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suspend method inside runInTransaction block

I have a compilation error using the code below:
Suspension functions can be called only within coroutine body
Can someone explain to me why? What do I need to do to make it work (without using the #Transaction annotation)?
override suspend fun replaceAccounts(newAccounts: List<Account>) {
database.runInTransaction {
database.accountDao().deleteAllAccounts() // I have the error on this line
database.accountDao().insertAccounts(newAccounts) // Here too
}
}
#Dao
abstract class AccountDao : BaseDao<AccountEntity> {
#Query("DELETE FROM Account")
abstract suspend fun deleteAllAccounts()
}
Thanks in advance for your help

For suspend functions you should use withTransaction instead of runInTransaction

IO-bound and other long-running operations (such as database or API calls) are restricted from running in the main thread directly (which could otherwise cause your program to become unresponsive). Coroutines are like light-weight threads which run, asynchronously, within a thread.
I suggest reading through the Coroutines guide at https://kotlinlang.org/docs/reference/coroutines/coroutine-context-and-dispatchers.html
To answer your question, you need to setup a coroutine scope, and dispatcher thread for your coroutine to run on. The simplest is something like:
GlobalScope.launch(Dispatchers.IO) {
replaceAccounts(newAccounts)
}
which would run your coroutine in the GlobalScope (the coroutine's "lifecycle" is bound to the lifecycle of the entire application), on the IO thread (a thread outside of the main thread which handles IO tasks).
EDIT
I do like #IR42's answer. To build upon that, the usage of withTransaction in this case allows Room to handle the thread in which the database operations are performed on, and helps to limit concurrency to the database.
GlobalScope.launch(Dispatchers.Main) {
replaceAccounts(newAccounts)
}
override suspend fun replaceAccounts(newAccounts: List<Account>) {
database.withTransaction {
database.accountDao().deleteAllAccounts() // I have the error on this line
database.accountDao().insertAccounts(newAccounts) // Here too
}
}
See more information on this article by one of Room's own: https://medium.com/androiddevelopers/threading-models-in-coroutines-and-android-sqlite-api-6cab11f7eb90

Kotlin in Android - Suspend Functions in Room

I was looking at the sample at https://github.com/android/architecture-samples/tree/dev-dagger/app/src/main/java/com/example/android/architecture/blueprints/todoapp/data/source/local of the dev-dagger branch and in the TasksLocalDataSource.kt file they have the following method:
override suspend fun getTasks(): Result<List<Task>> = withContext(ioDispatcher) {
return#withContext try {
Success(tasksDao.getTasks())
} catch (e: Exception) {
Error(e)
}
}
By using withContext with an IO as dispatcher, they want the coroutine to run on an IO thread. But the Room request tasksDao.getTasks() inside the method is a suspend function. And in the codelab at https://codelabs.developers.google.com/codelabs/kotlin-coroutines/#8, they say that Room takes care of running the request (here: getTasks() ) on a background thread when it is a suspend function which is the case here.
So, isn't too much to use also a withContext(ioDispatcher) ? Could I not rewrite the method above also like the following way ? :
override suspend fun getTasks(): Result<List<Task>> {
return Success(tasksDao.getTasks())
}

Yes, this is exactly what you should write. There still appears to be a lot of legacy in the docs, from the time before they introduced suspendable Room calls. It is a waste both in terms of readability and performance to force a suspendable function call into the IO dispatcher.

binding to a suspended function in android

I want to bind a button click to a suspended function inside viewmodel.
this is my code:
RegistrationActivityViewModel.kt
suspend fun register() {
if (validateFields()) {
val result = SourceplusplusApiService.invoke().registerUser(username.value!!, email.value!!, password.value!!).await()
isRegistrationCompleted.value = getResultValue(result)
}
}
activity_registration.xml
<Button
android:text="Register"
android:onClick="#{()->viewmodel.register()}"
android:textSize="16sp" />
i get a databinding error that says ActivityRegistrationBindingImpl not generated. after searching a lot i realized that when i remove the suspend keyword and comment the code inside, it works fine but it has to be a suspended function.
Does anyone know how to fix it?

You cannot data bind to a suspend function, and IMHO a viewmodel should not be exposing a suspend function in the first place. I recommend:
Step #1: Remove the suspend keyword from register()
Step #2: Rewrite register() to run your code in a suitable coroutine scope, so any suspend functions that it calls are handled properly:
fun register() {
viewModelScope.launch(Dispatchers.Main) {
if (validateFields()) {
val result = SourceplusplusApiService.invoke().registerUser(username.value!!, email.value!!, password.value!!).await()
isRegistrationCompleted.value = getResultValue(result)
}
}
}
Here, I am using the viewModelScope option provided by androidx.lifecycle:lifecycle-viewmodel-ktx version 2.1.0-alpha01 and newer. Alternatively, manage your own coroutine scope. Dispatchers.Main will ensure that any results of that work is available to you on the Android main application thread.
Now, your data binding expression can refer to register(), while you still have a coroutine scope for calling downstream suspend functions.

Is the Kotlin Structured Concurrency [coroutine] model scoped to UI suitable for DB writes?

I'm specifically concerned about inserting user initiated data into the local database.
The following pattern is prevalent in examples (including from official sources, e.g. JetBrains, Google/Android) for using Kotlin coroutines in conjunction with [Android Architecture Components] ViewModels.
class CoroutineScopedViewModel : ViewModel(), CoroutineScope {
private val _job = Job()
override val coroutineContext: CoroutineContext
get() = Dispatchers.Main + _job
override fun onCleared() {
super.onCleared()
_job.cancel()
}
fun thisIsCalledFromTheUI() = launch {
/* do some UI stuff on the main thread */
withContext(Dispatchers.IO) {
try {
/* do some IO, e.g. inserting into DB */
} catch (error: IOException) {
/* do some exception handling */
}
}
}
}
It's my understanding of the documentation that in the above example the coroutines started in the UI context (defined through coroutineContext) will be cancelled when the ViewModel is destroyed, but that the code in the withContext(Dispatchers.IO) block will get to run to completion.
But, before I go about refactoring my project from the (pre-1.0.0) globally scoped (launch/async) coroutine model, I feel I need to just have some things clarified:
Is my reading of the documentation correct? Or, will destruction of the viewmodel before the withContext(Dispatchers.IO) block runs to completion trigger cancellation of that job too? I.e. can this model be used for inserting data into my DB, or could some strange timing issue arise where the user hits back or otherwise causes the ViewModel owner to close that ends up losing the data?
I don't want to inadvertently introduce a timing bug because I misunderstood something and therefor converted my code to a model similar to the one shown above.
EDIT:
So, I decided to do a little test, and it seems to me that all those examples using this model to write to the database may have a fundamental bug.
Modifying the code to log what happens, as such:
class ChildViewModel : ViewModel(), CoroutineScope {
private val _job = Job()
override val coroutineContext: CoroutineContext
get() = Dispatchers.Main + _job
override fun onCleared() {
super.onCleared()
Log.d("onCleared", "Start")
_job.cancel()
Log.d("onCleared", "End")
}
fun thisIsCalledFromTheUI() = launch {
Log.d("thisIsCalledFromTheUI", "Start")
GlobalScope.launch(Dispatchers.IO) {
Log.d("GlobalScope", "Start")
delay(15000)
Log.d("GlobalScope", "End")
}
withContext(Dispatchers.IO) {
Log.d("withContext", "Start")
delay(10000)
Log.d("withContext", "End")
}
Log.d("thisIsCalledFromTheUI", "End")
}
}
Results in this, if you let it run to completion:
D/thisIsCalledFromTheUI: Start
D/GlobalScope: Start
D/withContext: Start
D/withContext: End
D/thisIsCalledFromTheUI: End
D/GlobalScope: End
But, if you close the Fragment/Activity (not the app) before withContext ends, you get this:
D/thisIsCalledFromTheUI: Start
D/GlobalScope: Start
D/withContext: Start
D/GlobalScope: End
Which indicates, to me at least, that you cannot use this to write non-transient data to the DB.

It's my understanding of the documentation that in the above example the coroutines started in the UI context (defined through coroutineContext) will be cancelled when the ViewModel is destroyed, but that the code in the withContext(Dispatchers.IO) block will get to run to completion.
This isn't a correct reading of the documentation. withContext doesn't start another coroutine, it just changes the current coroutine's context for the duration of its block. Therefore this coroutine will get cancelled, as well as all other coroutines you start without providing a new parent context that has a different job associated with it (or no job at all, like the GlobalScope).
However, your proposed idea to use the GlobalScope for persistent operations is just a local patch for the scenario you're testing, you're still not getting a guarantee it will run to completion. The user can exit the application completely and Android can kill the process.
Therefore, if your goal is building a truly robust application, you must accommodate the fact that, until the coroutine completes, no information was written to the DB. Hopefully you run the operation within a DB transaction that will automatically roll back if your program gets killed, otherwise it will be impossible to prevent inconsistencies.

Kotlin coroutines `runBlocking`

I am learning Kotlin coroutines. I've read that runBlocking is the way to bridge synchronous and asynchronous code. But what is the performance gain if the runBlocking stops the UI thread?
For example, I need to query a database in Android:
val result: Int
get() = runBlocking { queryDatabase().await() }
private fun queryDatabase(): Deferred<Int> {
return async {
var cursor: Cursor? = null
var queryResult: Int = 0
val sqlQuery = "SELECT COUNT(ID) FROM TABLE..."
try {
cursor = getHelper().readableDatabase.query(sqlQuery)
cursor?.moveToFirst()
queryResult = cursor?.getInt(0) ?: 0
} catch (e: Exception) {
Log.e(TAG, e.localizedMessage)
} finally {
cursor?.close()
}
return#async queryResult
}
}
Querying the database would stop the main thread, so it seems that it would take the same amount of time as synchronous code? Please correct me if I am missing something.

runBlocking is the way to bridge synchronous and asynchronous code
I keep bumping into this phrase and it's very misleading.
runBlocking is almost never a tool you use in production. It undoes the asynchronous, non-blocking nature of coroutines. You can use it if you happen to already have some coroutine-based code that you want to use in a context where coroutines provide no value: in blocking calls. One typical use is JUnit testing, where the test method must just sit and wait for the coroutine to complete.
You can also use it while playing around with coroutines, inside your main method.
The misuse of runBlocking has become so widespread that the Kotlin team actually tried to add a fail-fast check which would immediately crash your code if you call it on the UI thread. By the time they did this, it was already breaking so much code that they had to remove it.

Actually you use runBlocking to call suspending functions in "blocking" code that otherwise wouldn't be callable there or in other words: you use it to call suspend functions outside of the coroutine context (in your example the block passed to async is the suspend function). Also (more obvious, as the name itself implies already), the call then is a blocking call. So in your example it is executed as if there wasn't something like async in place. It waits (blocks interruptibly) until everything within the runBlocking-block is finished.
For example assume a function in your library as follows:
suspend fun demo() : Any = TODO()
This method would not be callable from, e.g. main. For such a case you use runBlocking then, e.g.:
fun main(args: Array<String>) {
// demo() // this alone wouldn't compile... Error:() Kotlin: Suspend function 'demo' should be called only from a coroutine or another suspend function
// whereas the following works as intended:
runBlocking {
demo()
} // it also waits until demo()-call is finished which wouldn't happen if you use launch
}
Regarding performance gain: actually your application may rather be more responsive instead of being more performant (sometimes also more performant, e.g. if you have multiple parallel actions instead of several sequential ones). In your example however you already block when you assign the variable, so I would say that your app doesn't get more responsive yet. You may rather want to call your query asynchronously and then update the UI as soon as the response is available. So you basically just omit runBlocking and rather use something like launch. You may also be interested in Guide to UI programming with coroutines.