kotlin委托类

清疚 2023-01-12 11:48 123阅读 0赞

# 委托类 #

委托模式为实现继承提供了一个好的选择，在java中还需要使用各种设计模式才能实现，而kotlin中提供了原生的支持。

interface MyInterface { 
        fun print()
    }
    
    class MyInterfaceImpl : MyInterface { 
        override fun print() { 
            println("hello world")
        }
    }
    
    class DelegationImpl(var myInterface: MyInterface) : MyInterface by myInterface
    
    fun main() { 
        var myInterfaceImpl = MyInterfaceImpl()
        var delegationImpl = DelegationImpl(myInterfaceImpl)
        delegationImpl.print()
    }

将DelegationImpl类反编译后的结果如下：

package com.morris.kotlin.delegation;
    
    import kotlin.Metadata;
    import kotlin.jvm.internal.Intrinsics;
    import org.jetbrains.annotations.NotNull;
    
    @Metadata(mv = { 1, 4, 0}, bv = { 1, 0, 3}, k = 1, d1 = { "\000\022\n\002\030\002\n\002\030\002\n\002\b\006\n\002\020\002\n\000\030\0002\0020\001B\r\022\006\020\002\032\0020\001\006\002\020\003J\t\020\007\032\0020\bH\001R\032\020\002\032\0020\001X\016\006\016\n\000\032\004\b\004\020\005\"\004\b\006\020\003\006\t"}, d2 = { "Lcom/morris/kotlin/delegation/DelegationImpl;", "Lcom/morris/kotlin/delegation/MyInterface;", "myInterface", "(Lcom/morris/kotlin/delegation/MyInterface;)V", "getMyInterface", "()Lcom/morris/kotlin/delegation/MyInterface;", "setMyInterface", "print", "", "untitled"})
    public final class DelegationImpl implements MyInterface { 
      @NotNull
      private MyInterface myInterface;
    
      @NotNull
      public final MyInterface getMyInterface() { 
        return this.myInterface;
      }
    
      public final void setMyInterface(@NotNull MyInterface <set-?>) { 
        Intrinsics.checkNotNullParameter(<set-?>, "<set-?>");
        this.myInterface = <set-?>;
      }
    
      public DelegationImpl(@NotNull MyInterface myInterface) { 
        this.$$delegate_0 = myInterface;
        this.myInterface = myInterface;
      }
    
      public void print() { 
        this.$$delegate_0.print();
      }
    }

可见DelegationImpl内部持有一个MyInterface的实例，然后具体的方法上面调用MyInterface的相应方法，从而实现委托。

如果委托类中实现了方法，那么就不会使用被委托类的方法：

interface Base { 
        fun printMessage()
    }
    
    class BaseImpl(val x: Int) : Base { 
        override fun printMessage() {  print(x) }
    }
    
    class Derived(b: Base) : Base by b { 
        override fun printMessage() {  print("abc") }
    }
    
    fun main() { 
        val b = BaseImpl(10)
        Derived(b).printMessage() // abc
    }

然而下面示例中message无法被委托。

interface Base2 { 
        val message: String
        fun print()
    }
    
    class BaseImpl2(val x: Int) : Base2 { 
        override val message = "BaseImpl: x = $x"
        override fun print() {  println(message) }
    }
    
    class Derived2(b: Base2) : Base2 by b { 
        // This property is not accessed from b's implementation of `print`
        override val message = "Message of Derived"
    }
    
    fun main() { 
        val b = BaseImpl2(10)
        val derived = Derived2(b)
        derived.print() // BaseImpl: x = 10
        println(derived.message) // Message of Derived
    }

Derived2反编译后的结果：

package com.morris.kotlin.delegation;
    
    import kotlin.Metadata;
    import org.jetbrains.annotations.NotNull;
    
    @Metadata(mv = { 1, 4, 0}, bv = { 1, 0, 3}, k = 1, d1 = { "\000\032\n\002\030\002\n\002\030\002\n\002\b\003\n\002\020\016\n\002\b\003\n\002\020\002\n\000\030\0002\0020\001B\r\022\006\020\002\032\0020\001\006\002\020\003J\t\020\b\032\0020\tH\001R\024\020\004\032\0020\005X\006\b\n\000\032\004\b\006\020\007\006\n"}, d2 = { "Lcom/morris/kotlin/delegation/Derived2;", "Lcom/morris/kotlin/delegation/Base2;", "b", "(Lcom/morris/kotlin/delegation/Base2;)V", "message", "", "getMessage", "()Ljava/lang/String;", "print", "", "untitled"})
    public final class Derived2 implements Base2 { 
      @NotNull
      private final String message;
    
      public void print() { 
        this.$$delegate_0.print();
      }
    
      public Derived2(@NotNull Base2 b) { 
        this.$$delegate_0 = b;
        this.message = "Message of Derived";
      }
    
      @NotNull
      public String getMessage() { 
        return this.message;
      }
    }