The new LiveData can be used as a replacement for RxJava's observables in some scenarios. However, unlike Observable, LiveData has no callback for errors.
My question is: How should I handle errors in LiveData, e.g. when it's backed by some network resource that can fail to be retrieved due to an IOException?
You can extend from MutableLiveData and create a holder Model to wrap your data.
This is your Wrapper Model
public class StateData<T> {
@NonNull
private DataStatus status;
@Nullable
private T data;
@Nullable
private Throwable error;
public StateData() {
this.status = DataStatus.CREATED;
this.data = null;
this.error = null;
}
public StateData<T> loading() {
this.status = DataStatus.LOADING;
this.data = null;
this.error = null;
return this;
}
public StateData<T> success(@NonNull T data) {
this.status = DataStatus.SUCCESS;
this.data = data;
this.error = null;
return this;
}
public StateData<T> error(@NonNull Throwable error) {
this.status = DataStatus.ERROR;
this.data = null;
this.error = error;
return this;
}
public StateData<T> complete() {
this.status = DataStatus.COMPLETE;
return this;
}
@NonNull
public DataStatus getStatus() {
return status;
}
@Nullable
public T getData() {
return data;
}
@Nullable
public Throwable getError() {
return error;
}
public enum DataStatus {
CREATED,
SUCCESS,
ERROR,
LOADING,
COMPLETE
}
}
This is your extended LiveData Object
public class StateLiveData<T> extends MutableLiveData<StateData<T>> {
/**
* Use this to put the Data on a LOADING Status
*/
public void postLoading() {
postValue(new StateData<T>().loading());
}
/**
* Use this to put the Data on a ERROR DataStatus
* @param throwable the error to be handled
*/
public void postError(Throwable throwable) {
postValue(new StateData<T>().error(throwable));
}
/**
* Use this to put the Data on a SUCCESS DataStatus
* @param data
*/
public void postSuccess(T data) {
postValue(new StateData<T>().success(data));
}
/**
* Use this to put the Data on a COMPLETE DataStatus
*/
public void postComplete() {
postValue(new StateData<T>().complete());
}
}
And this is how you use it
StateLiveData<List<Book>> bookListLiveData;
bookListLiveData.postLoading();
bookListLiveData.postSuccess(books);
bookListLiveData.postError(e);
And how it can be observed:
private void observeBooks() {
viewModel.getBookList().observe(this, this::handleBooks);
}
private void handleBooks(@NonNull StateData<List<Book>> books) {
switch (books.getStatus()) {
case SUCCESS:
List<Book> bookList = books.getData();
//TODO: Do something with your book data
break;
case ERROR:
Throwable e = books.getError();
//TODO: Do something with your error
break;
case LOADING:
//TODO: Do Loading stuff
break;
case COMPLETE:
//TODO: Do complete stuff if necessary
break;
}
}
Just some implementation of the method from Chris Cook's answer:
At first, we need the object that will contain response data and exceptions:
/**
* A generic class that holds a value with its loading status.
*
* @see <a href="https://github.com/android/architecture-components-samples/blob/master/GithubBrowserSample/app/src/main/java/com/android/example/github/vo/Resource.kt">Sample apps for Android Architecture Components</a>
*/
data class Resource<out T>(val status: Status, val data: T?, val exception: Throwable?) {
enum class Status {
LOADING,
SUCCESS,
ERROR,
}
companion object {
fun <T> success(data: T?): Resource<T> {
return Resource(Status.SUCCESS, data, null)
}
fun <T> error(exception: Throwable): Resource<T> {
return Resource(Status.ERROR, null, exception)
}
fun <T> loading(): Resource<T> {
return Resource(Status.LOADING, null, null)
}
}
}
And then my own invention - AsyncExecutor.
This small class do 3 important things:
import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
class AsyncExecutor {
companion object {
fun <T> run(callback: () -> T): LiveData<Resource<T>> {
val resourceData: MutableLiveData<Resource<T>> = MutableLiveData()
Thread(Runnable {
try {
resourceData.postValue(Resource.loading())
val callResult: T = callback()
resourceData.postValue(Resource.success(callResult))
} catch (e: Throwable) {
resourceData.postValue(Resource.error(e))
}
}).start()
return resourceData
}
}
}
Then you can create a LiveData in your ViewModel, contains the result of your callback or exception:
class GalleryViewModel : ViewModel() {
val myData: LiveData<Resource<MyData>>
init {
myData = AsyncExecutor.run {
// here you can do your synchronous operation and just throw any exceptions
return MyData()
}
}
}
And then you can get your data and any exceptions in the UI:
class GalleryFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
galleryViewModel = ViewModelProviders.of(this).get(GalleryViewModel::class.java)
// ...
// Subscribe to the data:
galleryViewModel.myData.observe(viewLifecycleOwner, Observer {
when {
it.status === Resource.Status.LOADING -> {
println("Data is loading...")
}
it.status === Resource.Status.ERROR -> {
it.exception!!.printStackTrace()
}
it.status === Resource.Status.SUCCESS -> {
println("Data has been received: " + it.data!!.someField)
}
}
})
return root
}
}
I have built a movie search app here in which I have used to different LiveData objects, one for the successful response from the network and one for the unsuccessful:
private val resultListObservable = MutableLiveData<List<String>>()
private val resultListErrorObservable = MutableLiveData<HttpException>()
fun findAddress(address: String) {
mainModel.fetchAddress(address)!!.subscribeOn(schedulersWrapper.io()).observeOn(schedulersWrapper.main()).subscribeWith(object : DisposableSingleObserver<List<MainModel.ResultEntity>?>() {
override fun onSuccess(t: List<MainModel.ResultEntity>) {
entityList = t
resultListObservable.postValue(fetchItemTextFrom(t))
}
override fun onError(e: Throwable) {
resultListErrorObservable.postValue(e as HttpException)
}
})
}