refactor: Introducing a CallbackPlugin to wrap the old style Callbacks

The goal is to unify the processing of Plugins and Callbacks.  We should consider depercating and removing the old Callbacks.

There are a bunch of cyclical dependencies caused by requests back to the agent to get specific Callbacks.  The next step will be to augmet the InvocationContext's PluginManager with the appropriate agent specific callbacks

PiperOrigin-RevId: 853407402

Author: google-genai-bot

core/src/main/java/com/google/adk/agents/BaseAgent.java

@@ -17,6 +17,7 @@
 package com.google.adk.agents;
 
 import static com.google.common.collect.ImmutableList.toImmutableList;
+import static java.util.Arrays.stream;
 
 import com.google.adk.Telemetry;
 import com.google.adk.agents.Callbacks.AfterAgentCallback;
@@ -36,7 +37,6 @@
 import java.util.List;
 import java.util.Optional;
 import java.util.function.Function;
-import java.util.stream.Stream;
 import org.jspecify.annotations.Nullable;
 
 /** Base class for all agents. */
@@ -59,8 +59,7 @@ public abstract class BaseAgent {
 
   private final List<? extends BaseAgent> subAgents;
 
-  private final Optional<List<? extends BeforeAgentCallback>> beforeAgentCallback;
-  private final Optional<List<? extends AfterAgentCallback>> afterAgentCallback;
+  protected final CallbackPlugin callbackPlugin;
 
   /**
    * Creates a new BaseAgent.
@@ -79,12 +78,35 @@ public BaseAgent(
       List<? extends BaseAgent> subAgents,
       List<? extends BeforeAgentCallback> beforeAgentCallback,
       List<? extends AfterAgentCallback> afterAgentCallback) {
+    this(
+        name,
+        description,
+        subAgents,
+        CallbackPlugin.builder()
+            .addBeforeAgentCallbacks(beforeAgentCallback)
+            .addAfterAgentCallbacks(afterAgentCallback)
+            .build());
+  }
+
+  /**
+   * Creates a new BaseAgent.
+   *
+   * @param name Unique agent name. Cannot be "user" (reserved).
+   * @param description Agent purpose.
+   * @param subAgents Agents managed by this agent.
+   * @param callbackPlugin The callback plugin for this agent.
+   */
+  protected BaseAgent(
+      String name,
+      String description,
+      List<? extends BaseAgent> subAgents,
+      CallbackPlugin callbackPlugin) {
     this.name = name;
     this.description = description;
     this.parentAgent = null;
     this.subAgents = subAgents != null ? subAgents : ImmutableList.of();
-    this.beforeAgentCallback = Optional.ofNullable(beforeAgentCallback);
-    this.afterAgentCallback = Optional.ofNullable(afterAgentCallback);
+    this.callbackPlugin =
+        callbackPlugin == null ? CallbackPlugin.builder().build() : callbackPlugin;
 
     // Establish parent relationships for all sub-agents if needed.
     for (BaseAgent subAgent : this.subAgents) {
@@ -172,11 +194,15 @@ public List<? extends BaseAgent> subAgents() {
   }
 
   public Optional<List<? extends BeforeAgentCallback>> beforeAgentCallback() {
-    return beforeAgentCallback;
+    return Optional.of(callbackPlugin.getBeforeAgentCallback());
   }
 
   public Optional<List<? extends AfterAgentCallback>> afterAgentCallback() {
-    return afterAgentCallback;
+    return Optional.of(callbackPlugin.getAfterAgentCallback());
+  }
+
+  public Plugin getPlugin() {
+    return callbackPlugin;
   }
 
   /**
@@ -221,8 +247,7 @@ public Flowable<Event> runAsync(InvocationContext parentContext) {
               () ->
                   callCallback(
                           beforeCallbacksToFunctions(
-                              invocationContext.pluginManager(),
-                              beforeAgentCallback.orElse(ImmutableList.of())),
+                              invocationContext.pluginManager(), callbackPlugin),
                           invocationContext)
                       .flatMapPublisher(
                           beforeEventOpt -> {
@@ -239,7 +264,7 @@ public Flowable<Event> runAsync(InvocationContext parentContext) {
                                         callCallback(
                                                 afterCallbacksToFunctions(
                                                     invocationContext.pluginManager(),
-                                                    afterAgentCallback.orElse(ImmutableList.of())),
+                                                    callbackPlugin),
                                                 invocationContext)
                                             .flatMapPublisher(Flowable::fromOptional));
 
@@ -251,30 +276,27 @@ public Flowable<Event> runAsync(InvocationContext parentContext) {
   /**
    * Converts before-agent callbacks to functions.
    *
-   * @param callbacks Before-agent callbacks.
    * @return callback functions.
    */
   private ImmutableList<Function<CallbackContext, Maybe<Content>>> beforeCallbacksToFunctions(
-      Plugin pluginManager, List<? extends BeforeAgentCallback> callbacks) {
-    return Stream.concat(
-            Stream.of(ctx -> pluginManager.beforeAgentCallback(this, ctx)),
-            callbacks.stream()
-                .map(callback -> (Function<CallbackContext, Maybe<Content>>) callback::call))
+      Plugin... plugins) {
+    return stream(plugins)
+        .map(
+            p ->
+                (Function<CallbackContext, Maybe<Content>>) ctx -> p.beforeAgentCallback(this, ctx))
         .collect(toImmutableList());
   }
 
   /**
    * Converts after-agent callbacks to functions.
    *
-   * @param callbacks After-agent callbacks.
    * @return callback functions.
    */
   private ImmutableList<Function<CallbackContext, Maybe<Content>>> afterCallbacksToFunctions(
-      Plugin pluginManager, List<? extends AfterAgentCallback> callbacks) {
-    return Stream.concat(
-            Stream.of(ctx -> pluginManager.afterAgentCallback(this, ctx)),
-            callbacks.stream()
-                .map(callback -> (Function<CallbackContext, Maybe<Content>>) callback::call))
+      Plugin... plugins) {
+    return stream(plugins)
+        .map(
+            p -> (Function<CallbackContext, Maybe<Content>>) ctx -> p.afterAgentCallback(this, ctx))
         .collect(toImmutableList());
   }
 
@@ -399,8 +421,11 @@ public abstract static class Builder<B extends Builder<B>> {
     protected String name;
     protected String description;
     protected ImmutableList<BaseAgent> subAgents;
-    protected ImmutableList<BeforeAgentCallback> beforeAgentCallback;
-    protected ImmutableList<AfterAgentCallback> afterAgentCallback;
+    protected final CallbackPlugin.Builder callbackPluginBuilder = CallbackPlugin.builder();
+
+    protected CallbackPlugin.Builder callbackPluginBuilder() {
+      return callbackPluginBuilder;
+    }
 
     /** This is a safe cast to the concrete builder type. */
     @SuppressWarnings("unchecked")
@@ -434,25 +459,25 @@ public B subAgents(BaseAgent... subAgents) {
 
     @CanIgnoreReturnValue
     public B beforeAgentCallback(BeforeAgentCallback beforeAgentCallback) {
-      this.beforeAgentCallback = ImmutableList.of(beforeAgentCallback);
+      callbackPluginBuilder.addBeforeAgentCallback(beforeAgentCallback);
       return self();
     }
 
     @CanIgnoreReturnValue
     public B beforeAgentCallback(List<Callbacks.BeforeAgentCallbackBase> beforeAgentCallback) {
-      this.beforeAgentCallback = CallbackUtil.getBeforeAgentCallbacks(beforeAgentCallback);
+      callbackPluginBuilder.addBeforeAgentCallbacks(beforeAgentCallback);
       return self();
     }
 
     @CanIgnoreReturnValue
     public B afterAgentCallback(AfterAgentCallback afterAgentCallback) {
-      this.afterAgentCallback = ImmutableList.of(afterAgentCallback);
+      callbackPluginBuilder.addAfterAgentCallback(afterAgentCallback);
       return self();
     }
 
     @CanIgnoreReturnValue
     public B afterAgentCallback(List<Callbacks.AfterAgentCallbackBase> afterAgentCallback) {
-      this.afterAgentCallback = CallbackUtil.getAfterAgentCallbacks(afterAgentCallback);
+      callbackPluginBuilder.addAfterAgentCallbacks(afterAgentCallback);
       return self();
     }