feat(in_app_messaging): add custom Flutter display for campaigns

Author: SelaseKay

docs/in-app-messaging/_customize-messages.md

@@ -31,3 +31,28 @@ The action's format depends on which message layout you choose. Modals get
 action buttons with customizable button text content, text color, and background
 color. Images and top banners, on the other hand, become interactive and invoke
 the specified action when tapped.
+
+## Render campaigns with Flutter widgets
+
+By default, In-App Messaging draws native templates (banner, modal, card,
+image-only). To render campaigns with your own Flutter UI instead, opt in to
+custom display, listen for the campaign payload, and report impression, click,
+and dismiss back to the SDK:
+
+```dart
+FirebaseInAppMessaging.instance.onMessageDisplay.listen((message) async {
+  await message.impress();
+  // Draw your own widgets using message.title, body, imageUrl, actions, data.
+  // The plugin does not open action URLs — your app should.
+  await message.click(message.primaryAction ?? message.action!);
+  await message.dismiss();
+});
+
+await FirebaseInAppMessaging.instance.setCustomDisplayEnabled(true);
+```
+
+Until `setCustomDisplayEnabled(true)` is called, native templates keep working.
+Listening to `onMessageDisplay` alone does not replace native UI.
+
+If you also listen to `onMessageClicked` / `onMessageImpression`, those
+lifecycle streams still fire when your Flutter UI reports click and impress.

docs/in-app-messaging/_modify-message-behavior.md

@@ -48,9 +48,8 @@ In your campaigns, you can specify custom data in a series of key/value pairs.
 When users interact with messages, this data is available for you to, for example,
 display a promo code.
 
-To do so, you will have to use the platform-native APIs.
-See the documentation for [iOS](/docs/in-app-messaging/modify-message-behavior?platform=ios#use_campaign_custom_metadata)
-and [Android](/docs/in-app-messaging/modify-message-behavior?platform=android#use_campaign_custom_metadata).
+When you opt in to [custom Flutter display](/docs/in-app-messaging/customize-messages?platform=flutter),
+those key/value pairs are available on `InAppMessage.data`.
 
 
 ## Temporarily disable in-app messages

packages/firebase_in_app_messaging/firebase_in_app_messaging/README.md

@@ -28,6 +28,24 @@ FirebaseInAppMessaging.instance.onMessageClicked.listen((event) {
 `onMessageImpression`, `onMessageDismissed` and `onMessageDisplayError` report
 the rest of the message lifecycle.
 
+### Custom Flutter display
+
+To render campaigns with your own widgets instead of the native templates:
+
+```dart
+FirebaseInAppMessaging.instance.onMessageDisplay.listen((message) async {
+  await message.impress();
+  // Draw your own UI, then:
+  // await message.click(message.action!);
+  // await message.dismiss();
+});
+
+await FirebaseInAppMessaging.instance.setCustomDisplayEnabled(true);
+```
+
+The plugin does not open action URLs. You must report impress / click / dismiss
+so campaign analytics keep working.
+
 ## Issues and feedback
 
 Please file FlutterFire specific issues, bugs, or feature requests in our [issue tracker](https://github.com/firebase/flutterfire/issues/new).