[pigeon] Kotlin implementation for ProxyApis

[bparrishMines]

Kotlin portion of flutter/flutter#134777

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[bparrishMines]

@camsim99 FYI

@stuartmorgan @tarrinneal This is one of the places where the design changes from the implementation in the doc. I wanted to improve the scenario of when a method returns a ProxyApi or a list/map of a ProxyApi:

class Basket {
  Apple giveMeAnApple();
  List<Apple> giveMeApples();
}

Typically we solve this by manually handling adding a new instance when we implement the api:

class PigeonApiBasketImpl {
  Apple giveMeAnApple(Basket instance) {
    final Apple apple = instance.giveMeAnApple();
    if (!instanceManager.containsInstance(apple)) {
      instanceManager.addHostCreatedInstance(apple);
    }

    return apple;
  }

  List<Apple> giveMeApples(Basket instance) {
    final List<Apple> apples = instance.giveMeApples();
    for (Apple apple in apples) {
      if (!instanceManager.containsInstance(apple)) {
        instanceManager.addHostCreatedInstance(apple);
      }
    }

    return apples;
  }
}

This change makes the codec the central class that contains all the proxy api implementations, so the writeValue can handle creating new dart instances automatically by calling pigeon_newInstance. And pigeon_newInstance would handle the logic shown in the methods above.

This also:

• Creates a registrar like class that ensures a user implements each proxy api.
• Provides access to a central class with every api implementation which prevents the need for multiple instances of the same api implementation.

[tarrinneal]

This may end up overlapping with some of the codec work I'm about to do as well. I think it's generally a good idea to create a central codec though. We may end up with multiple codecs implementing each other.

[tarrinneal]

My main concern at this point is that the number of custom inputs is limited to around 120, so if there are a lot of proxy apis, enums, and or data classes, we may not be able to accommodate them all.

[bparrishMines]

As of right now, the generation of the ProxyApi codec is separate from Host/Flutter APIs codecs. Mainly because:

1. ProxyApis don't support data classes
2. ProxyApis support other ProxyApis in the codec and Host/Flutter APIs don't.

So they currently don't really share anything. If you create a central custom codec for all the APIs, then ProxyApis should only need one dedicated spot that uses the InstanceManager: https://github.com/flutter/packages/blob/main/packages/pigeon/platform_tests/shared_test_plugin_code/lib/src/generated/proxy_api_tests.gen.dart#L374.

Also, as a side note, since all data classes have an encode/decode could you create a base data class for all the data classes and only use one spot. I realized this wouldn't work since the reader wouldn't know what class to call this on.

[tarrinneal]

Are you planning to add the full implementation and test suite to this pr?

[bparrishMines]

Are you planning to add the full implementation and test suite to this pr?

I wasn't planning on doing that for this PR, but I can if you prefer the full implementation. This PR just adds the Kotlin ProxyApi utility classes, including the InstanceManager and the tests for it. And also the bare minimum ProxyApi class declaration.

[tarrinneal]

@bparrishMines I think there needs to be some plumbing added to have the kotlin files generate using the tool/generate script

[bparrishMines]

@tarrinneal What do you mean? I have been using dart run tool/generate.dart to generate the Kotlin code successfully. Is this the script you are referring to?

[tarrinneal]

@tarrinneal What do you mean? I have been using dart run tool/generate.dart to generate the Kotlin code successfully. Is this the script you are referring to?

yep, If it's working then don't worry about it :)

[tarrinneal]

A few more things here, should be easy.

[tarrinneal]

Why was this change necessary?

[tarrinneal]

I think it's at a place where we can land it and start making incremental changes as needed. Good job with it all, and thanks for your patience!