The official Android documentation states that using allowMainThreadQueries() is not recommended because it could lock the UI for a long period of time and trigger an ANR.
But Kotlin coroutines gave us the possibility to perform some operation in the main thread without effectively blocking the UI.
So I'm asking: is it safe to use allowMainThreadQueries() and access the database in a couroutine scope running on the main thread? Like in the following:
// WITH allowMainThreadQueries()
val activityJob = Job()
val mainScope = CoroutineScope(Dispatchers.Main + activityJob)
mainscope.launch {
// access room database and retrieve some data
// update UI with data retrived
}
Or we should stick to the old way of not allowing main thread queries and performing database queries in another thread?
// WITHOUT allowMainThreadQueries()
val activityJob = Job()
val defaultScope = CoroutineScope(Dispatchers.Default + activityJob)
val mainScope = CoroutineScope(Dispatchers.Main + activityJob)
defaultScope.launch {
// access room database and retrieve some data
mainScope.launch {
// update UI with data retrived
}
}
I'm asking because the former way (with allowMainThreadQueries()):
You shouldn't need allowMainThreadQueries() for this to work. A scoped coroutine executs in its thread.
This is what I did not long ago:
@UiThread
fun getUsers(context: Context): LiveData<List<User>> {
if (!::users.isInitialized) {
users = MutableLiveData()
users.postValue(MyDatabase.get(context).users().getAll())
GlobalScope.launch(Dispatchers.Main) {
val usersFromDb: List<User> = async(Dispatchers.IO) {
return@async MyDatabase.get(context).users().getAll()
}.await()
users.value = usersFromDb
}
}
return users
}
You can see this getUsers() method gets called from main thread, returning a LiveData (which is handy in this case). The database query happens in GlobalScope.launch().
So yes, your design is one I personaly like. And one that works. But I don't think that you'll need allowMainThreadQueries() at all. Feel free to read (my) blog post: https://proandroiddev.com/android-viewmodel-livedata-coroutines-contraption-e1e44af690a6
It is recommended to access the database in your ViewModel inside viewmodelScope.
If you are you need to access room database from activity or fragment use
lifecyclescope.launch{ // access database dao functions here which are suspend in definition. }
Or inside the lifecycleScope change your thread using withContext(Dispatchers.IO){}