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Arraylist not assigned with the value on app start from api query [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>

Get user by userId in ROOM

I want to take event by id but it returns unit.
Entity:
#Dao
interface EventDao {
#Query("SELECT * FROM event_table WHERE eventId= :myeventId")
fun getEventByEventId(myeventId: Int): LiveData<Event>
}
Repository:
class EventRepository(private val eventDao: EventDao){
fun getEventById(myeventId: Int): LiveData<Event>{
return eventDao.getEventByEventId(myeventId = myeventId)
}
}
Viewmodel:
class EventViewModel (application: Application): AndroidViewModel(application) {
private val readEventById = MutableLiveData<LiveData<Event>>()
fun getEventById(eventId: Int) {
viewModelScope.launch(Dispatchers.IO) {
readEventById.postValue(eventRepository.getEventById(eventId))
}
}
}
I am calling it on user fragment:
lifecycleScope.launch {
val event = eventViewModel.getEventById(currentUserId)
}
but it returns unit. How can i return event by userId?

In your ViewModel class, you should include a public LiveData<Event> value that returns the readEventById live data object:
val selectedEvent: LiveData<Event>
get() = readEventById
Then, in the user Fragment, you should instead add an Observer on eventViewModel.selectedEvent. When you call eventViewModel.getEventById(currentUserId), you don't worry about the result. Instead, the Observer will let you know the LiveData<Event> was updated and you can handle the value this way.
This is the proper approach since you're getting the data from the database asynchronously.

We can use withContext for returning values from a coroutine scope, it is defination from documentation : "Calls the specified suspending block with a given coroutine context, suspends until it completes, and returns the result." for details visit the documentation withContext
in your case you can use like this
suspend fun getEventById(eventId: Int): LiveData<Event> {
return withContext(Dispatchers.IO) {
eventRepository.getEventById(eventId)
}
}

How to restrict function invocation to a particular thread/executor

I'm currently designing database for a mobile application. Recently I found very useful function to access database in background:
private val IO_EXECUTOR = Executors.newSingleThreadExecutor()
fun ioThread(f : () -> Unit) {
IO_EXECUTOR.execute(f)
}
Besides that I figured out that don't need synchronization code as the database will be accessed only in one thread (i.e the thread used by SingleThreadExecutor).
The only issue is that the following methods have to be restricted to be invoked only through ioThread function (or using IO_EXECUTOR).
abstract class MyDatabase : RoomDatabase() {
companion object {
fun init(context: Context) { ... }
fun getInstance() { ... }
}
Is it possible to achieve this in Kotlin/Java?
UPDATE: for now I have this implementation but think there should be better ones
// App.kt file
private val IO_EXECUTOR = Executors.newSingleThreadExecutor()
private var IO_THREAD_ID: Long = -1L
private fun getIOThreadId(): Long {
if (IO_THREAD_ID == -1L)
IO_THREAD_ID = IO_EXECUTOR.submit(Callable<Long> { Thread.currentThread().id }).get()
return IO_THREAD_ID
}
fun notInIOThread() = Thread.currentThread().id != getIOThreadId()
fun ioThread(f : () -> Unit) {
IO_EXECUTOR.execute(f)
}
and then use notInIOThread() in init() and getInstance() functions

If you absolutely need to make sure that the code is running on the correct thread, you could make use of a custom thread and then checking Thread.currentThread() for the interface.
private interface MarkedIOThread // Marker interface
private val IO_EXECUTOR = Executors.newSingleThreadExecutor { r ->
return object : Thread(r), MarkedIOThread
}
fun notInIOThread(): Boolean = Thread.currentThread() !is MarkedIOThread

Yes, you can use android annotations`s Worker Thread annotation.
When you annotate a method or class with #WorkerThread, android will give you lint errors if you call it from the UI thread.
You can read more about the #WorkerThread here: https://developer.android.com/reference/android/support/annotation/WorkerThread
And more about android annotations here: https://developer.android.com/studio/write/annotations
I would suggest that you should check room library: https://developer.android.com/topic/libraries/architecture/room
It is very powerful, if you don't have any specific reason to create a database library, room is your best bet.

I assume you want the functions to be called only inside ioThread code block, otherwise there'd be a type error. First make them member functions of a class with user-code-inaccessible constructor so others cannot call it directly:
class MyDslClass internal constructor() {
fun init(context: Context) { ... }
fun getInstance() { ... }
}
And ioThread should be:
fun ioThread(f : MyDslClass.() -> Unit) {
val dsl = MyDslClass()
IO_EXECUTOR.execute { dsl.f() }
}
Then you can restrict calls to those functions only inside ioThread block.
fun main(args: Array<String>) {
ioThread {
getInstance() // Ok
}
// cannot call `getInstance` since I cannot construct a `MyDslClass`
}

Fetching data from local and remote simultaneously using RxJava

So I'm a beginner with RxJava but here's what I want to accomplish:
MainViewModel talks to the Repository. Repository has both LocalDataStore (that talks with the database) and RemoteDataStore (Retrofit) Both are different implementations of interface DataStore).
What I want to achieve is have a single call fetchData from the Repository that returns an Observable but:
it takes it from the RemoteDataStore at first
after fetching every single thing (onNext()), it inserts it into the database
if it fails, it returns results from the LocalDataStore.
However, I don't know how to implement this logic. Subscription happens on the ViewModel's end, but I cannot really change the observable to LocalDataStore from Repository end (?). Upserting data into the database also returns an Observable (Single to be precise) and for it to work it needs a subscription.
Could someone explain it to me or point me in a good direction?
My code (problem in repository comments):
Remote data store
override fun getData(): Observable<SomeData> = api
.getData(token)
.flatMapIterable { x -> x }
Local data store
override fun saveData(data: SomeData): Single<SomeData> {
return database.upsert(data)
}
Repository
fun getData(): Observable<SomeData> {
return
remoteDataStore.getData()
.doOnError {
localDataStore.getData() //? this will invoke but nothing happens because I'm not subscribed to it
}
.doOnNext {
saveData(it) //The same as before, nothing will happen
}
}
ViewModel
override fun fetchData() {
repository.getData()
.observeOn(androidScheduler)
.subscribeOn(threadScheduler)
.subscribe(
{ data: SomeData ->
dataList.add(data)
},
{ throwable: Throwable? ->
handleError(throwable)
},
{
//send data to view
},
{ disposable: Disposable ->
compositeDisposable.add(disposable)
}
)
}
Thank you for your time.

You need to use one of onErrorResumeNext methods. I would also suggest to change your stream type from Observable to Single as nature of your data seems like Get data once or throw error. It's just a good API design.
In your particular case I would implement the repository this way:
class RepositoryImpl #Inject constructor(private val localRepository: Repository, private val remoteRepository: Repository) : Repository {
override fun getData(): Single<Data> = remoteRepository.getData()
.onErrorResumeNext { throwable ->
if (throwable is IOException) {
return localRepository.getData()
}
return Single.error(throwable)
}
}
You might ask why only catch IOException? I usually handle only this exception to not miss anything critical but only unimportant network errors. If you will catch every exception you might miss, for example, a NullPointerException.

onErrorResumeNext is what you're looking for. doOnError invokes a side-effecting action, doesn't replace the original Observable with another one.

Realm returning stale data

I don't have a lot of experience writing Android apps. For fun I am writing an app that will upload my call logs to my server. This entire app is running as a Service. The Service (starts when the phone is booted) is the one that registers the ContentObserver, which then calls my custom CallLog class. I use ContentObserver to listen for content change events. Unfortunately, the ContentObserver is called multiple times when I e.g. dial a number.
For that reason, I have function that I call after a successful upload (I use Retrofit) called markAsUploaded(). This function creates a RealmObject called CallLogUploaded (which is different from my regular CallLog model). This CallLogUploaded simply has one identifier which is the dateTime of the call, which should be unique enough. Then, when I am iterating through the list of all the call logs, I check every single call log against a isDataUploaded() function, which does a Realm query and checks to see if there is already a call log with that dateTime stored in the database (realm). In theory, it should work.
However, I have noticed that it does not always work. It seems that very often my data is stale. When I do realm.isAutoRefresh(), it returns false (although I swear it returned true once). In my isDataUploaded function, even when I do a findAll() on the Realm, I do not see all of my data - but the data definitely did hit the markDataAsUploaded function.
Here's my code - it's in Kotlin but should be easy to understand:
val callLogCall = service.sendCalLLogs(childId, dataToUpload)
callLogCall.enqueue(object : Callback<Void> {
override fun onResponse(call: Call<Void>, response: Response<Void>) {
if (response.isSuccessful) {
Log.i(AppConstants.LOG_TAG, "Call log data uploaded successfully!")
this#CallLogData.markDataAsUploaded(dataToUpload)
} else {
Log.w(AppConstants.LOG_TAG, "Call log data upload failed")
}
}
override fun onFailure(call: Call<Void>, t: Throwable) {
Log.w(AppConstants.LOG_TAG, "Call log data upload error (onFailure) called")
}
})
// This function simply stores a Realm model for all the data that has been uploaded to the server
private fun markDataAsUploaded(dataToUpload: List<CallLog>) {
realm = Realm.getDefaultInstance()
for (data in dataToUpload) {
realm.beginTransaction()
val callLogUploaded = realm.createObject(CallLogUploaded::class.java)
callLogUploaded.callDate = data.callDate
realm.commitTransaction()
}
}
// This function checks to see if the data is already uploaded.
private fun isDataUploaded(callLog: CallLog) : Boolean {
return realm
.where(CallLogUploaded::class.java)
.equalTo("callDate", callLog.callDate)
.count() > 0L
}
// Gets the call logs - not the entire function
for (call in callLogs) {
val callLog = CallLog()
callLog.id = call.id
callLog.callDate = Utilities.getTimestampAsSeconds(call.callDate)
if (this.isDataUploaded(callLog)) {
continue
}
callLog.name = call.name
callLog.number = call.number
}
I am very new to Realm and fairly new to Android development, so I would appreciate any help you can give me. Thanks!

That's because
// This function simply stores a Realm model for all the data that has been uploaded to the server
private fun markDataAsUploaded(dataToUpload: List<CallLog>) {
realm = Realm.getDefaultInstance()
for (data in dataToUpload) {
realm.beginTransaction()
val callLogUploaded = realm.createObject(CallLogUploaded::class.java)
callLogUploaded.callDate = data.callDate
realm.commitTransaction()
}
}
This method has quite a few errors that I had written about a long time ago
The Realm instance is opened, but never closed
There is a new transaction per every element, rather than inserting all elements in a single transaction
If you don't close the Realm instance (each instance requires its own close() call), then your Realm instance will never update unless you actually begin a transaction and do things inside the transaction.
You have three solutions:
1.) do your logic on the background thread inside a transaction, if there is nothing to be done, then cancel the transaction - queries made in transactions are never stale
2.) make sure the Realm instance is closed properly (although this is definitely a necessity on any non-autoupdating thread)
3.) a hacky solution is to call RealmRefresh.refreshRealm() after getDefaultInstance() according to my answer on Stack Overflow which relies on package-private API, but it works to solve this issue
Typically you need to open the Realm instance at the beginning of the thread, and close it at the end of the thread.
so, it's basically one big try(Realm realm = Realm.getDefaultInstance() { ... } for onHandleIntent().
enqueue(new Callback() { #Override public void onSuccess(..) {...} runs on the UI thread. To run it on the current thread, you should use call.execute().
instead of
for (data in dataToUpload) {
realm.beginTransaction()
val callLogUploaded = realm.createObject(CallLogUploaded::class.java)
callLogUploaded.callDate = data.callDate
realm.commitTransaction()
}
do
realm.beginTransaction()
for (data in dataToUpload) {
val callLogUploaded = realm.createObject(CallLogUploaded::class.java)
callLogUploaded.callDate = data.callDate
}
realm.commitTransaction()
In order to understand about version retention you can read https://medium.com/#Zhuinden/understanding-realm-version-retention-and-synchronization-9a513c2445bb .