MissingPluginException when firebase_messaging creates a background FlutterEngine

[Goddchen]

I did a lot of debugging to find this, with the help of Copilot. So I also let Copilot write up this summary of my hour long debugging journey and the findings. In case anything is unclear, let me know!

Description

Using dotlottie_flutter: ^0.1.7 with firebase_messaging on Android can cause DotLottie method calls to fail with MissingPluginException.

The problem occurs when Firebase Messaging creates its background FlutterEngine. That engine registers dotlottie_flutter too, which overwrites the BinaryMessenger stored by the plugin for the main UI engine.

The DotLottie platform view can still be created successfully, but method-channel calls to it are then routed through the background engine's messenger and fail.

Environment

• Flutter: 3.44.6
• Dart: 3.12.2
• Android
• dotlottie_flutter: 0.1.7
• firebase_messaging: 16.3.0
• Main activity: FlutterFragmentActivity

Steps to reproduce

1. Create a Flutter Android app using dotlottie_flutter: ^0.1.7.
2. Add and configure firebase_messaging, including a background message handler.
3. Display a DotLottieView in the main Flutter engine.
4. Call a DotLottie method, for example:

await controller.stateMachineSetBooleanInput(
  'isLightMode',
  true,
);

Expected behavior

DotLottie method calls from the UI Flutter engine continue working after Firebase Messaging creates and initializes its background engine.

Actual behavior

A call such as stateMachineSetBooleanInput fails:

MissingPluginException(
  No implementation found for method stateMachineSetBooleanInput
  on channel dotlottie_view_0
)

The native libraries load and the platform view is created, but subsequent calls use the wrong Flutter engine messenger.

Root cause

The Android plugin stores its BinaryMessenger in a shared static field:

companion object {
    lateinit var binaryMessenger: BinaryMessenger
}

When the plugin attaches to another engine, it overwrites that field:

override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) {
    binaryMessenger = binding.binaryMessenger
    binding.platformViewRegistry.registerViewFactory(
        "dotlottie_view",
        DotLottieViewFactory(),
    )
}

firebase_messaging creates a separate background FlutterEngine and registers plugins there. This replaces the main UI engine's messenger in the static field.

When DotLottieViewFactory subsequently creates a view for the UI engine, it reads the shared messenger and binds the platform-view channels to the background engine instead.

Suggested fix

Avoid storing the engine-specific BinaryMessenger in a static/companion-object field.

Pass the messenger belonging to the attached engine into the view factory and platform view:

class DotLottieViewFactory(
    private val binaryMessenger: BinaryMessenger,
) : PlatformViewFactory(StandardMessageCodec.INSTANCE)

Register it per engine:

binding.platformViewRegistry.registerViewFactory(
    "dotlottie_view",
    DotLottieViewFactory(binding.binaryMessenger),
)

Then pass that messenger to the corresponding DotLottiePlatformView when it is created.

This keeps each platform view bound to the correct Flutter engine and supports multi-engine apps such as those using firebase_messaging.