Kotlin Android - correct way to dispatch to main thread-CodePudding

I am using OkHttp library to download some data from the internet in my androidx.lifecycle.ViewModel I then want to update my LiveData. It seems that doing it from background thread throws exception like so:

2022-01-17 15:47:59.589 7354-7396/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: OkHttp Dispatcher
    Process: com.example.myapplication, PID: 7354
    java.lang.IllegalStateException: Cannot invoke setValue on a background thread
        at androidx.lifecycle.LiveData.assertMainThread(LiveData.java:487)
        at androidx.lifecycle.LiveData.setValue(LiveData.java:306)
        at androidx.lifecycle.MutableLiveData.setValue(MutableLiveData.java:50)
        at com.example.myapplication.MainActivityViewModel$getOneMoreCat$1.invoke(MainActivityViewModel.kt:86)
        at com.example.myapplication.MainActivityViewModel$getOneMoreCat$1.invoke(MainActivityViewModel.kt:39)
        at com.example.myapplication.singleton.CommunicationManager$sendRequest$1.onResponse(CommunicationManager.kt:24)
        at okhttp3.internal.connection.RealCall$AsyncCall.run(RealCall.kt:519)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
        at java.lang.Thread.run(Thread.java:923)

Now I found two different ways to dispatch to main thread from ViewModel (which has no reference to Context as per AAC guidelines), see here:

            GlobalScope.launch {
                withContext(Dispatchers.Main) {
                    // do whatever, e.g. update LiveData
                }
            }

            Handler(Looper.getMainLooper()).post(Runnable {
                   // do whatever, e.g. update LiveData
            })

Which is the correct way? That is, least impactful at runtime.

Update I did find that I can also do myLiveData.post() and it works from background thread.

Still, I'd like to know what is the correct way to dispatch work to main thread in modern Android under kotlin

CodePudding user response：

Inside viewmodel,

private val _downloading = MutableLiveData<Result<Boolean>>()
val downloading: LiveData<Result<Boolean>>
    get() = _downloading

fun downloadFile() {
    viewModelScope.launch {
        try {
            _downloading.value = Result.Loading
            val result = withContext(Dispatchers.IO) {
                // download something
            }
            _downloading.value = Result.Success(true)
        } catch (ex: Exception) {
            _downloading.value = Result.Failure(ex)
        }
    }
}

In activity/fragment,

 viewModel.downloading.observe(this, {
        when (it) {
            is Result.Failure -> TODO()
            Result.Loading -> TODO()
            is Result.Success -> TODO()
        }
    })

Result is a sealed class to capture state, which in turn will help us update the UI accordingly. Also viewmodelscope is used instead of GlobalScope since we don't want the download to go on when the viewmodel is destroyed.

CodePudding user response：

there are many ways to do that you can simply post value to live data, using dispatcher's and handler which is running on main thread as you provide looper of main thread.

Another way is you can use high order functions to update the viewmodels which is easy to use and give it a try.