added new optional param classInstance to Serializer and Deserializer helper classes

[kdrakon]

These are defaulted to perform the normal behaviour: reflective construction of the specified classes. The param can then be set so that one can specify (provide) logic that defines how an instance of the class should be constructed. The primary use case for this was special serializers that take constructor parameters.

Addresses Issue #4

[kdrakon]

As an example of the usecase using Confluents Avro deserializer:

import io.confluent.kafka.schemaregistry.client.CachedSchemaRegistryClient
import io.confluent.kafka.serializers.KafkaAvroDeserializer

val schemaRegistryClient = new CachedSchemaRegistryClient("http://localhost:8081", 42)

implicit val avroDeserializer =
  Deserializer[AnyRef](
    "io.confluent.kafka.serializers.KafkaAvroDeserializer",
    classOf[KafkaAvroDeserializer],
    (d) => new KafkaAvroDeserializer(schemaRegistryClient)
  )

[alexandru]

Looks good, I just have this little obsession against verbose naming and if you could change ClassInstanceProvider to Constructor, that would be great.

Thanks for the PR.

[alexandru]

Default parameter looks ugly (personal pet peeve). I'm fairly certain that Scala accepts this:

classInstance: Serializer.ClassInstanceProvider[A] = _.classType.newInstance()

While we are at it classInstance: Serializer.ClassInstanceProvider is a little redundant and verbose. So lets simplify to:

constructor: Serializer.Constructor[A] = _.classType.newInstance()

// ... 

object Serializer {
  type Constructor[A] = (Serializer[A]) => KafkaSerializer[A]
  // ...

Naming, right? :-) I don't care that much, so it's up to you.

[alexandru]

@kdrakon can I bother you with also adding a comment for that new type alias and for the params of the Serializer and Deserializer classes? The config is has a good Scaladoc, but not these.

[kdrakon]

@alexandru unfortunately, I couldn't simplify this to _.classType.newInstance(). There is a

missing parameter type for expanded function

warning, but because -Xfatal-warnings is enabled, this makes it impossible to compile.

[kdrakon]

Other than that, the rest of the suggestions have been pushed to the PR.

[alexandru]

Thanks @kdrakon, I'm merging it, I hope you tested it :-)

[kdrakon]

Ran the tests locally a few times, but tested it more with my localised project 😅