Handle errors

As we saw in the previous step, Apollo had onFailure and onResponse callback methods.

Now let's go back to the getMyTodoQueryCloud function that you wrote in the previous step.

private fun getMyTodoQueryCloud() {
// Init Query
getMyTodosQuery = GetMyTodosQuery.builder().build()
// Apollo runs query on background thread
Network.apolloClient.query(getMyTodosQuery)?.enqueue(object : ApolloCall.Callback<GetMyTodosQuery.Data>() {
override fun onFailure(error: ApolloException) {
Log.d("Todo", error.toString() )
}
override fun onResponse(@NotNull response: Response<GetMyTodosQuery.Data>) {
// Changing UI must be on UI thread
Log.d("Todo", response.data().toString() )
listItems = response.data()?.todos()?.toMutableList()
activity?.runOnUiThread { updateTabs() }
}
})
}

When this function is executed in the onCreate, we handle the completion of it from the onError or onResponse callback methods.

Now, the query could also end up in an error state due to various reasons. Sometimes the graphql query could be wrong, or the server isn't responding. Whatever may be the reason, the user facing UI should show something to convey that an error has occurred. Here we are just logging the error, but you can do retry or redirect logic here. In this error state, typically you can send these error messages to third-party services to track what went wrong.

All said and done, these are two important states that need to be handled inside your fragment. What you have written above is basic, but sufficient for this tutorial.