Combining/merging Kotlin 中的数据类

Question

有没有办法在不指定所有属性的情况下合并 kotlin 数据类？

data class MyDataClass(val prop1: String, val prop2: Int, ...//many props)

具有以下签名的函数：

fun merge(left: MyDataClass, right: MyDataClass): MyDataClass

此函数在类上检查每个属性并且它们不同的地方使用左侧参数创建一个新的 MyDataClass。

是否可以使用 kotlin-reflect 或其他方式实现？

编辑：更清晰

这是对我希望能够做的事情的更好描述

  data class Bob(
        val name: String?,
        val age: Int?,
        val remoteId: String?,
        val id: String)

@Test
fun bob(){

    val original = Bob(id = "local_id", name = null, age = null, remoteId = null)
    val withName = original.copy(name = "Ben")
    val withAge = original.copy(age = 1)
    val withRemoteId = original.copy(remoteId = "remote_id")

    //TODO: merge without accessing all properties
    // val result = 
    assertThat(result).isEqualTo(Bob(id = "local_id", name = "Ben", age=1, remoteId = "remote_id"))
}

Answer 1

infix fun <T : Any> T.merge(mapping: KProperty1<T, *>.() -> Any?): T {
    //data class always has primary constructor ---v
    val constructor = this::class.primaryConstructor!!
    //calculate the property order
    val order = constructor.parameters.mapIndexed { index, it -> it.name to index }
                                      .associate { it };

    // merge properties
    @Suppress("UNCHECKED_CAST")
    val merged = (this::class as KClass<T>).declaredMemberProperties
                                           .sortedWith(compareBy{ order[it.name]})
                                           .map { it.mapping() }
                                           .toTypedArray()


    return constructor.call(*merged);
}

编辑

infix fun <T : Any> T.merge(right: T): T {
    val left = this;
    return left merge mapping@ {
        //    v--- implement your own merge strategy
        return@mapping this.get(left) ?: this.get(right);
    };
}

示例

val original = Bob(id = "local_id", name = null, age = null, remoteId = null)
val withName = original.copy(name = "Ben")
val withAge = original.copy(age = 1)
val withRemoteId = original.copy(remoteId = "remote_id")

val result = withName merge withAge merge withRemoteId;

Answer 2

您的要求与复制left值完全相同：

fun merge(left: MyDataClass, right: MyDataClass) = left.copy()

也许其中一种用途没有正确理解另一种用途。如果这不是您想要的，请详细说明。

请注意，由于未使用 right，您可以将其设为可变参数，并且 "merge" 随心所欲:)

fun merge(left: MyDataClass, vararg right: MyDataClass) = left.copy()

val totallyNewData = merge(data1, data2, data3, data4, ...)

编辑

类在 Kotlin 中不跟踪它们的增量。想想你在经历这个过程时会得到什么。第一次更改后你有

current = Bob("Ben", null, null, "local_id")
next = Bob(null, 1, null, "local_id")

它怎么知道您希望 next 将更改应用到 age 而不是 name？如果您只是根据可空性进行更新， @mfulton 有一个很好的答案。否则您需要自己提供信息。

Answer 3

如果你想在左边的值是 null 时从右边复制值，那么你可以执行以下操作：

inline infix fun <reified T : Any> T.merge(other: T): T {
    val propertiesByName = T::class.declaredMemberProperties.associateBy { it.name }
    val primaryConstructor = T::class.primaryConstructor
        ?: throw IllegalArgumentException("merge type must have a primary constructor")
    val args = primaryConstructor.parameters.associateWith { parameter ->
        val property = propertiesByName[parameter.name]
            ?: throw IllegalStateException("no declared member property found with name '${parameter.name}'")
        (property.get(this) ?: property.get(other))
    }
    return primaryConstructor.callBy(args)
}

用法：

data class MyDataClass(val prop1: String?, val prop2: Int?)
val a = MyDataClass(null, 1)
val b = MyDataClass("b", 2)
val c = a merge b // MyDataClass(prop1=b, prop2=1)

Answer 4

当我们可以定义要合并的字段时，class-specific 合并数据的方法类将是：

data class SomeData(val dataA: Int?, val dataB: String?, val dataC: Boolean?) {
    fun combine(newData: SomeData): SomeData {        
        //Let values of new data replace corresponding values of this instance, otherwise fall back on the current values.
        return this.copy(dataA = newData.dataA ?: dataA,
                dataB = newData.dataB ?: dataB,
                dataC = newData.dataC ?: dataC)
    }
}

Answer 5

@mfulton26 的解决方案合并了仅属于主构造函数的属性。我已将其扩展为支持所有属性

inline infix fun <reified T : Any> T.merge(other: T): T {
    val nameToProperty = T::class.declaredMemberProperties.associateBy { it.name }
    val primaryConstructor = T::class.primaryConstructor!!
    val args = primaryConstructor.parameters.associate { parameter ->
        val property = nameToProperty[parameter.name]!!
        parameter to (property.get(other) ?: property.get(this))
    }
    val mergedObject = primaryConstructor.callBy(args)
    nameToProperty.values.forEach { it ->
        run {
            val property = it as KMutableProperty<*>
            val value = property.javaGetter!!.invoke(other) ?: property.javaGetter!!.invoke(this)
            property.javaSetter!!.invoke(mergedObject, value)
        }
    }
    return mergedObject
}

Combining/merging Kotlin 中的数据 类

Combining/merging data classes in Kotlin

kotlin

kotlin-reflect

编辑

示例

Combining/merging Kotlin 中的数据类