I have a function that runs on the Android main thread (UI thread):
fun someFunc() {
...
doSomething1()
doSomething2()
doSomething3()
...
}
I want to run doSomething2() asynchronously in a background thread and make sure that someFunc() is suspended (not blocked) until this execution completes. Once done, someFunc() should resume in the main thread (from doSomething3()). During this background thread execution, I want to make sure that main thread is free and not blocked.
I know this can be done using Futures, but I'm wondering how to do this using coroutines/async?
You can call those functions in a coroutine, launched using viewModelScope in a ViewModel class or lifecycleScope in Activity/Fragment:
fun someFunc() = viewModelScope.launch {
doSomething1()
doSomething2()
doSomething3()
}
suspend fun doSomething2() = withContext(Dispatchers.IO) {
...
}
To run doSomething2() in a background thread we need to switch context of the coroutine to Dispatchers.IO using withContext() function.
Related
Currently my code looks like this i have a ViewModel that calls the repository to do some background computations and return a result.
ViewModel function is run with viewModelScope.launch(Dispatchers.IO) and then the repository one is a suspend function.
Do I have to use return withContext{} to ensure that everything will be done sequentially? I checked and it is indeed sequential, but in the documentation i found that it doesn't have to be?
Let's dissect the viewModelScope first from it's source code. It uses a custom implementation of CouroutineScope. The context comprises of a SupervisorJob and a Dispatchers.Main dispatcher. This ensures that the coroutine is launched on the main thread and it's failure doesn't affect other coroutines in the scope.
CloseableCoroutineScope(SupervisorJob() + Dispatchers.Main.immediate))
Couple of examples worth exploring.
viewModelScope.launch {
Log.d("ViewModel", "Just viewModelScope: ${Thread.currentThread().name}")
}
// Output: Just viewModelScope: main
viewModelScope.launch(Dispatchers.IO) {
Log.d("ViewModel", "IO viewModelScope: ${Thread.currentThread().name}")
}
// Output: IO viewModelScope: DefaultDispatcher-worker-3
viewModelScope.launch {
Log.d("ViewModel", "viewModelScope thread: ${Thread.currentThread().name}")
withContext(Dispatchers.IO) {
delay(3000)
Log.d("ViewModel", "withContext thread: ${Thread.currentThread().name}")
}
Log.d("ViewModel", "I'm finished!")
}
// Output:
// viewModelScope thread: main
// withContext thread: DefaultDispatcher-worker-4
I checked and it is indeed sequential, but in the documentation i
found that it doesn't have to be.
withContext() is a suspending operation and the coroutine will suspend till it's completion and then proceed ahead. That is apparent from the third example above.
In summary, viewModelScope will use main thread to execute a coroutine whose cancellation won't affect other couroutines. Use withContext when you want to do heavy task off of main thread in a suspending fashion; dispatch it using an appropriate dispatcher. Kotlin Coroutine guide is worth a read.
Edit:
Consider the below code as a single unit of execution. This illustrates the fact that when using withContext(), the caller thread is suspending, but it is not blocked, which allows it to go ahead and pick up some other pending work. The interleaving of the output loggers is of interest to us.
viewModelScope.launch {
Log.d("ViewModel", "viewModelScope thread: ${Thread.currentThread().name}")
withContext(Dispatchers.IO) {
delay(3000)
Log.d("ViewModel", "withContext thread: ${Thread.currentThread().name}")
}
Log.d("ViewModel", "I'm finished!")
}
viewModelScope.launch {
Log.d("ViewModel", "I'm not blocked: ${Thread.currentThread().name}")
}
// Output:
// viewModelScope thread: main
// I'm not blocked: main
// withContext thread: DefaultDispatcher-worker-2
// I'm finished!
I am using kotlin Coroutines to perform async network operations to avoid NetworkOnMainThreadException.
The problem is the lag that happens when i use runBlocking,that take sometime to complete current thread.
How can i prevent this delay or lag,and allow the async operation to be done without delay
runBlocking {
val job = async (Dispatchers.IO) {
try{
//Network operations are here
}catch(){
}
}
}
By using runBlocking you are blocking the main thread until the coroutine finishes.
The NetworkOnMainThread exception is not thrown because technically the request is done on a background thread, but by making the main thread wait until the background thread is done, this is just as bad!
To fix this you could launch a coroutine, and any code that depends on the network request can be done inside the coroutine. This way code may still be executed on the main thread, but it never blocks.
// put this scope in your activity or fragment so you can cancel it in onDestroy()
val scope = MainScope()
// launch coroutine within scope
scope.launch(Dispachers.Main) {
try {
val result = withContext(Dispachters.IO) {
// do blocking networking on IO thread
""
}
// now back on the main thread and we can use 'result'. But it never blocked!
} catch(e: Exception) {
}
}
If you don't care about the result and just want to run some code on a different thread, this can be simplified to:
GlobalScope.launch(Dispatchers.IO) {
try {
// code on io thread
} catch(e: Exception) {
}
}
Note: if you are using variables or methods from the enclosing class you should still use your own scope so it can be cancelled in time.
I am new to multithreading and looking for solution for this problem.
I am launching a method in coroutine which updates data in my database and if it is updated I would like to update the UI for users. How to this? I cannot put runOnUiThread inside a coroutine. Is there some type of magic like -> when coroutine finished -> then -> runOnUi?
Greetings
You don't need to call runOnUiThread as the coroutine will have the main dispatcher as the context.
Let's say you have this helper function to offload work to the I/O thread.
suspend fun <T> withIO(block: suspend CoroutineScope.() -> T) = withContext(Dispatchers.IO, block)
If you are using a ViewModel, then you can call it like this
viewModelScope.launch {
val result = withIO {
// You are on IO thread here.
update your database
}
// The block will be suspended until the above task is done.
// You are on UI thread now.
// Update your UI.
}
If you are not using a ViewModel, you can also use
withContext(Disptachers.Main) {
val result = withIO {
// You are on IO thread
}
// You are back on the main thread with the result from the task
}
Coroutine are task that work on different thread.
What you really want is wating for changes in database. Coroutine in this idea could work for insert data in db, but listening part is role of ViewModel pattern.
I recently answer similar question to yours:
AutocompleteTextView with room
More specific could be this answer from another user:
Wait until Kotlin coroutine finishes in onCreateView()
So the basic problem is to jumping back to main thread after co-routine finishes
this can be done multiple ways
using launch(Dispatcher.Main)
from main thread init co-routine
something like this
//launches coroutine running on main thread
GlobalScope.launch(Dispatchers.Main) {
updateDb()
}
suspend fun updateDb(){
//runs on worker thread and returns data
val value = withContext(Dispatchers.IO){
saveDataInDb();
}
//runs back on main thread
updateUI(value);
}
However global scope should not be used
You can read about that here https://medium.com/#elizarov/the-reason-to-avoid-globalscope-835337445abc
using async await
suspend fun saveInDb() {
val value = GlobalScope.async {
delay(1000)
println("thread running on [${Thread.currentThread().name}]")
10
}
println("value = ${value.await()} thread running on [${Thread.currentThread().name}]")
}
output:
thread running on [DefaultDispatcher-worker-1]
value = 10 thread running on [main]
thread running on [main]
fetchData() is suspendCoroutine function, so it is implemented on other thread.
viewModelScope is bound to Dispatchers.Main: this should be used only for interacting with the UI and performing quick work.
So should I have delay() in Dispatcher.Main or should I move out it?
fun loadData() {
viewModelScope.launch {
delay(START_DELAY)
when (val result = fetchData()) {
is Response.Success<IData> -> {}
is Response.Failure -> {}
}
}
}
fun fetchData(){
return suspendCoroutine { cont ->}
}
Answer is it can stay, delay will not hurt Main thread, it will not block it. This coroutine inside Main Dispatcher will be suspended, while other coroutines inside Main will continue to run.
I'm new to coroutines and I'm wondering if it's possible to switch from coroutineScope (GlobalScope) to UI scope for the code below. My problem is that the steps inside the coroutine launch body must be executed in a worker thread, otherwise the listener notification must be executed in the ui thread in order to avoid to call runOnUiThread in my activity code.
override suspend fun startRent(name: String, bikeMode: BikeMode, listener: StartRentListener) {
var bleDevice : RxBleDevice
val scanFilter: ScanFilter = ScanFilter.Builder().setDeviceName(name).build()
val scanSettings: ScanSettings = ScanSettings.Builder().build()
val job = GlobalScope.launch {
try {
bleDevice = rxBleClient.scanBleDevicesExt(rxBleClient, scanSettings, scanFilter)
val bleConnection = bleDevice.establishConnectionExt()
// write handshake
connectionManager.writeHandshake(bleDevice, bleConnection)
// open lock
openLock(bleDevice, bikeMode, bleConnection)
// getting user position
apiHelper.sendLockRequest(bleDevice.name, getPosition())
bleDevice.disconnect()
// this should be called on main thread once all the previous operations are finished
listener.onSuccess()
} catch (e: Exception) {
listener.onError(e)
}
}
job.join()
}
A snippet of my current activity code:
bikeAccessClient.startRent(bikeBLEName, BikeMode.HYBRID, object :
StartRentListener {
override fun onSuccess() {
runOnUiThread {
// UI update here
}
}
You may use withContext(Dispatchers.Main) {..} function to execute a part of your code with the other Coroutine Dispatcher.
kotlinx.coroutines.android contains the definition of the Dispatchers.Main function and it integrates correctly with Android UI.
Using explicit Dispatcher in your code is quite error-prone. Instead, I would recommend designing the code with fewer explicit requirements.
I would wrote something like that:
fun uiActionHandlerToStartTheProcess() {
launch(Dispatchers.Main) {
val result = startRent(...) // no callback here, suspend function
//UI Update Here
}
}
suspend fun CoroutineScope.startRent() : SomeResultOfWork {
//that function offloads the execution to a IO (aka brackground) thread
return withContext(Dispatchers.IO){
//here goes your code from `startRent`
//use `suspendCancellableCoroutine {cont -> .. }` if you need to handle callbacks from it
SomeResultOfWork()
}
The code in the launch(Dispatchers.Main){..} block is executed in the UI thread. The call to startRent suspend function suspends the execution in the UI thread. Once the startRent is ready with the reply (from a background thread) it resumes the execution (which is done by the Dispatchers.Main and equivalent to the runOnUiThread {...}) and executes the UI update from the right thread