Modifying ktors call co-routine context - kotlin

So I'm using ktor and want to have some data available throughout the context, via a ThreadLocal. What I am looking at is:
val dataThreadLocal = ThreadLocal<String>()
suspend fun fromOtherFunction() = "From other function -- ${dataThreadLocal.get()}"
routing {
get("/hello") {
// [1]
launch(this.coroutineContext + dataThreadLocal.asContextElement(value = "world")) {
val fromHere = async {
"ThreadLocal value: ${dataThreadLocal.get()}"
}
val fromThere = async {
fromOtherFunction()
}
call.respond(fromHere.await() + "," + fromThere.await())
}
}
}
What I am doing is that making sure all and any functions called from the parent launch (labelled [1]) have access to this kind of "scoped" data.
However, my application is quite big and I want this to work for all requests handled by routing and not want to have every route require this wrapping.
What would be really great is:
intercept(ApplicationCallPipeline.Features) {
launch(this.coroutineContext + dataThreadLocal.asContextElement(value = "world")) {
...
}
}
routing {
get("/hello") {
val threadLocalValue = dataThreadLocal.get()
...
Which obviously does not work, becase the coroutine scope in the intercept does not enclose the route's coroutine scope.
What I believe happens under the hood is that every call is launched in a parent co-routine scope (similar to how we have 'request threads'). Is there a way for me to modify the context in that? Is there a way where I can tell ApplicationEngine to + an additional context element whenever this new co-routine context is started?

Related

Kotlin flows SharedFlow not received in collectInLifeCycle inside Fragment

I am observing inside a fragment the events of a sharedflow such as this:
myEvent.collectInLifeCycle(viewLifecycleOwner) { event ->
when (state) {
//check the event. The event emited form onStart is never reached here :(
}
}
Whereas in the viewmodel I have
private val _myEvent = MutableSharedFlow<MyEvent>()
val myEvent: SharedFlow<MyEvent> = _myEvent
fun loadData() =
viewModelScope.launch {
getDataUseCase
.safePrepare(onGenericError = { _event.emit(Event.Error(null)) })
.onStart { _event.emit(Event.Loading) }
.onEach { result ->
result.onSuccess { response ->
_event.emit(Event.Something)
}
}
.launchIn(viewModelScope)
}
So the problem is that only the Event.Something is the one being properly collected from the fragment, whereas _event.emit(Event.Loading) is not being collected... If I debug it goes to the onStart, but it is never called in the fragment.
Your SharedFlow needs to have a replay so that collectors always get at least the most recent value. Otherwise, if you emit to the Flow before the collector is registered, it will never see anything emitted. Do this:
private val _myEvent = MutableSharedFlow<MyEvent>(replay = 1)
Personally, unless I'm missing some detail here that would change my mind, I would simplify all your code to avoid having to manually call loadData(). Something like this but I'm guessing a bit because I don't know all your types and functions.
val myEvent: SharedFlow<MyEvent> = flow {
emit(Event.Loading)
emitAll(
getDataUseCase
.transform { result ->
result.onSuccess { response ->
emit(Event.Something)
}
}
.catch { error -> emit(Event.Error(null)) }
)
}.shareIn(viewModelScope, SharingStarted.Lazily, replay = 1)

Appropriate use of coroutines for parallel API calls

I'm trying to learn the right way to use Kotlin coroutines to do some parallel API calls. I've been able to get the expected results/process, but am wondering if there is a "more correct" way to do so.
Here's my contrived example of what I'm trying to do. Use case is essentially an inbound web request, and that request in turn needing to make a couple of API calls. Digging around led me to using launch or async:
// using launch
var recordExists = false
var otherRecordExists = false
coroutineScope {
launch {
recordExists = someHttpService.doesRecordExist(123)
}
launch {
otherRecordExists = someHttpService.doesRecordExist(456)
}
}
if (!recordExists || !otherRecordExists) {...}
vs
// using async
var recordExists = false
var otherRecordExists = false
coroutineScope {
val recordDeferred = async { someHttpService.doesRecordExist(123) }
val otherRecordDeferred = async { someHttpService.doesRecordExist(456) }
recordExists = recordDeferred.await()
otherRecordExists = otherRecordDeferred.await()
}
if (!recordExists || !otherRecordExists) {...}
I landed on using launch because it felt cleaner, but not sure if that was the most appropriate decision reasoning...
Is there a specific reason to use one over the other that I'm overlooking?
Your code is okay but as a general practice, I would recommend avoiding using var as much as possible when dealing with concurrency and parallelism. Having a shared mutable state is very risky and can often lead to race-conditions. In your case you can do something like:
coroutineScope {
val recordDeferred = async { someHttpService.doesRecordExist(123) }
val otherRecordDeferred = async { someHttpService.doesRecordExist(456) }
val recordExists = recordDeferred.await()
val otherRecordExists = otherRecordDeferred.await()
if (!recordExists || !otherRecordExists) {
...
}
}
Here you can only use async as you need to get some data back from the coroutine.

Unable to trigger re-composition after re-assigning mutableStateList inside a coroutine scope

I'm still a bit of a beginner with Jetpack compose and understanding how re-composition works.
So I have a piece of code calls below inside a ViewModel.
SnapshotStateList
var mutableStateTodoList = mutableStateListOf<TodoModel>()
private set
during construction of the view model, I execute a room database call
init {
viewModelScope.launch {
fetchTodoUseCase.execute()
.collect { listTypeTodo ->
mutableStateTodoList = listTypeTodo.toMutableStateList()
}
}
}
then I have an action from a the ui that triggers adding of a new Todo to the list and expecting a re-composition from the ui that shows a card composable
fun onFabClick() {
todoList.add(TodoModel())
}
I can't figure out why it doesn't trigger re-composition.
However if I modify the init code block below, and invoke the onFabClick() action, it triggers re-composition
init {
viewModelScope.launch {
fetchTodoUseCase.execute()
.collect { listTypeTodo ->
mutableStateTodoList.addAll(listTypeTodo)
}
}
}
or this, taking out the re-assigning of the mutableStateList outside of the coroutine scope also works (triggers re-composition).
init {
// just trying to test a re-assigning of the mutableStateList property
mutableStateTodoList = emptyList<TodoModel>().toMutableStateList()
}
Not quite sure where the problem if it is within the context of coroutine or SnapshotStateList itself.
Everything is also working as expected when the code was implemented this way below, using standard list inside a wrapper and performing a copy (creating new reference) and re-assigning the list inside the wrapper.
var todoStateWrapper by mutableStateOf<TodoStateWrapper>(TodoStateWrapper)
private set
Same init{...} call
init {
viewModelScope.launch {
fetchTodoUseCase.execute()
.collect { listTypeTodo ->
todoStateWrapper = todoStateWrapper.copy (
todoList = listTypeTodo
)
}
}
}
To summarize, inside a coroutine scope, why this works
// mutableStateList
todoList.addAll(it)
while this one does not?
// mutableStateList
todoList = it.toMutableStateList()
also why does ordinary list inside a wrapper and doing copy() works?
The mutable state in Compose can only keep track of updates to the containing value. Here is simplified code on how MutableState could be implemented:
class MutableState<T>(initial: T) {
private var _value: T = initial
private var listeners = MutableList<Listener>
var value: T
get() = _value
set(value) {
if (value != _value) {
_value = value
listeners.forEach {
it.triggerRecomposition()
}
}
}
fun addListener(listener: Listener) {
listeners.add(listener)
}
}
When the state is used by some view, this view subscribes to updates of this particular state.
So, if you declare the property as follows:
var state = MutableState(1)
and try to update it with state = 2.toMutableState() (this is analogous to your mutableStateTodoList = listTypeTodo.toMutableStateList()), triggerRecomposition cannot be called because you create a new object which resets all the listeners. Instead, to trigger recomposition you should update it with state.value = 2.
With mutableStateList, the analog of updating value is any method of MutableList interface that updates containing list, including addAll.
Inside init it works because no view is subscribed to this state so far, and that's the only place where methods such as toMutableStateList should be used.
It is important to always define mutable states as immutable property with val in order to prevent such mistakes. To make it mutable only from view model, you can define it like this, and make updates on _mutableStateTodoList:
private val _mutableStateTodoList = mutableStateListOf<TodoModel>()
val mutableStateTodoList: List<TodoModel> = _mutableStateTodoList
The only exception when you can use var is using mutableStateOf with delegation - this is where you can use it with private set because in that case the delegation does the work for you by not modifying the container, but only it's value property. Such method cannot be applied to mutableStateListOf, because there's no single value field that's responsible for the data in case of list.
var someValue by mutableStateOf(1)
private set

Kotlin Coroutine/Flow Timeout without cancelling the running coroutine?

I am trying to create a Flow that emits a value after a timeout, without cancelling the underlying coroutine. The idea is that the network call has X time to complete and emit a value and after that timeout has been reached, emit some initial value without cancelling the underlying work (eventually emitting the value from the network call, assuming it succeeds).
Something like this seems like it might work, but it would cancel the underlying coroutine when the timeout is reached. It also doesn't handle emitting some default value on timeout.
val someFlow = MutableStateFlow("someInitialValue")
val deferred = async {
val networkCallValue = someNetworkCall()
someFlow.emit(networkCallValue)
}
withTimeout(SOME_NUMBER_MILLIS) {
deferred.await()
}
I'd like to be able to emit the value returned by the network call at any point, and if the timeout is reached just emit some default value. How would I accomplish this with Flow/Coroutines?
One way to do this is with a simple select clause:
import kotlinx.coroutines.selects.*
val someFlow = MutableStateFlow("someInitialValue")
val deferred = async {
someFlow.value = someNetworkCall()
}
// await the first of the 2 things, without cancelling anything
select<Unit> {
deferred.onAwait {}
onTimeout(SOME_NUMBER_MILLIS) {
someFlow.value = someDefaultValue
}
}
You would have to watch out for race conditions though, if this runs on a multi-threaded dispatcher. If the async finished just after the timeout, there is a chance the default value overwrites the network response.
One way to prevent that, if you know the network can't return the same value as the initial value (and if no other coroutine is changing the state) is with the atomic update method:
val deferred = async {
val networkCallValue = someNetworkCall()
someFlow.update { networkCallValue }
}
// await the first of the 2 things, without cancelling anything
val initialValue = someFlow.value
select<Unit> {
deferred.onAwait {}
onTimeout(300) {
someFlow.update { current ->
if (current == initialValue) {
"someDefaultValue"
} else {
current // don't overwrite the network result
}
}
}
}
If you can't rely on comparisons of the state, you can protect access to the flow with a Mutex and a boolean:
val someFlow = MutableStateFlow("someInitialValue")
val mutex = Mutex()
var networkCallDone = false
val deferred = async {
val networkCallValue = someNetworkCall()
mutex.withLock {
someFlow.value = networkCallValue
networkCallDone = true
}
}
// await the first of the 2 things, without cancelling anything
select<Unit> {
deferred.onAwait {}
onTimeout(300) {
mutex.withLock {
if (!networkCallDone) {
someFlow.value = "someDefaultValue"
}
}
}
}
Probably the easiest way to solve the race condition is to use select() as in #Joffrey's answer. select() guarantees to execute only a single branch.
However, I believe mutating a shared flow concurrently complicates the situation and introduces another race condition that we need to solve. Instead, we can do it really very easily:
flow {
val network = async { someNetworkCall() }
select {
network.onAwait{ emit(it) }
onTimeout(1000) {
emit("initial")
emit(network.await())
}
}
}
There are no race conditions to handle. We have just two simple execution branches, depending on what happened first.
If we need a StateFlow then we can use stateIn() to convert a regular flow. Or we can use a MutableStateFlow as in the question, but mutate it only inside select(), similarly to above:
select {
network.onAwait{ someFlow.value = it }
onTimeout(1000) {
someFlow.value = "initial"
someFlow.value = network.await()
}
}
You can launch two coroutines simultaneously and cancel the Job of the first one, which responsible for emitting default value, in the second one:
val someFlow = MutableStateFlow("someInitialValue")
val firstJob = launch {
delay(SOME_NUMBER_MILLIS)
ensureActive() // Ensures that current Job is active.
someFlow.update {"DefaultValue"}
}
launch {
val networkCallValue = someNetworkCall()
firstJob.cancelAndJoin()
someFlow.update { networkCallValue }
}
You can send the network request and start the timeout delay simultaneously. When the network call succeeds, update the StateFlow with the response. And, when the timeout finishes and we haven't received the response, update the StateFlow with the default value.
val someFlow = MutableStateFlow(initialValue)
suspend fun getData() {
launch {
someFlow.value = someNetworkCall()
}
delay(TIMEOUT_MILLIS)
if(someFlow.value == initialValue)
someFlow.value = defaultValue
}
If the response of the network call can be same as the initialValue, you can create a new Boolean to check the completion of network request. Another option can be to store a reference of the Job returned by launch and check if job.isActive after the timeout.
Edit: In case you want to cancel delay when the network request completes, you can do something like:
val someFlow = MutableStateFlow(initialValue)
suspend fun getData() {
val job = launch {
delay(TIMEOUT_MILLIS)
someFlow.value = defaultValue
}
someFlow.value = someNetworkCall()
job.cancel()
}
And to solve the possible concurrency issue, you can use MutableStateFlow.update for atomic updates.

Variable value is still null even after assigning a value inside the listener block [duplicate]

(Disclaimer: There are a ton of questions which arise from people asking about data being null/incorrect when using asynchronous operations through requests such as facebook,firebase, etc. My intention for this question was to provide a simple answer for that problem to everyone starting out with asynchronous operations in android)
I'm trying to get data from one of my operations, when I debug it using breakpoints or logs, the values are there, but when I run it they are always null, how can I solve this ?
Firebase
firebaseFirestore.collection("some collection").get()
.addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
#Override
public void onSuccess(QuerySnapshot documentSnapshots) {
//I want to return these values I receive here...
});
//...and use the returned value here.
Facebook
GraphRequest request = GraphRequest.newGraphPathRequest(
accessToken,
"some path",
new GraphRequest.Callback() {
#Override
public void onCompleted(GraphResponse response) {
//I want to return these values I receive here...
}
});
request.executeAsync();
//...and use the returned value here.
Kotlin coroutine
var result: SomeResultType? = null
someScope.launch {
result = someSuspendFunctionToRetrieveSomething()
//I want to return the value I received here...
}
Log.d("result", result.toString()) //...but it is still null here.
Etc.
What is a Synchronous/Asynchronous operation ?
Well, Synchronous waits until the task has completed. Your code executes "top-down" in this situation.
Asynchronous completes a task in the background and can notify you when it is complete.
If you want to return the values from an async operation through a method/function, you can define your own callbacks in your method/function to use these values as they are returned from these operations.
Here's how for Java
Start off by defining an interface :
interface Callback {
void myResponseCallback(YourReturnType result);//whatever your return type is: string, integer, etc.
}
next, change your method signature to be like this :
public void foo(final Callback callback) { // make your method, which was previously returning something, return void, and add in the new callback interface.
next up, wherever you previously wanted to use those values, add this line :
callback.myResponseCallback(yourResponseObject);
as an example :
#Override
public void onSuccess(QuerySnapshot documentSnapshots) {
// create your object you want to return here
String bar = document.get("something").toString();
callback.myResponseCallback(bar);
})
now, where you were previously calling your method called foo:
foo(new Callback() {
#Override
public void myResponseCallback(YourReturnType result) {
//here, this result parameter that comes through is your api call result to use, so use this result right here to do any operation you previously wanted to do.
}
});
}
How do you do this for Kotlin ?
(as a basic example where you only care for a single result)
start off by changing your method signature to something like this:
fun foo(callback:(YourReturnType) -> Unit) {
.....
then, inside your asynchronous operation's result :
firestore.collection("something")
.document("document").get()
.addOnSuccessListener {
val bar = it.get("something").toString()
callback(bar)
}
then, where you would have previously called your method called foo, you now do this :
foo() { result->
// here, this result parameter that comes through is
// whatever you passed to the callback in the code aboce,
// so use this result right here to do any operation
// you previously wanted to do.
}
// Be aware that code outside the callback here will run
// BEFORE the code above, and cannot rely on any data that may
// be set inside the callback.
if your foo method previously took in parameters :
fun foo(value:SomeType, callback:(YourType) -> Unit)
you simply change it to :
foo(yourValueHere) { result ->
// here, this result parameter that comes through is
// whatever you passed to the callback in the code aboce,
// so use this result right here to do any operation
// you previously wanted to do.
}
these solutions show how you can create a method/function to return values from async operations you've performed through the use of callbacks.
However, it is important to understand that, should you not be interested in creating a method/function for these:
#Override
public void onSuccess(SomeApiObjectType someApiResult) {
// here, this `onSuccess` callback provided by the api
// already has the data you're looking for (in this example,
// that data would be `someApiResult`).
// you can simply add all your relevant code which would
// be using this result inside this block here, this will
// include any manipulation of data, populating adapters, etc.
// this is the only place where you will have access to the
// data returned by the api call, assuming your api follows
// this pattern
})
There's a particular pattern of this nature I've seen repeatedly, and I think an explanation of what's happening would help. The pattern is a function/method that calls an API, assigning the result to a variable in the callback, and returns that variable.
The following function/method always returns null, even if the result from the API is not null.
Kotlin
fun foo(): String? {
var myReturnValue: String? = null
someApi.addOnSuccessListener { result ->
myReturnValue = result.value
}.execute()
return myReturnValue
}
Kotlin coroutine
fun foo(): String? {
var myReturnValue: String? = null
lifecycleScope.launch {
myReturnValue = someApiSuspendFunction()
}
return myReturnValue
}
Java 8
private String fooValue = null;
private String foo() {
someApi.addOnSuccessListener(result -> fooValue = result.getValue())
.execute();
return fooValue;
}
Java 7
private String fooValue = null;
private String foo() {
someApi.addOnSuccessListener(new OnSuccessListener<String>() {
public void onSuccess(Result<String> result) {
fooValue = result.getValue();
}
}).execute();
return fooValue;
}
The reason is that when you pass a callback or listener to an API function, that callback code will only be run some time in the future, when the API is done with its work. By passing the callback to the API function, you are queuing up work, but the current function (foo() in this case) returns immediately before that work begins and before that callback code is run.
Or in the case of the coroutine example above, the launched coroutine is very unlikely to complete before the function that started it.
Your function that calls the API cannot return the result that is returned in the callback (unless it's a Kotlin coroutine suspend function). The solution, explained in the other answer, is to make your own function take a callback parameter and not return anything.
Alternatively, if you're working with coroutines, you can make your function suspend instead of launching a separate coroutine. When you have suspend functions, somewhere in your code you must launch a coroutine and handle the results within the coroutine. Typically, you would launch a coroutine in a lifecycle function like onCreate(), or in a UI callback like in an OnClickListener.
Other answer explains how to consume APIs based on callbacks by exposing a similar callbacks-based API in the outer function. However, recently Kotlin coroutines become more and more popular, especially on Android and while using them, callbacks are generally discouraged for such purposes. Kotlin approach is to use suspend functions instead. Therefore, if our application uses coroutines already, I suggest not propagating callbacks APIs from 3rd party libraries to the rest of our code, but converting them to suspend functions.
Converting callbacks to suspend
Let's assume we have this callback API:
interface Service {
fun getData(callback: Callback<String>)
}
interface Callback<in T> {
fun onSuccess(value: T)
fun onFailure(throwable: Throwable)
}
We can convert it to suspend function using suspendCoroutine():
private val service: Service
suspend fun getData(): String {
return suspendCoroutine { cont ->
service.getData(object : Callback<String> {
override fun onSuccess(value: String) {
cont.resume(value)
}
override fun onFailure(throwable: Throwable) {
cont.resumeWithException(throwable)
}
})
}
}
This way getData() can return the data directly and synchronously, so other suspend functions can use it very easily:
suspend fun otherFunction() {
val data = getData()
println(data)
}
Note that we don't have to use withContext(Dispatchers.IO) { ... } here. We can even invoke getData() from the main thread as long as we are inside the coroutine context (e.g. inside Dispatchers.Main) - main thread won't be blocked.
Cancellations
If the callback service supports cancelling of background tasks then it is best to cancel when the calling coroutine is itself cancelled. Let's add a cancelling feature to our callback API:
interface Service {
fun getData(callback: Callback<String>): Task
}
interface Task {
fun cancel();
}
Now, Service.getData() returns Task that we can use to cancel the operation. We can consume it almost the same as previously, but with small changes:
suspend fun getData(): String {
return suspendCancellableCoroutine { cont ->
val task = service.getData(object : Callback<String> {
...
})
cont.invokeOnCancellation {
task.cancel()
}
}
}
We only need to switch from suspendCoroutine() to suspendCancellableCoroutine() and add invokeOnCancellation() block.
Example using Retrofit
interface GitHubService {
#GET("users/{user}/repos")
fun listRepos(#Path("user") user: String): Call<List<Repo>>
}
suspend fun listRepos(user: String): List<Repo> {
val retrofit = Retrofit.Builder()
.baseUrl("https://api.github.com/")
.build()
val service = retrofit.create<GitHubService>()
return suspendCancellableCoroutine { cont ->
val call = service.listRepos(user)
call.enqueue(object : Callback<List<Repo>> {
override fun onResponse(call: Call<List<Repo>>, response: Response<List<Repo>>) {
if (response.isSuccessful) {
cont.resume(response.body()!!)
} else {
// just an example
cont.resumeWithException(Exception("Received error response: ${response.message()}"))
}
}
override fun onFailure(call: Call<List<Repo>>, t: Throwable) {
cont.resumeWithException(t)
}
})
cont.invokeOnCancellation {
call.cancel()
}
}
}
Native support
Before we start converting callbacks to suspend functions, it is worth checking whether the library that we use does support suspend functions already: natively or with some extension. Many popular libraries like Retrofit or Firebase support coroutines and suspend functions. Usually, they either provide/handle suspend functions directly or they provide suspendable waiting on top of their asynchronous task/call/etc. object. Such waiting is very often named await().
For example, Retrofit supports suspend functions directly since 2.6.0:
interface GitHubService {
#GET("users/{user}/repos")
suspend fun listRepos(#Path("user") user: String): List<Repo>
}
Note that we not only added suspend, but also we no longer return Call, but the result directly. Now, we can use it without all this enqueue() boilerplate:
val repos = service.listRepos(user)
TL;DR The code you pass to these APIs (e.g. in the onSuccessListener) is a callback, and it runs asynchronously (not in the order it is written in your file). It runs at some point later in the future to "call back" into your code. Without using a coroutine to suspend the program, you cannot "return" data retrieved in a callback from a function.
What is a callback?
A callback is a piece of code you pass to some third party library that it will run later when some event happens (e.g. when it gets data from a server). It is important to remember that the callback is not run in the order you wrote it - it may be run much later in the future, could run multiple times, or may never run at all. The example callback below will run Point A, start the server fetching process, run Point C, exit the function, then some time in the distant future may run Point B when the data is retrieved. The printout at Point C will always be empty.
fun getResult() {
// Point A
var r = ""
doc.get().addOnSuccessListener { result ->
// The code inside the {} here is the "callback"
// Point B - handle result
r = result // don't do this!
}
// Point C - r="" still here, point B hasn't run yet
println(r)
}
How do I get the data from the callback then?
Make your own interface/callback
Making your own custom interface/callback can sometimes make things cleaner looking but it doesn't really help with the core question of how to use the data outside the callback - it just moves the aysnc call to another location. It can help if the primary API call is somewhere else (e.g. in another class).
// you made your own callback to use in the
// async API
fun getResultImpl(callback: (String)->Unit) {
doc.get().addOnSuccessListener { result ->
callback(result)
}
}
// but if you use it like this, you still have
// the EXACT same problem as before - the printout
// will always be empty
fun getResult() {
var r = ""
getResultImpl { result ->
// this part is STILL an async callback,
// and runs later in the future
r = result
}
println(r) // always empty here
}
// you still have to do things INSIDE the callback,
// you could move getResultImpl to another class now,
// but still have the same potential pitfalls as before
fun getResult() {
getResultImpl { result ->
println(result)
}
}
Some examples of how to properly use a custom callback: example 1, example 2, example 3
Make the callback a suspend function
Another option is to turn the async method into a suspend function using coroutines so it can wait for the callback to complete. This lets you write linear-looking functions again.
suspend fun getResult() {
val result = suspendCoroutine { cont ->
doc.get().addOnSuccessListener { result ->
cont.resume(result)
}
}
// the first line will suspend the coroutine and wait
// until the async method returns a result. If the
// callback could be called multiple times this may not
// be the best pattern to use
println(result)
}
Re-arrange your program into smaller functions
Instead of writing monolithic linear functions, break the work up into several functions and call them from within the callbacks. You should not try to modify local variables within the callback and return or use them after the callback (e.g. Point C). You have to move away from the idea of returning data from a function when it comes from an async API - without a coroutine this generally isn't possible.
For example, you could handle the async data in a separate method (a "processing method") and do as little as possible in the callback itself other than call the processing method with the received result. This helps avoid a lot of the common errors with async APIs where you attempt to modify local variables declared outside the callback scope or try to return things modified from within the callback. When you call getResult it starts the process of getting the data. When that process is complete (some time in the future) the callback calls showResult to show it.
fun getResult() {
doc.get().addOnSuccessListener { result ->
showResult(result)
}
// don't try to show or return the result here!
}
fun showResult(result: String) {
println(result)
}
Example
As a concrete example here is a minimal ViewModel showing how one could include an async API into a program flow to fetch data, process it, and display it in an Activity or Fragment. This is written in Kotlin but is equally applicable to Java.
class MainViewModel : ViewModel() {
private val textLiveData = MutableLiveData<String>()
val text: LiveData<String>
get() = textLiveData
fun fetchData() {
// Use a coroutine here to make a dummy async call,
// this is where you could call Firestore or other API
// Note that this method does not _return_ the requested data!
viewModelScope.launch {
delay(3000)
// pretend this is a slow network call, this part
// won't run until 3000 ms later
val t = Calendar.getInstance().time
processData(t.toString())
}
// anything out here will run immediately, it will not
// wait for the "slow" code above to run first
}
private fun processData(d: String) {
// Once you get the data you may want to modify it before displaying it.
val p = "The time is $d"
textLiveData.postValue(p)
}
}
A real API call in fetchData() might look something more like this
fun fetchData() {
firestoreDB.collection("data")
.document("mydoc")
.get()
.addOnCompleteListener { task ->
if (task.isSuccessful) {
val data = task.result.data
processData(data["time"])
}
else {
textLiveData.postValue("ERROR")
}
}
}
The Activity or Fragment that goes along with this doesn't need to know anything about these calls, it just passes actions in by calling methods on the ViewModel and observes the LiveData to update its views when new data is available. It cannot assume that the data is available immediately after a call to fetchData(), but with this pattern it doesn't need to.
The view layer can also do things like show and hide a progress bar while the data is being loaded so the user knows it's working in the background.
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
val model: MainViewModel by viewModels()
// Observe the LiveData and when it changes, update the
// state of the Views
model.text.observe(this) { processedData ->
binding.text.text = processedData
binding.progress.visibility = View.GONE
}
// When the user clicks the button, pass that action to the
// ViewModel by calling "fetchData()"
binding.getText.setOnClickListener {
binding.progress.visibility = View.VISIBLE
model.fetchData()
}
binding.progress.visibility = View.GONE
}
}
The ViewModel is not strictly necessary for this type of async workflow - here is an example of how to do the same thing in the activity
class MainActivity : AppCompatActivity() {
private lateinit var binding: ActivityMainBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
// When the user clicks the button, trigger the async
// data call
binding.getText.setOnClickListener {
binding.progress.visibility = View.VISIBLE
fetchData()
}
binding.progress.visibility = View.GONE
}
private fun fetchData() {
lifecycleScope.launch {
delay(3000)
val t = Calendar.getInstance().time
processData(t.toString())
}
}
private fun processData(d: String) {
binding.progress.visibility = View.GONE
val p = "The time is $d"
binding.text.text = p
}
}
(and, for completeness, the activity XML)
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:id="#+id/text"
android:layout_margin="16dp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"/>
<Button
android:id="#+id/get_text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="16dp"
android:text="Get Text"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="#+id/text"
/>
<ProgressBar
android:id="#+id/progress"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="48dp"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="#+id/get_text"
/>
</androidx.constraintlayout.widget.ConstraintLayout>