feat: add oneShot parameter for initialization configuration change listener

eppo/src/androidTest/java/cloud/eppo/android/EppoClientTest.java

@@ -396,6 +396,50 @@ public void testConfigurationChangeListener() throws ExecutionException, Interru
     assertEquals(3, received.size());
   }
 
+  @Test
+  public void testConfigurationChangeListenerOneShot()
+      throws ExecutionException, InterruptedException {
+    List<Configuration> received = new ArrayList<>();
+
+    // Set up a changing response from the "server"
+    EppoHttpClient mockHttpClient = mock(EppoHttpClient.class);
+
+    // Mock sync get to return empty
+    when(mockHttpClient.get(anyString())).thenReturn(EMPTY_CONFIG);
+
+    // Mock async get to return empty
+    CompletableFuture<byte[]> emptyResponse = CompletableFuture.completedFuture(EMPTY_CONFIG);
+    when(mockHttpClient.getAsync(anyString())).thenReturn(emptyResponse);
+
+    setBaseClientHttpClientOverrideField(mockHttpClient);
+
+    EppoClient.Builder clientBuilder =
+        new EppoClient.Builder(DUMMY_API_KEY, ApplicationProvider.getApplicationContext())
+            .forceReinitialize(true)
+            .onConfigurationChange(received::add, true) // oneShot = true
+            .isGracefulMode(false);
+
+    // Initialize and no exception should be thrown.
+    EppoClient eppoClient = clientBuilder.buildAndInitAsync().get();
+
+    verify(mockHttpClient, times(1)).getAsync(anyString());
+    assertEquals(1, received.size());
+
+    // Now, return the boolean flag config so that the config has changed.
+    when(mockHttpClient.get(anyString())).thenReturn(BOOL_FLAG_CONFIG);
+
+    // Trigger a reload of the client
+    eppoClient.loadConfiguration();
+
+    // Callback should NOT be invoked again because it was oneShot
+    assertEquals(1, received.size());
+
+    // Reload the client again to verify it's still not called
+    eppoClient.loadConfiguration();
+
+    assertEquals(1, received.size());
+  }
+
   @Test
   public void testPollingClient() throws ExecutionException, InterruptedException {
     EppoHttpClient mockHttpClient = mock(EppoHttpClient.class);

eppo/src/main/java/cloud/eppo/android/EppoClient.java

@@ -157,6 +157,31 @@ public static class Builder {
     private IAssignmentCache assignmentCache = new LRUAssignmentCache(100);
     @Nullable private Consumer<Configuration> configChangeCallback;
 
+    /**
+     * Wrapper for one-shot configuration change callbacks that auto-unsubscribes after first
+     * invocation.
+     */
+    private static class OneShotConfigCallback implements Consumer<Configuration> {
+      private final Consumer<Configuration> delegate;
+      private Runnable unsubscriber;
+
+      OneShotConfigCallback(Consumer<Configuration> delegate) {
+        this.delegate = delegate;
+      }
+
+      void setUnsubscriber(Runnable unsubscriber) {
+        this.unsubscriber = unsubscriber;
+      }
+
+      @Override
+      public void accept(Configuration config) {
+        if (unsubscriber != null) {
+          unsubscriber.run();
+        }
+        delegate.accept(config);
+      }
+    }
+
     public Builder(@NonNull String apiKey, @NonNull Application application) {
       this.application = application;
       this.apiKey = apiKey;
@@ -261,6 +286,20 @@ public Builder onConfigurationChange(Consumer<Configuration> configChangeCallbac
       return this;
     }
 
+    /**
+     * Registers a callback for when a new configuration is applied to the `EppoClient` instance.
+     *
+     * @param configChangeCallback The callback to invoke when configuration changes
+     * @param oneShot If true, the callback will be automatically unsubscribed after the first
+     *     invocation
+     */
+    public Builder onConfigurationChange(
+        Consumer<Configuration> configChangeCallback, boolean oneShot) {
+      this.configChangeCallback =
+          oneShot ? new OneShotConfigCallback(configChangeCallback) : configChangeCallback;
+      return this;
+    }
+
     public CompletableFuture<EppoClient> buildAndInitAsync() {
       if (application == null) {
         throw new MissingApplicationException();
@@ -311,7 +350,11 @@ public CompletableFuture<EppoClient> buildAndInitAsync() {
               assignmentCache);
 
       if (configChangeCallback != null) {
-        instance.onConfigurationChange(configChangeCallback);
+        Runnable unsubscriber = instance.onConfigurationChange(configChangeCallback);
+        // If this is a one-shot callback, set the unsubscriber so it can clean up after first call
+        if (configChangeCallback instanceof OneShotConfigCallback) {
+          ((OneShotConfigCallback) configChangeCallback).setUnsubscriber(unsubscriber);
+        }
       }
 
       final CompletableFuture<EppoClient> ret = new CompletableFuture<>();