Reference outside the sealed class in Kotlin?

问题:

I'm trying to create a class that uses its own state to operate on the state of an external object that it holds a reference to. The external object can be of class A or B, which are similar, but not controlled by the author. So a sealed class is created to access their common attributes, per this earlier answer from @SimY4.

// *** DOES NOT COMPILE *** class A {   // foreign class whose structure is not modifiable   val prop get()= "some string made the Class-A way" } class B {   // foreign class whose structure is not modifiable   val prop get()= "some string made the Class-B way" } data class ABTool (val obj:AB, val i:Int, val j:Int) {   // class that manipulates i and j and uses them to do   // things with AB's "common" attributes through the sealed class AB   sealed class AB {   // substitute for a common interface     abstract val prop: String     abstract val addmagic: String     data class BoxA(val o:A) : AB() {       override val prop get()= o.prop       override val addmagic get() = prop + [email protected]???.magic  // HOW TO REFERENCE?     }     data class BoxB(val o:B) : AB() {       override val prop get()= o.prop       override val addmagic get() = [email protected]???.magic + prop  // HOW TO REFERENCE?     }   }   val magic get()= "magic: ${i*j}" } 

The problem now is that I've figured out I can't operate on the external object in the way I want, because a sealed class can't refer to its outer class members. Is there a better way to make this work, even if using a different approach (other than sealed class), while:

• not changing foreign classes A or B;
• respecting that A and B (and many others in the real case) are similar, so I'm trying to write one tool that calculates and adds magic to A and B with the same code base; and
• noting that although the ABTool tools are the same, the way they are applied to add magic is slightly different in A vs. B, just as the to access the conceptually common elements of A and B may be different.

Any thoughts on this or a similar workaround? Maybe a more functional approach that I haven't conceived yet?

回答1:

If ABTool being a sealed class is something you can give up, then here's a solution:

1. Replace sealed with inner abstract at the ABTool declaration;
2. Mark BoxA and BoxB as inner as well;

data class ABTool(val obj: AB, val i: Int, val j: Int) {     inner abstract class AB {         abstract val prop: String         abstract val addmagic: String          inner class BoxA(val o: A) : AB() {             override val prop get() = o.prop             override val addmagic get() = prop + magic         }          inner class BoxB(val o: B) : AB() {             override val prop get() = o.prop             override val addmagic get() = magic + prop         }     }      val magic get() = "magic: ${i * j}" } 

(alternatively, instead of marking AB as inner, move BoxA and BoxB out of it to the scope of ABTool)

回答2:

An alternative would be to add an ABTool field to AB:

sealed class AB(val tool: ABTool) {   abstract val prop: String   abstract val addmagic: String   data class BoxA(val o:A, tool: ABTool) : AB(tool) {     override val prop get()= o.prop     override val addmagic get() = prop + tool.magic   }   data class BoxB(val o:B, tool: ABTool) : AB(tool) {     override val prop get()= o.prop     override val addmagic get() = tool.magic + prop   } } 

and pass this when creating it from ABTool. That's just what inner really does, after all.

In this specific case the field happens to be unused in AB itself and so you can remove it from there and make it val in BoxA and BoxB.

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