innerSet.isEmpty();
   }


   @NotNull
   public Iterator iterator() {
      return this.innerSet.iterator();
   }


   public boolean remove(Object element) {
      return this.innerSet.remove(element);
   }


   public boolean removeAll(@NotNull Collection elements) {
      Intrinsics.checkNotNullParameter(elements, "elements");
      return this.innerSet.removeAll(elements);
   }


   public boolean retainAll(@NotNull Collection elements) {
      Intrinsics.checkNotNullParameter(elements, "elements");
      return this.innerSet.retainAll(elements);
   }


   public Object[] toArray() {
      return CollectionToArray.toArray(this);
   }


   public Object[] toArray(Object[] var1) {
      return CollectionToArray.toArray(this, var1);
   }
}

委托属性

class Foo {
    var p: Type by Delegate()
}

等价于

class Foo {
    private val delegate = Delegate()
    var p: Type
        set(value: Type) = delegate.setValue(this, ..., value)
        get() = delegate.getValue(this, ...)
}

实例如下

class Person6 {
    var name: String by Delegate3()
    var lastname: String by Delegate3()
    var updateCount = 0
}


fun main() {
    val person = Person6()
    person.name = "peter"
    person.lastname = "wang"
    println("name=${person.name}")
    println("lastname=${person.lastname}")
    println("updateCount=${person.updateCount}")
}
/*

打印如下：

name=Peter-5

lastname=Wang-4

updateCount=2

*/

class Delegate3 : ReadWriteProperty {
    var formattedString = ""
    override fun getValue(thisRef: Any, property: KProperty<*>): String {
        return formattedString + "-" + formattedString.length
    }


    override fun setValue(thisRef: Any, property: KProperty<*>, value: String) {
        if (thisRef is Person6) {
            thisRef.updateCount++
        }
        formattedString = value.lowercase().replaceFirstChar { it.uppercase() }
    }
}

反编译后对应java代码如下

public final class Delegate3 implements ReadWriteProperty {
   @NotNull
   private String formattedString = "";


   @NotNull
   public final String getFormattedString() {
      return this.formattedString;
   }


   public final void setFormattedString(@NotNull String var1) {
      this.formattedString = var1;
   }


   @NotNull
   public String getValue(@NotNull Object thisRef, @NotNull KProperty property) {
      return this.formattedString + "-" + this.formattedString.length();
   }


   // $FF: synthetic method
   // $FF: bridge method
   public Object getValue(Object var1, KProperty var2) {
      return this.getValue(var1, var2);
   }


   public void setValue(@NotNull Object thisRef, @NotNull KProperty property, @NotNull String value) {
      if (thisRef instanceof Person6) {
         ((Person6)thisRef).setUpdateCount(((Person6)thisRef).getUpdateCount() + 1);
      }


      Delegate3 var10000 = this;
      boolean var5 = false;
      String var10001 = value.toLowerCase(Locale.ROOT);
      Intrinsics.checkNotNullExpressionValue(var10001, "(this as java.lang.Strin….toLowerCase(Locale.ROOT)");
      String var4 = var10001;
      var5 = false;
      CharSequence var6 = (CharSequence)var4;
      boolean var7 = false;
      if (var6.length() > 0) {
         StringBuilder var20 = new StringBuilder();
         char it = var4.charAt(0);
         StringBuilder var15 = var20;
         int var9 = false;
         boolean var11 = false;
         String var12 = String.valueOf(it);
         boolean var13 = false;
         if (var12 == null) {
            throw new NullPointerException("null cannot be cast to non-null type java.lang.String");
         }


         String var19 = var12.toUpperCase(Locale.ROOT);
         Intrinsics.checkNotNullExpressionValue(var19, "(this as java.lang.Strin….toUpperCase(Locale.ROOT)");
         String var16 = var19;
         var10000 = this;
         var20 = var15.append(var16.toString());
         byte var17 = 1;
         boolean var18 = false;
         if (var4 == null) {
            throw new NullPointerException("null cannot be cast to non-null type java.lang.String");
         }


         String var10002 = var4.substring(var17);
         Intrinsics.checkNotNullExpressionValue(var10002, "(this as java.lang.String).substring(startIndex)");
         var10001 = var20.append(var10002).toString();
      } else {
         var10001 = var4;
      }


      var10000.formattedString = var10001;
   }


   // $FF: synthetic method
   // $FF: bridge method
   public void setValue(Object var1, KProperty var2, Object var3) {
      this.setValue(var1, var2, (String)var3);
   }
}




public final class Person6 {
   // $FF: synthetic field
   static final KProperty[] $$delegatedProperties = new KProperty[]{
   (KProperty)Reflection.mutableProperty1(new MutablePropertyReference1Impl(Person6.class, "name", "getName()Ljava/lang/String;", 0)),
   (KProperty)Reflection.mutableProperty1(new MutablePropertyReference1Impl(Person6.class, "lastname", "getLastname()Ljava/lang/String;", 0))};
   @NotNull
   private final Delegate3 name$delegate = new Delegate3();
   @NotNull
   private final Delegate3 lastname$delegate = new Delegate3();
   private int updateCount;


   @NotNull
   public final String getName() {
      return this.name$delegate.getValue(this, $$delegatedProperties[0]);
   }


   public final void setName(@NotNull String var1) {
      this.name$delegate.setValue(this, $$delegatedProperties[0], var1);
   }


   @NotNull
   public final String getLastname() {
      return this.lastname$delegate.getValue(this, $$delegatedProperties[1]);
   }


   public final void setLastname(@NotNull String var1) {
      this.lastname$delegate.setValue(this, $$delegatedProperties[1], var1);
   }


   public final int getUpdateCount() {
      return this.updateCount;
   }


   public final void setUpdateCount(int var1) {
      this.updateCount = var1;
   }
}

lateinit与by lazy

如下写法会编译不通过

class Demo {
  var value: String
  
  fun printValue() {
    println(value)
  }}

改为
lateinit var value: String
运行抛异常
lateinit修饰的成员变量等价于

// 字节码等价代码
fun printValue() {
  val tempValue = this.value
  if(tempValue == null) {
    throw UninitializedPropertyAccessException("lateinit property value has not been initialized")
  }
  println(tempValue)}

val lazyProp: String by lazy {
    println("Hello，第一次调用才会执行我！")
    "西哥！"
}


// 打印lazyProp 3次，查看结果
fun main() {
    println(lazyProp)
    println(lazyProp)
    println(lazyProp)
}
Hello，第一次调用才会执行我！
西哥！
西哥！
西哥！

扩展函数与扩展属性

如果扩展函数和被扩展类中的成员函数有相同的接收类型、名字和参数，那么这种情况下 ** 总是取成员**。例如：

class Dog {


    fun showName(){
        print("Dog")
    }
}


fun Dog.showName(){
    print("Cat")
}


fun main(args: Array) {
    Dog().showName()
}
    

如果我们调用 Dog 类的 showName(),它将输出“Dog”,而不是“Cat”

扩展函数实质上是调用了新生成的一个类的静态方法，如

fun String.times(t:Int){
    val sb = StringBuilder()
    for (i in 0 until t) {
        sb.append(this)
    }
    println(sb.toString())}
``
其等价于
```kotlin
public final class TestObjectKt {
   public static final void times(@NotNull String $receiver, int t) {
      Intrinsics.checkParameterIsNotNull($receiver, "$receiver");
      StringBuilder sb = new StringBuilder();
      IntRange var10000 = RangesKt.until(0, t);
      int i = var10000.getFirst();
      int var4 = var10000.getLast();
      if(i <= var4) {
         while(true) {
            sb.append($receiver);
            if(i == var4) {
               break;
            }


            ++i;
         }
      }


      String var5 = sb.toString();
      System.out.println(var5);
   }}

扩展属性
扩展属性不能初始化

//错误：扩展属性不能有初始化
val Snake.bbb = 1

扩展属性并没有field可以用来存储value，其实际作用是使用通过value来操作调用者，即this

fun main(args: Array) {
    val str = "aa"
    //没有backing field，不能存储值，其实际是通过setXXX(str，10)操作str
    //输出：aa10
    str.s = 10

    //输出：2
    println(str.s)}

var String.s: Int
    get() = this.length
    set(value){
        //set方法并没有field可以用来存储value，
        //其实际作用是使用通过value来操作调用者，即this
        println(this.plus(value))
    }

对应java

public final class ExtendsKt {
   public static final void main(@NotNull String[] args) {
      Intrinsics.checkParameterIsNotNull(args, "args");
      String str = "aa";
      setS(str, 10);
      int var2 = getS(str);
      System.out.println(var2);
   }

   public static final int getS(@NotNull String $receiver) {
      Intrinsics.checkParameterIsNotNull($receiver, "$receiver");
      return $receiver.length();
   }

   public static final void setS(@NotNull String $receiver, int value) {
      Intrinsics.checkParameterIsNotNull($receiver, "$receiver");
      String var2 = $receiver + value;
      System.out.println(var2);
   }}

let,run,apply

let需要it指代统一的引用
run不需要it
apply可以有返回值

//let
val result = object?.let{//表示object不为null的条件下，才会去执行let函数体
   it.todo()
    1000
}


//with
val result = with(user, {
        println("my name is $name, I am $age years old, my phone number is $phoneNum")
        1000
    })

l
//run
fun main(args: Array) {
    val user = User("Kotlin", 1, "1111111")

    val result = user.run {
        println("my name is $name, I am $age years old, my phone number is $phoneNum")
        1000
    }
    println("result: $result")
}

//apply
fun main(args: Array) {
    val user = User("Kotlin", 1, "1111111")


    val result = user.apply {
        println("my name is $name, I am $age years old, my phone number is $phoneNum")
        1000
    }
    println("result: $result")
}


//also
fun main(args: Array) {
    val result = "testLet".also {
        println(it.length)
        1000
    }
    println(result)
}

Flow与Channel

Channel类似于阻塞队列

Flow类似于Rxjava

https://juejin.cn/post/6983673515526324237

    fun produceAndConsume() {
        GlobalScope.launch {
            val channel = Channel()


            val producer = GlobalScope.launch {
                var i = 0
                while (true) {
                    Log.i(tag, "生产者生产了:$i")
                    channel.send(i++)
                    delay(1000)
                }
            }


            val consumer = GlobalScope.launch {
                while (true) {
                    val element = channel.receive()
                    Log.i(tag, "消费者消费了:$element")
                }
            }
            producer.join()
            consumer.join()
        }
    }
//输出
I/ProduceAndConsume: 生产者生产了:0
I/ProduceAndConsume: 消费者消费了:0
I/ProduceAndConsume: 生产者生产了:1
I/ProduceAndConsume: 消费者消费了:1
I/ProduceAndConsume: 生产者生产了:2
I/ProduceAndConsume: 消费者消费了:2
I/ProduceAndConsume: 生产者生产了:3
I/ProduceAndConsume: 消费者消费了:3

Flow有以下特点：

1. 冷数据流，不消费则不生产,这一点与Channel正相反：Channel的发送端并不依赖于接收端。
2. Flow通过flowOn改变数据发射的线程，数据消费线程则由协程所在线程决定
3. 与RxJava类似,支持通过catch捕获异常,通过onCompletion 回调完成
4. Flow没有提供取消方法，可以通过取消Flow所在协程的方式来取消

lifecycleScope.launch {
    flow {
        for (i in 1..10) {
            emit(i)
        }
    }.flowOn(Dispatchers.Main)
        .catch {
            //异常处理
        }
        .onCompletion {
            //完成回调
        }
        .collect { num ->
            // 具体的消费处理
            // 只有collect时才会生产数据
            // ...
        }
}