[in_app_purchase] Switch to Kotlin Pigeon (#11608)

Replaces the Java Pigeon generator with the Kotlin Pigeon generator, and adjusts the project accordingly:
- Adds Kotlin build setings to Gradle.
- Updates API signatures and number handling for Kotlin/Java generator differences.
- Adds generic Java/Kotlin compat shim to create Result objects from Java, since those haven't been added to the Pigeon generator yet.
- Updates tests to use constructors instead of builders, since the Kotlin generator doesn't create builders.
- Updates tests to use a fake result value, instead of mocking the Java Pigeon response object.

Most of this conversion was done by Gemini; I gave it my previous "Switch to Kotlin Pigeon" PRs for context, and let it do most of the work. Once it had a passing conversion I did some manual cleanup and warning fixing.

This also converts Translator.java, which does SDK<->Pigeon type conversion, to Kotlin, per discussion in the linked issue:
- I used Android Studio's auto-converter.
- I did some manual cleanup (e.g., fixing nullability where it assumed nullable because the Java wasn't annotated, simplifying some switch return statements).
- I added parameter names to all the constructors, so it's easy to see that the params are being passed correctly in review both now and in future changes.
- I had Gemini do a pass over it to make the Kotlin simpler or more idiomatic. The big improvement was it replacing a bunch of manual list construction with `map`s.
- Removed `currencyCodeFromSymbol`, which has been dead code for years.

These steps are separate commits in case it's useful to review them separately

Fixes flutter/flutter#185545

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: stuartmorgan-g

packages/in_app_purchase/in_app_purchase_android/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.4.0+11
+
+* Updates internal implementation to use Kotlin Pigeon.
+
 ## 0.4.0+10
 
 * Fixes dartdoc comments that accidentally used HTML.

packages/in_app_purchase/in_app_purchase_android/android/build.gradle.kts

@@ -1,14 +1,18 @@
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+
 group = "io.flutter.plugins.inapppurchase"
 version = "1.0-SNAPSHOT"
 
 buildscript {
+    val kotlinVersion = "2.3.20"
     repositories {
         google()
         mavenCentral()
     }
 
     dependencies {
         classpath("com.android.tools.build:gradle:8.13.1")
+        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion")
     }
 }
 
@@ -21,6 +25,13 @@ allprojects {
 
 plugins {
     id("com.android.library")
+    id("kotlin-android")
+}
+
+kotlin {
+    compilerOptions {
+        jvmTarget = JvmTarget.fromTarget(JavaVersion.VERSION_17.toString())
+    }
 }
 
 android {

packages/in_app_purchase/in_app_purchase_android/android/src/main/java/io/flutter/plugins/inapppurchase/BillingClientFactory.java

@@ -7,7 +7,6 @@
 import android.content.Context;
 import androidx.annotation.NonNull;
 import com.android.billingclient.api.BillingClient;
-import io.flutter.plugins.inapppurchase.Messages.PlatformBillingChoiceMode;
 
 /** Responsible for creating a {@link BillingClient} object. */
 interface BillingClientFactory {
@@ -25,7 +24,7 @@ interface BillingClientFactory {
    */
   BillingClient createBillingClient(
       @NonNull Context context,
-      @NonNull Messages.InAppPurchaseCallbackApi callbackApi,
+      @NonNull InAppPurchaseCallbackApi callbackApi,
       PlatformBillingChoiceMode billingChoiceMode,
-      Messages.PlatformPendingPurchasesParams pendingPurchasesParams);
+      PlatformPendingPurchasesParams pendingPurchasesParams);
 }

packages/in_app_purchase/in_app_purchase_android/android/src/main/java/io/flutter/plugins/inapppurchase/BillingClientFactoryImpl.java

@@ -4,26 +4,26 @@
 
 package io.flutter.plugins.inapppurchase;
 
-import static io.flutter.plugins.inapppurchase.Translator.fromUserChoiceDetails;
-import static io.flutter.plugins.inapppurchase.Translator.toPendingPurchasesParams;
+import static io.flutter.plugins.inapppurchase.TranslatorKt.fromUserChoiceDetails;
+import static io.flutter.plugins.inapppurchase.TranslatorKt.toPendingPurchasesParams;
 
 import android.content.Context;
 import androidx.annotation.NonNull;
 import androidx.annotation.VisibleForTesting;
 import com.android.billingclient.api.BillingClient;
 import com.android.billingclient.api.UserChoiceBillingListener;
 import io.flutter.Log;
-import io.flutter.plugins.inapppurchase.Messages.PlatformBillingChoiceMode;
+import kotlin.Unit;
 
 /** The implementation for {@link BillingClientFactory} for the plugin. */
 final class BillingClientFactoryImpl implements BillingClientFactory {
 
   @Override
   public BillingClient createBillingClient(
       @NonNull Context context,
-      @NonNull Messages.InAppPurchaseCallbackApi callbackApi,
+      @NonNull InAppPurchaseCallbackApi callbackApi,
       PlatformBillingChoiceMode billingChoiceMode,
-      Messages.PlatformPendingPurchasesParams pendingPurchasesParams) {
+      PlatformPendingPurchasesParams pendingPurchasesParams) {
     BillingClient.Builder builder =
         BillingClient.newBuilder(context)
             .enablePendingPurchases(toPendingPurchasesParams(pendingPurchasesParams));
@@ -49,19 +49,19 @@ public BillingClient createBillingClient(
 
   @VisibleForTesting
   /* package */ UserChoiceBillingListener createUserChoiceBillingListener(
-      @NonNull Messages.InAppPurchaseCallbackApi callbackApi) {
+      @NonNull InAppPurchaseCallbackApi callbackApi) {
     return userChoiceDetails ->
         callbackApi.userSelectedalternativeBilling(
             fromUserChoiceDetails(userChoiceDetails),
-            new Messages.VoidResult() {
-              @Override
-              public void success() {}
-
-              @Override
-              public void error(@NonNull Throwable error) {
-                io.flutter.Log.e(
-                    "IN_APP_PURCHASE", "userSelectedalternativeBilling handler error: " + error);
-              }
-            });
+            ResultCompat.asCompatCallback(
+                result -> {
+                  Throwable error = result.exceptionOrNull();
+                  if (error != null) {
+                    io.flutter.Log.e(
+                        "IN_APP_PURCHASE",
+                        "userSelectedalternativeBilling handler error: " + error);
+                  }
+                  return Unit.INSTANCE;
+                }));
   }
 }

packages/in_app_purchase/in_app_purchase_android/android/src/main/java/io/flutter/plugins/inapppurchase/InAppPurchasePlugin.java

@@ -58,15 +58,15 @@ public void onDetachedFromActivityForConfigChanges() {
   }
 
   private void setUpMethodChannel(BinaryMessenger messenger, Context context) {
-    Messages.InAppPurchaseCallbackApi handler = new Messages.InAppPurchaseCallbackApi(messenger);
+    InAppPurchaseCallbackApi handler = new InAppPurchaseCallbackApi(messenger, "");
     methodCallHandler =
         new MethodCallHandlerImpl(
             /* activity= */ null, context, handler, new BillingClientFactoryImpl());
-    Messages.InAppPurchaseApi.setUp(messenger, methodCallHandler);
+    InAppPurchaseApi.Companion.setUp(messenger, methodCallHandler);
   }
 
   private void teardownMethodChannel(BinaryMessenger messenger) {
-    Messages.InAppPurchaseApi.setUp(messenger, null);
+    InAppPurchaseApi.Companion.setUp(messenger, null);
     methodCallHandler = null;
   }