I have been trying to create a decorator for a lambda function in Kotlin. The requirements for the decorator are following:
The decorator must be able to REPEAT the process passed in the form of lambda function
The decorator must be able to TIMEOUT after a certain time which will be passed along in the decorator.
This is what I have been able to come up so far:
inline fun <T, R> testDecorator(input: T,
timesToRepeat: Int,
timeout: Long,
crossinline work: (T) -> R): R {
repeat(timesToRepeat - 1) {
try {
return runBlocking {
withTimeout(timeout) {
work(input)
}
} catch (ex: Exception) {
// Deal with exception
}
}
return work(input)
}
Right now, the work function will only be making a http request but later on I plan to add some more work.
Sample work:
fun sampleWork() {
val client = HttpClient.newBuilder().build();
val request = HttpRequest.newBuilder() .uri(URI.create("some url")) .build();
val response = client.send(request, HttpResponse.BodyHandlers.ofString());
// Parse respone
return parsedResponse;
}
The problem is that the timeout is not working as expected and the function only ends after all the processes in the lambda have ended. Can someone tell me what am I missing here?
(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>
I'm making a call to the API and the response body is assigned to an object inside Retrofit's enqueue(), the problem is that enqueue finishes too quickly for the value to be assigned before the return statement of the function body is called.
Previously, I was using MutableLiveData before and it took care of that because it's always observing the data and when it changes it assigns it with no problem but now I don't want to use any MutableLiveData or Observables because I'm trying to prepare the data before any UI is actually drawn on the screen.
fun getResponse(
weatherLocationCoordinates: WeatherLocation
): RequestResponse {
weatherApiService.getCurrentWeather(
weatherLocationCoordinates.latitude,
weatherLocationCoordinates.longitude
).enqueue(object : Callback<WeatherResponse> {
override fun onResponse(
call: Call<WeatherResponse>,
response: Response<WeatherResponse>
) {
if (response.isSuccessful) {
// This where I do the assigning
requestResponse = RequestResponse(response.body(), true)
}
}
override fun onFailure(call: Call<WeatherResponse>, t: Throwable) {
requestResponse = RequestResponse(null, false)
}
})
// When this is called, enqueue is still not finished
// therefore I get the wrong value, I get the previously set initialization value of the obj.
return requestResponse
}
Should I be using Callbacks or something else? I'm not sure on how to implement the callback.
Following up on the comments here's an approach with callbacks:
Let's suppose we change the method signature to:
fun getResponse(
weatherLocationCoordinates: WeatherLocation,
onSuccess: (WeatherResponse) -> Unit = {},
onError: (Throwable) -> Unit = {}
) {
weatherApiService.getCurrentWeather(
weatherLocationCoordinates.latitude,
weatherLocationCoordinates.longitude
).enqueue(object : Callback<WeatherResponse> {
override fun onResponse(
call: Call<WeatherResponse>,
response: Response<WeatherResponse>
) {
if (response.isSuccessful) {
onSuccess(response.body())
} else {
onError(CustomHttpExceptionWithErrorDescription(response))
}
}
override fun onFailure(call: Call<WeatherResponse>, t: Throwable) {
onError(t)
}
})
}
CustomHttpExceptionWithErrorDescription will have to be something you code that can simply parse the error gotten from the server. Anything that is not 2XX status code
This method accepts 2 extra parameters - one gets called upon success the other on error. The idea is to call it like:
getResponse(
weatherLocationCoordinates,
onSuccess = {
// do something with response
},
onError = {
// do something with the error
}
)
Because they have default parameters you actually don't need to specify both callbacks. Just the one you are interested in. Examples:
// react only to successes
getResponse(
weatherLocationCoordinates,
onSuccess = {
// do something with response
}
)
// react only to errors
getResponse(weatherLocationCoordinates) {
// do something with the error
}
// just call the network calls and don't care about success or error
getResponse(weatherLocationCoordinates)
So I have some asynchronous operations happening, I can create some lambada, call a function and pass that value to them. But what i want is not to have the result of the operation as a parameter, I want to return them.
As a example, I have a class A with some listeners, if there is a result all listeners are notified. So basically the asyncFunction should return a result if there is one otherwise be suspended.
object A {
val listeners = mutableListOf<(Int) -> Unit>()
fun onResult(value: Int) {
listeners.forEach { it(value) }
}
}
fun asyncFunction(): Deferred<Int> {
return async {
A.listeners.add({ result ->
})
return result
}
}
What I'm thinking right now (maybe I'm completely on the wrong track), is to have something like a Deferred, to which i can send the result and it returns. Is there something like that? Can I implement a Deffered myself?
class A {
private val awaiter: ??? // can this be a Deferred ?
fun onResult(result: Int) {
awaiter.putResult(result)
}
fun awaitResult(): Int {
return awaiter.await()
}
}
val a = A()
launch {
val result = a.awaitResult()
}
launch {
a.onResult(42)
}
So I do know that with callbacks this can be handled but it would be cleaner and easier to have it that way.
I hope there is a nice and clean solution im just missing.
Your asyncFunction should in fact be a suspendable function:
suspend fun suspendFunction(): Int =
suspendCoroutine { cont -> A.listeners.add { cont.resume(it) } }
Note that it returns the Int result and suspends until it's available.
However, this is just a fix for your immediate problem. It will still malfunction in many ways:
the listener's purpose is served as soon as it gets the first result, but it stays in the listener list forever, resulting in a memory leak
if the result arrived before you called suspendFunction, it will miss it and hang.
You can keep improving it manually (it's a good way to learn) or switch to a solid solution provided by the standard library. The library solution is CompletableDeferred:
object A {
val result = CompletableDeferred<Int>()
fun provideResult(r: Int) {
result.complete(r)
}
}
suspend fun suspendFunction(): Int = A.result.await()
I create a mock of a class with mockk.
On this mock I now call a method that gets a lambda as a parameter.
This lambda serves as a callback to deliver state changes of the callback to the caller of the method.
class ObjectToMock() {
fun methodToCall(someValue: String?, observer: (State) -> Unit) {
...
}
}
How do I configure the mock to call the passed lambda?
You can use answers:
val otm: ObjectToMock = mockk()
every { otm.methodToCall(any(), any())} answers {
secondArg<(String) -> Unit>().invoke("anything")
}
otm.methodToCall("bla"){
println("invoked with $it") //invoked with anything
}
Within the answers scope you can access firstArg, secondArg etc and get it in the expected type by providing it as a generic argument. Note that I explicitly used invoke here to make it more readable, it may also be omitted.
I had to look for a bit more example for the callback and found some example in Kotlin Test with Mockk. In my case, it's a bit more specific.
I wanted to check and mock the onFailure and onSuccess case of a a custom callback implementation MyCustomCallback implementing the ListenableFutureCallback.
The code would look like that for my ExampleProducer class that would have a send function:
fun send(data: String) {
val responseFuture = kafkaTemplate.send(topic, data)
responseFuture.addCallback(MyCustomCallback())
}
So here who would the test go:
#Test
fun onFailureTest() {
kafkaTemplate: KafkaTemplate<String, String> = mockk()
val captureCallback = slot<ListenableFutureCallback<SendResult<String, String>>>()
every { callback.addCallback(capture(captureCallback)) } answers {
captureCallback.captured.onFailure(Throwable())
}
every { kafkaTemplate.send(any()) } returns callback
val prod: ExampleProducer = ExampleProducer()
prod.send("test")
// Then you can verify behaviour or check your captureCallback.captured
verify { kafkaTemplate.send(any()) }
assertNotNull(captureCallback.captured)
}
Maybe not exactly what you ask about, but you can use the funciton type for the mock:
val observerMock = mockk<(State) -> Unit>()