Scala Pickling：为嵌套结构编写自定义pickler / unpickler

Question

我正在尝试编写一个自定义的 SPickler / Unpickler 对来解决当前 scala-pickling 的一些限制。 我试图腌制的数据类型是一个案例类，其中一些字段已经有自己的 SPickler 和 Unpickler 实例。 我想在我的自定义pickler 中使用这些实例，但我不知道如何。

这是我的意思的一个例子：

// Here's a class for which I want a custom SPickler / Unpickler.
// One of its fields can already be pickled, so I'd like to reuse that logic.
case class MyClass[A: SPickler: Unpickler: FastTypeTag](myString: String, a: A)

// Here's my custom pickler.
class MyClassPickler[A: SPickler: Unpickler: FastTypeTag](
  implicit val format: PickleFormat) extends SPickler[MyClass[A]] with Unpickler[MyClass[A]] {
  override def pickle(
    picklee: MyClass[A],
    builder: PBuilder) {
    builder.beginEntry(picklee)

    // Here we save `myString` in some custom way.
    builder.putField(
      "mySpecialPickler",
      b => b.hintTag(FastTypeTag.ScalaString).beginEntry(
        picklee.myString).endEntry())

    // Now we need to save `a`, which has an implicit SPickler.
    // But how do we use it?

    builder.endEntry()
  }

  override def unpickle(
    tag: => FastTypeTag[_],
    reader: PReader): MyClass[A] = {
    reader.beginEntry()

    // First we read the string.
    val myString = reader.readField("mySpecialPickler").unpickle[String]

    // Now we need to read `a`, which has an implicit Unpickler.
    // But how do we use it?
    val a: A = ???

    reader.endEntry()

    MyClass(myString, a)
  }
}

我非常感谢一个工作示例。 谢谢！

Answer 1

这是一个工作示例：

case class MyClass[A](myString: String, a: A)

注意MyClass 的类型参数不需要上下文边界。只有自定义pickler类需要相应的隐式：

class MyClassPickler[A](implicit val format: PickleFormat, aTypeTag: FastTypeTag[A],
                                     aPickler: SPickler[A], aUnpickler: Unpickler[A])
  extends SPickler[MyClass[A]] with Unpickler[MyClass[A]] {

  private val stringUnpickler = implicitly[Unpickler[String]]

  override def pickle(picklee: MyClass[A], builder: PBuilder) = {
    builder.beginEntry(picklee)

    builder.putField("myString",
      b => b.hintTag(FastTypeTag.ScalaString).beginEntry(picklee.myString).endEntry()
    )

    builder.putField("a",
      b => {      
        b.hintTag(aTypeTag)
        aPickler.pickle(picklee.a, b)
      }
    )

    builder.endEntry()
  }

  override def unpickle(tag: => FastTypeTag[_], reader: PReader): MyClass[A] = {
    reader.hintTag(FastTypeTag.ScalaString)
    val tag = reader.beginEntry()
    val myStringUnpickled = stringUnpickler.unpickle(tag, reader).asInstanceOf[String]
    reader.endEntry()

    reader.hintTag(aTypeTag)
    val aTag = reader.beginEntry()
    val aUnpickled = aUnpickler.unpickle(aTag, reader).asInstanceOf[A]
    reader.endEntry()

    MyClass(myStringUnpickled, aUnpickled)
  }

}

除了自定义pickler类，我们还需要一个隐式def，它返回一个专门用于具体类型参数的pickler实例：

implicit def myClassPickler[A: SPickler: Unpickler: FastTypeTag](implicit pf: PickleFormat) =
  new MyClassPickler