Provide easy access to context within `dataProperty`

[jeggy]

Currently if I want to get access to the context within a dataLoader resolver I need to do the following:

dataProperty<Pair<Int, Int>, List<Value>>("valueList") {
  prepare { parent, ctx: Context -> ctx.userId to parent.id }
  loader { keys ->
    val dataMap: Map<Int, List<Value>> = loadValues(keys.first().first, keys.map { it.second })
    keys.map { ExecutionResult.Success(dataMap.getValue(it.second)) }
  }
}

It would be great if I didn't need to pass anything from the context from the prepare to the loader, so it could look more like something this:

dataProperty<Int, List<Value>>("valueList") {
  prepare { it.id }
  loader { parentIds: List<Int>, ctx: Context ->
    val dataMap: Map<Int, List<Value>> = loadValues(ctx.userId, parentIds)
    keys.map { ExecutionResult.Success(dataMap.getValue(it)) }
  }
}

[jeggy]

Maybe the whole idea of having a prepare lambda should be completely removed?

So it would be

type<User> {
  dataProperty<List<Value>>("valueList") {
    loader { parents: List<User>, ctx: Context ->
      val dataMap: Map<Int, List<Value>> = loadValues(ctx.userId, parentIds.map(User::id)
      keys.map { ExecutionResult.Success(dataMap.getValue(it)) }
    }
  }
}

Which means the dataProperty would be defined like this:

• dataProperty<R>(property: String)
• Then the loader like this:
  • <T, R> loader(block: (T) -> R),
  • <T, R, E> loader(block: (T, E) -> R),
  • <T, R, E, W> loader(block: (T, E, W) -> R)
  • and so on.