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 + this@???.magic // HOW TO REFERENCE?
}
data class BoxB(val o:B) : AB() {
override val prop get()= o.prop
override val addmagic get() = this@???.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:
- Replace
sealed
withinner abstract
at theABTool
declaration; - Mark
BoxA
andBoxB
asinner
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
.
来源:https://stackoverflow.com/questions/47361279/reference-outside-the-sealed-class-in-kotlin