Kotlin coroutines exception handler on main thread

Question

I have a coroutine with an exception handler to call an API. I want to update UI if there is any exception but i got bellow error:

Caused by: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

So is there any way to update UI in exception handler?

Here is my code :

val exceptionHandler = CoroutineExceptionHandler { coroutineContext, exception ->

            binding.textview.text = exception.message

        }
        myJob = CoroutineScope(Dispatchers.IO).launch(exceptionHandler) {

            val result = webServices.getTest()

            withContext(Dispatchers.Main) {
                binding.textview.text = "$result"
            }
        }

as well as you did in the "normal" launch case you need to switch context to Dispatchers.Main — Nicola Gallazzi
– Nicola Gallazzi, Commented Dec 24, 2020 at 11:35

ChristianB · Accepted Answer · 2020-12-24 12:55:19Z

3

I think there is no way that ensures CoroutineExceptionHandler is called on Main thread. Because you can be on any thread at any time where the crash can happen.

Documentation

From CoroutineExceptionHandler

CoroutineExceptionHandler can be invoked from an arbitrary thread.

See also this Issue on GitHub

Workaround

You could launch a coroutine on main dispatcher, as Roman Elizarov mentioned, in your CoroutineExceptionHandler:

GlobalScope.launch(Dispatchers.Main) { ... /* put code here */ ...  }

or if you have a lifecycleScope (because GlobalScope is not recommended):

lifecycleScope.launch(Dispatchers.Main) { ... /* put code here */ ...  }

edited Dec 24, 2020 at 12:55

answered Dec 24, 2020 at 11:39

ChristianB

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4 Comments

darshan Over a year ago

Use lifecycleScope.launch {} by adding its relevant dependencies instead, GlobalScope.launch {} is not recommended.

ChristianB Over a year ago

@DarShan you are right, I updated my answer.

Michael Osofsky Over a year ago

What if I don't have access to any lifecycleScope? I'm developing an SDK and the use of Coroutines needs to be an implementation detail; the SDK callers shouldn't need to know about it. My CoroutineExceptionHandler calls back to the SDK user's code. I want to switch back to Dispatchers.Main before calling back. Should I use GlobalScope.launch for this exceptional case?

Michael Osofsky Over a year ago

Another case where I don't have access to lifecycleScope is in Android instrumented tests. How should we launch coroutines from Espresso tests?

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