How to Coroutine LiveData in ViewModel

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As we know lately . . Kotlin Coroutine become hot topic in Android development. Usually i’m using RxJava in my Android project, and i decide to take a look on Coroutine for a simple project and share my “experience” in this story.

I’m using Retrofit (2.6.1) in my demo project, which already support for suspend function. Let’s take a look in my MovieApiService below :

retrofit interface which using suspend function

From the snippet above, we see that all of the function return object Retrofit Response not Deferred.

And then, i have a BaseDataSource which is the base abstract class for the DataSource, and here’s the snippet code :

BaseDataSource

This BaseDataSource, we wrap the Retrofit Response into 2 type in the getResult function, ResponseResult.Success and ResponseResult.Error.

In resultLiveData function, we gonna wrap the suspend function result as LiveData by emitting the object.

MovieRemoteDataSource extends BaseDataSource

Take a look on snippet above. Both of function will return LiveData using the function from the BaseDataSource. So when those function invoked, it should give LiveData immediately right? ummmm . . . not quite right though.

So here’s my experience i mentioned earlier.

Usually, when we work using ViewModel & LiveData, we put like . . . let say 2 variable which will store value to be observe :

My first approach, but fail :(

On the snippet above will give null result when we observe movieResult . . but why?

But why??

So it seem our DataSource won’t emitting data because the LiveData (from our remote data source) mark as no active observer. And then i change the approach, for the function in our viewmodel become not return Unit but return LiveData instead.

My 2nd approach, and it works :)

When we observe the LiveData by observing discoverAllMovies() in our Activity, it will give correct result.

Observing LiveData in Activity

Please take a look at :

dashboardViewModel.discoverAllMovies().onResult {
    vfDashboardContent.displayedChild = when(it) {
        is ResponseResult.Success -> {
            val data = it.result.data?.mapToMovieItemList()
            movieAdapter.setListMovieData(data)
            DashboardStateView.STATE_CONTENT
        }
        is ResponseResult.Loading -> {
            DashboardStateView.STATE_LOADING
        }
        is ResponseResult.Error -> {
            tvCommonError.text = it.msg.errorMsg.orEmpty()
            DashboardStateView.STATE_ERROR
        }
    }
}

On above block, the live data can be observed (not null) and result will be handled by the state, then update current UI.

After this demo project, i realise that the livedata{} on coroutine need to be observe directly, so it can emitting data.

You can find the full source code in my github.

herisulistiyanto/Simple-Retrofit-Coroutine

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