[Flutter 3.22 & 3.24] AR crashes on Android < 13 - NoClassDefFoundError: Landroid/window/OnBackInvokedCallback

[timbotimbo]

[UPDATE]

This is now in the Unity issue tracker.
Anyone affect by this should probably upvote this in the issuetracker.

Description

Since Flutter 3.22, Unity will crash on certain Android versions when ARFoundation is activated.
This only happens on Android <13.
This might also affect other unity plugins using native code, I just noticed it using ARFoundation.

Unity works fine for 3D scenes but crashes when I switch to a scene that includes ARFoundation and its ARSession.

Error in the console:
(unity 2022.3)

E/Unity   ( 7029): AndroidJavaException: java.lang.NoClassDefFoundError: Failed resolution of: Landroid/window/OnBackInvokedCallback;
E/Unity   ( 7029): java.lang.NoClassDefFoundError: Failed resolution of: Landroid/window/OnBackInvokedCallback;
E/Unity   ( 7029): 	at java.lang.reflect.Executable.getMethodReturnTypeInternal(Native Method)
E/Unity   ( 7029): 	at java.lang.reflect.Method.getReturnType(Method.java:148)
E/Unity   ( 7029): 	at java.lang.Class.getDeclaredMethods(Class.java:1880)
E/Unity   ( 7029): 	at com.unity3d.player.ReflectionHelper.getMethodID(Unknown Source:26)
E/Unity   ( 7029): 	at com.unity3d.player.UnityPlayer.nativeRender(Native Method)
E/Unity   ( 7029): 	at com.unity3d.player.UnityPlayer.-$$Nest$mnativeRender(Unknown Source:0)
E/Unity   ( 7029): 	at com.unity3d.player.UnityPlayer$F$a.handleMessage(Unknown Source:122)
E/Unity   ( 7029): 	at android.os.Handler.dispatchMessage(Handler.java:102)
E/Unity   ( 7029): 	at android.os.Looper.loop(Looper.java:214)
E/Unity   ( 7029): 	at com.unity3d.player.UnityPlayer$F.run(Unknown Source:24)
E/Unity   ( 7029): Caused by: java.lang.ClassNotFoundException: Didn't find class "android.window.OnBackInvokedCallback" on path: DexPathList[[zip file "

(unity 2023.2)

E/Unity   (17280): AndroidJavaException: java.lang.NoClassDefFoundError: Failed resolution of: Landroid/window/OnBackInvokedCallback;
E/Unity   (17280): java.lang.NoClassDefFoundError: Failed resolution of: Landroid/window/OnBackInvokedCallback;
E/Unity   (17280):   at java.lang.reflect.Executable.getMethodReturnTypeInternal(Native Method)
E/Unity   (17280):   at java.lang.reflect.Method.getReturnType(Method.java:148)
E/Unity   (17280):   at java.lang.Class.getDeclaredMethods(Class.java:1880)
E/Unity   (17280):   at com.unity3d.player.ReflectionHelper.getMethodID(Unknown Source:26)
E/Unity   (17280):   at com.unity3d.player.UnityPlayerForActivityOrService.nativeRender(Native Method)
E/Unity   (17280):   at com.unity3d.player.S.handleMessage(Unknown Source:140)
E/Unity   (17280):   at android.os.Handler.dispatchMessage(Handler.java:102)
E/Unity   (17280):   at android.os.Looper.loop(Looper.java:214)
E/Unity   (17280):   at com.unity3d.player.V.run(Unknown Source:24)
E/Unity   (17280): Caused by: java.lang.ClassNotFoundException: Didn't find class "android.window.OnBackInvokedCallback" on path: DexPathList[

More info

Looking up OnBackInvokedCallback, it is added in API 33 (android 13) and is related to the predictive back gesture in Android 14.

I narrowed it down to this commit on Flutter master, which includes the engine PR Platform channel for predictive back in route transitions on android.
Any Flutter master commit before this one won't crash.

I can also reproduce this with the flutter_embed_unity plugin. So it isn't specific to code in this plugin.

Flutter version

Crashed on:

• Flutter 3.22,0, 3.22.2
• master (3.23.0-13.0.pre.265 revision a9f554ac39)

Did not crash on:

• Flutter 3.13.9
• Flutter 3.16.9
• Flutter 3.19.6.

Unity version

Only tested with:
2022.3.21f1, 2022.3.33f1, 2023.2.20f1.
ArFoundation 5.1.2, 5.1.4 and 5.1.5

Android devices

Devices without crash:

• Samsung S20FE, Android 13
• Xiaomi redmi note 10 pro, Android 13
• Wilefox Swift, Android 7, (doesn't support AR)

Devices with crash:

• Samsung S10, Android 12
• Nokia 8.1, Android 11
• Lenovo tab M10, Android 10
• Samsung S8, Android 9

Some things I've tried

• Adding proguard rules like -keep class android.window.** { *; } -keep class android.window.OnBackInvokedCallback { *; }.
• Adding android:enableOnBackInvokedCallback="false" to all androidmanifest activities and applications.
• Updating to the latest Unity 2022.3.33
• Updating Android Studio to Jellyfish 2023.3.1 Patch2 and updating all SDK tools
• Adding implementation "androidx.activity:activity:1.6.0 in gradle
  (OnbackInvokedCallback is mentioned in the changes.)

[KailaashJeevan]

Yup, got the same for Vuforia integration. The app successfully runs on android version 14, but not on anything else. It results on black screen.

[timbotimbo]

After a report in the Flutter repo this can be labeled as a Unity issue.

The cause

Once AR boots up, Unity uses the getMethodID function in the ReflectionHelper class of unity-classes.jar .
This function gets a list of all function in the current Activity using Java reflection getDeclaredMethods(), and then tries to match function signatures.
It then repeats this for all super classes of the current Activity.

The getDeclaredMethods call instantly crashes on Android <13 because it can't handle the OnBackInvokedCallback return type that is defined in API 33.

What now

Flutter won't stop using this class for a few people using the niche use-case of embedding Unity with AR.
So AR is broken from now on unless this can be fixed on the Unity side.

With some luck a try-catch around getDeclaredMethods might be enough.

However this means either:

1. Getting Unity to modify their reflection usage.
2. Decompiling unity-classes.jar to add a try-catch.

I've tried to decompile unity-classes.jar, but none of the decompilers that I could find returned valid code that could compile again without errors.

Unity reproduction

To replicate this issue witthout Flutter:

1. export a Unity project using AR as an Android project.
2. edit the UnityPlayerActivity.java file.
   Either add a parent Activity that includes this function, or add this function to the Activity itself.

import android.annotation.TargetApi;
import android.window.OnBackInvokedCallback;

   @TargetApi(33)
    protected OnBackInvokedCallback getOnBackInvokedCallback() {
        return null;
    }

3. Now run this using Android Studio on on a device with Android API <33.

TLDR (too long didn't read)

AR is broken with Flutter 3.22+. Don't expect this to be fixed for months, if at all.

[timbotimbo]

This is now in the Unity issue tracker.

They seem to have omitted most details from my bug report, but I guess it is a first step.

Anyone affect by this should probably upvote this in the issuetracker.

[t5ujiri]

note that if you want to use AR with flutter 3.19.6 on android, make sure useAndroidViewSurface: true or the player view will freeze

[timbotimbo]

Unity got back to me on the bug report, calling it "By Design".

Given that Flutter devs point me at Unity, and Unity considers it by design, AR is likely a lost cause on Android < 13.

Since 3.16 and 3.19 contain various serious bugs (#912, #929) the last reliable version is 3.13.9, which is getting old.