Inconsistent Flutter behavior with native iOS exceptions

Question

In my Flutter app, I have main() set up to catch runtime errors, like this:

void main() async
{
    runZoned< Future >(() async {
        runApp ( MyApp() );
    }, onError: ( error, stackTrace ) async {
        print("Got an error");
        // ...
    });
}

This works fine for all Flutter-level errors, and also works fine for code that runs natively on Android, but the onError: handler is never called for iOS apps.

An example:

My Flutter app calls the following native method through a method channel:

void _onButtonClick() async {
    final channel = const MethodChannel('my-custom-channel');
    await channel.invokeMethod('testMethod');
}

Here's the Android implementation:

@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    GeneratedPluginRegistrant.registerWith(this);

    MethodChannel channel = new MethodChannel(getFlutterView(), "my-custom-channel");

    channel.setMethodCallHandler(new MethodCallHandler() {
        @Override
        public void onMethodCall(MethodCall call, Result result) {

            if ( call.method.equals("testMethod") ) {
                throw new IllegalStateException("This is a native Android exception.");
            } else {
                result.notImplemented();
            }

        }
    });

}

When this code executes, the IllegalStateException is "propagated" back up to the onError: handler in Flutter's main() function, and I get some info and a bit of a stack trace, as expected.

Here is the iOS implementation:

- (BOOL) application:(UIApplication*)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions
{
    FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;

    FlutterMethodChannel* channel = [FlutterMethodChannel
        methodChannelWithName:@"my-custom-channel"
        binaryMessenger:controller];

    [channel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {

        if ( [call.method isEqualToString:@"testMethod"] )
        {
            @throw([NSException
                exceptionWithName:@"iOS Exception"
                reason:@"Testing native iOS exceptions"
                userInfo:nil]);

            result(FlutterMethodNotImplemented);
        }
        else
        {
            result(FlutterMethodNotImplemented);
        }

    }];

    [GeneratedPluginRegistrant registerWithRegistry:self];
    return [super application:application didFinishLaunchingWithOptions:launchOptions];
}

In this case, the onError: handler in Flutter's main() function is never called. The console output is the following:

Lost connection to device.
*** First throw call stack:
(
    0   CoreFoundation                      0x00000001065cf1bb __exceptionPreprocess + 331
    1   libobjc.A.dylib                     0x0000000105b6d735 objc_exception_throw + 48
    2   Runner                              0x0000000103cd0772 __57-[AppDelegate application:didFinishLaunchingWithOptions:]_block_invoke + 226
    3   Flutter                             0x0000000103d7b9a2 __45-[FlutterMethodChannel setMethodCallHandler:]_block_invoke + 115
    4   Flutter                             0x0000000103d98616 _ZNK5shell21PlatformMessageRouter21HandlePlatformMessageEN3fml6RefPtrIN5blink15PlatformMessageEEE + 166
    5   Flutter                             0x0000000103d9bfbe _ZN5shell15PlatformViewIOS21HandlePlatformMessageEN3fml6RefPtrIN5blink15PlatformMessageEEE + 38
    6   Flutter                             0x0000000103dec8e9 _ZNSt3__110__function6__funcIZN5shell5Shell29OnEngineHandle<…>

That looks like a bit of a stack trace output from iOS land, but nothing ever "propagates" back up to the Flutter onError: handler, like it does with Android.

It's worth mentioning that this only happens for native NSExceptions, and also happens if I force a crash with something like assert(false) (both in the simulator and on physical devices), but it does not happen if I use [FlutterError errorWithCode: ... ] instead of native NSExceptions on the iOS side.

So, is there a way to have native iOS exceptions and crashes propagate back up to the onError: handler in Flutter?

Inconsistent Flutter behavior with native iOS exceptions

0 Answers