Kotlin extension function with Generic doesn't work with Any? - android

I want to have an extension function from the RealmList class:
private inline fun RealmList<Any?>.saveAll() {
this.forEach {
item -> Realm.getDefaultInstance().insert(item!! as RealmModel)
}
}
But whenever I use it, this error appears:

To achieve that add out to generic declaration of your extension function.
It would work if such declaration was stated in the RealmList
private inline fun RealmList<out Any?>.saveAll() {
this.forEach {
item -> Realm.getDefaultInstance().insert(item!! as RealmModel)
}
}

Your code is generally unsafe. Please fix your code, read the documentation, stuff like that.
Also, RealmList expects ? extends RealmModel, so you need to use T: RealmModel with out.
fun <T: RealmModel> RealmList<out T?>.saveAll() {
Realm.getDefaultInstance().use { realm ->
val wasInTransaction = realm.isInTransaction()
try {
if(!wasInTransaction) {
realm.beginTransaction()
}
this.forEach {
item -> item?.let { realm.insert(it) }
}
if(!wasInTransaction) {
realm.commitTransaction()
}
} catch(e: Throwable) {
if(realm.isInTransaction()) {
realm.cancelTransaction()
}
}
}
}

Maybe you should use Realms method to insert instead of performing your loop? That way your extension becomes a simple call:
fun <T: RealmModel> RealmList<out T?>.saveAll() = Realm.getDefaultInstance().insert(this)

Related

UseCases or Interactors with Kt Flow and Retrofit

Context
I started working on a new project and I've decided to move from RxJava to Kotlin Coroutines. I'm using an MVVM clean architecture, meaning that my ViewModels communicate to UseCases classes, and these UseCases classes use one or many Repositories to fetch data from network.
Let me give you an example. Let's say we have a screen that is supposed to show the user profile information. So we have the UserProfileViewModel:
#HiltViewModel
class UserProfileViewModel #Inject constructor(
private val getUserProfileUseCase: GetUserProfileUseCase
) : ViewModel() {
sealed class State {
data SuccessfullyFetchedUser(
user: ExampleUser
) : State()
}
// ...
val state = SingleLiveEvent<UserProfileViewModel.State>()
// ...
fun fetchUserProfile() {
viewModelScope.launch {
// ⚠️ We trigger the use case to fetch the user profile info
getUserProfileUseCase()
.collect {
when (it) {
is GetUserProfileUseCase.Result.UserProfileFetched -> {
state.postValue(State.SuccessfullyFetchedUser(it.user))
}
is GetUserProfileUseCase.Result.ErrorFetchingUserProfile -> {
// ...
}
}
}
}
}
}
The GetUserProfileUseCase use case would look like this:
interface GetUserProfileUseCase {
sealed class Result {
object ErrorFetchingUserProfile : Result()
data class UserProfileFetched(
val user: ExampleUser
) : Result()
}
suspend operator fun invoke(email: String): Flow<Result>
}
class GetUserProfileUseCaseImpl(
private val userRepository: UserRepository
) : GetUserProfileUseCase {
override suspend fun invoke(email: String): Flow<GetUserProfileUseCase.Result> {
// ⚠️ Hit the repository to fetch the info. Notice that if we have more
// complex scenarios, we might require zipping repository calls together, or
// flatmap responses.
return userRepository.getUserProfile().flatMapMerge {
when (it) {
is ResultData.Success -> {
flow { emit(GetUserProfileUseCase.Result.UserProfileFetched(it.data.toUserExampleModel())) }
}
is ResultData.Error -> {
flow { emit(GetUserProfileUseCase.Result.ErrorFetchingUserProfile) }
}
}
}
}
}
The UserRepository repository would look like this:
interface UserRepository {
fun getUserProfile(): Flow<ResultData<ApiUserProfileResponse>>
}
class UserRepositoryImpl(
private val retrofitApi: RetrofitApi
) : UserRepository {
override fun getUserProfile(): Flow<ResultData<ApiUserProfileResponse>> {
return flow {
val response = retrofitApi.getUserProfileFromApi()
if (response.isSuccessful) {
emit(ResultData.Success(response.body()!!))
} else {
emit(ResultData.Error(RetrofitNetworkError(response.code())))
}
}
}
}
And finally, the RetrofitApi and the response class to model the backend API response would look like this:
data class ApiUserProfileResponse(
#SerializedName("user_name") val userName: String
// ...
)
interface RetrofitApi {
#GET("api/user/profile")
suspend fun getUserProfileFromApi(): Response<ApiUserProfileResponse>
}
Everything has been working fine so far, but I've started to run into some issues when implementing more complex features.
For example, there's a use case where I need to (1) post to a POST /send_email_link endpoint when the user first signs in, this endpoint will check if the email that I send in the body already exists, if it doesn't it will return a 404 error code, and (2) if everything goes okay, I'm supposed to hit a POST /peek endpoint that will return some info about the user account.
This is what I've implemented so far for this UserAccountVerificationUseCase:
interface UserAccountVerificationUseCase {
sealed class Result {
object ErrorVerifyingUserEmail : Result()
object ErrorEmailDoesNotExist : Result()
data class UserEmailVerifiedSuccessfully(
val canSignIn: Boolean
) : Result()
}
suspend operator fun invoke(email: String): Flow<Result>
}
class UserAccountVerificationUseCaseImpl(
private val userRepository: UserRepository
) : UserAccountVerificationUseCase {
override suspend fun invoke(email: String): Flow<UserAccountVerificationUseCase.Result> {
return userRepository.postSendEmailLink().flatMapMerge {
when (it) {
is ResultData.Success -> {
userRepository.postPeek().flatMapMerge {
when (it) {
is ResultData.Success -> {
val canSignIn = it.data?.userName == "Something"
flow { emit(UserAccountVerificationUseCase.Result.UserEmailVerifiedSuccessfully(canSignIn)) }
} else {
flow { emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail) }
}
}
}
}
is ResultData.Error -> {
if (it.exception is RetrofitNetworkError) {
if (it.exception.errorCode == 404) {
flow { emit(UserAccountVerificationUseCase.Result.ErrorEmailDoesNotExist) }
} else {
flow { emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail) }
}
} else {
flow { emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail) }
}
}
}
}
}
}
Issue
The above solution is working as expected, if the first API call to the POST /send_email_link ever returns a 404, the use case will behave as expected and return the ErrorEmailDoesNotExist response so the ViewModel can pass that back to the UI and show the expected UX.
The problem as you can see is that this solution requires a ton of boilerplate code, I thought using Kotlin Coroutines would make things simpler than with RxJava, but it hasn't turned out like that yet. I'm quite sure that this is because I'm missing something or I haven't quite learned how to use Flow properly.
What I've tried so far
I've tried to change the way I emit the elements from the repositories, from this:
...
override fun getUserProfile(): Flow<ResultData<ApiUserProfileResponse>> {
return flow {
val response = retrofitApi.getUserProfileFromApi()
if (response.isSuccessful) {
emit(ResultData.Success(response.body()!!))
} else {
emit(ResultData.Error(RetrofitNetworkError(response.code())))
}
}
}
...
To something like this:
...
override fun getUserProfile(): Flow<ResultData<ApiUserProfileResponse>> {
return flow {
val response = retrofitApi.getUserProfileFromApi()
if (response.isSuccessful) {
emit(ResultData.Success(response.body()!!))
} else {
error(RetrofitNetworkError(response.code()))
}
}
}
..
So I can use the catch() function like I'd with RxJava's onErrorResume():
class UserAccountVerificationUseCaseImpl(
private val userRepository: UserRepository
) : UserAccountVerificationUseCase {
override suspend fun invoke(email: String): Flow<UserAccountVerificationUseCase.Result> {
return userRepository.postSendEmailLink()
.catch { e ->
if (e is RetrofitNetworkError) {
if (e.errorCode == 404) {
flow { emit(UserAccountVerificationUseCase.Result.ErrorEmailDoesNotExist) }
} else {
flow { emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail) }
}
} else {
flow { emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail) }
}
}
.flatMapMerge {
userRepository.postPeek().flatMapMerge {
when (it) {
is ResultData.Success -> {
val canSignIn = it.data?.userName == "Something"
flow { emit(UserAccountVerificationUseCase.Result.UserEmailVerifiedSuccessfully(canSignIn)) }
} else -> {
flow { emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail) }
}
}
}
}
}
}
}
This does reduce the boilerplate code a bit, but I haven't been able to get it working because as soon as I try to run the use case like this I start getting errors saying that I shouldn't emit items in the catch().
Even if I could get this working, still, there's way too much boilerplate code here. I though doing things like this with Kotlin Coroutines would mean having much more simple, and readable, use cases. Something like:
...
class UserAccountVerificationUseCaseImpl(
private val userRepository: AuthRepository
) : UserAccountVerificationUseCase {
override suspend fun invoke(email: String): Flow<UserAccountVerificationUseCase.Result> {
return flow {
coroutineScope {
val sendLinksResponse = userRepository.postSendEmailLink()
if (sendLinksResponse is ResultData.Success) {
val peekAccount = userRepository.postPeek()
if (peekAccount is ResultData.Success) {
emit(UserAccountVerificationUseCase.Result.UserEmailVerifiedSuccessfully())
} else {
emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail)
}
} else {
if (sendLinksResponse is ResultData.Error) {
if (sendLinksResponse.error == 404) {
emit(UserAccountVerificationUseCase.Result.ErrorEmailDoesNotExist)
} else {
emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail)
}
} else {
emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail)
}
}
}
}
}
}
...
This is what I had pictured about working with Kotlin Coroutines. Ditching RxJava's zip(), contact(), delayError(), onErrorResume() and all those Observable functions in favor of something more readable.
Question
How can I reduce the amount of boilerplate code and make my use cases look more Coroutine-like?
Notes
I know some people just call the repositories directly from the ViewModel layer, but I like having this UseCase layer in the middle so I can contain all the code related to switching streams and handling errors here.
Any feedback is appreciated! Thanks!
Edit #1
Based on #Joffrey response, I've changed the code so it works like this:
The Retrofit API layer keeps returning suspendable function.
data class ApiUserProfileResponse(
#SerializedName("user_name") val userName: String
// ...
)
interface RetrofitApi {
#GET("api/user/profile")
suspend fun getUserProfileFromApi(): Response<ApiUserProfileResponse>
}
The repository now returns a suspendable function and I've removed the Flow wrapper:
interface UserRepository {
suspend fun getUserProfile(): ResultData<ApiUserProfileResponse>
}
class UserRepositoryImpl(
private val retrofitApi: RetrofitApi
) : UserRepository {
override suspend fun getUserProfile(): ResultData<ApiUserProfileResponse> {
val response = retrofitApi.getUserProfileFromApi()
return if (response.isSuccessful) {
ResultData.Success(response.body()!!)
} else {
ResultData.Error(RetrofitNetworkError(response.code()))
}
}
}
The use case keeps returning a Flow since I might also plug calls to a Room DB here:
interface GetUserProfileUseCase {
sealed class Result {
object ErrorFetchingUserProfile : Result()
data class UserProfileFetched(
val user: ExampleUser
) : Result()
}
suspend operator fun invoke(email: String): Flow<Result>
}
class GetUserProfileUseCaseImpl(
private val userRepository: UserRepository
) : GetUserProfileUseCase {
override suspend fun invoke(email: String): Flow<GetUserProfileUseCase.Result> {
return flow {
val userProfileResponse = userRepository.getUserProfile()
when (userProfileResponse) {
is ResultData.Success -> {
emit(GetUserProfileUseCase.Result.UserProfileFetched(it.toUserModel()))
}
is ResultData.Error -> {
emit(GetUserProfileUseCase.Result.ErrorFetchingUserProfile)
}
}
}
}
}
This looks much more clean. Now, applying the same thing to the UserAccountVerificationUseCase:
interface UserAccountVerificationUseCase {
sealed class Result {
object ErrorVerifyingUserEmail : Result()
object ErrorEmailDoesNotExist : Result()
data class UserEmailVerifiedSuccessfully(
val canSignIn: Boolean
) : Result()
}
suspend operator fun invoke(email: String): Flow<Result>
}
class UserAccountVerificationUseCaseImpl(
private val userRepository: UserRepository
) : UserAccountVerificationUseCase {
override suspend fun invoke(email: String): Flow<UserAccountVerificationUseCase.Result> {
return flow {
val sendEmailLinkResponse = userRepository.postSendEmailLink()
when (sendEmailLinkResponse) {
is ResultData.Success -> {
val peekResponse = userRepository.postPeek()
when (peekResponse) {
is ResultData.Success -> {
val canSignIn = peekResponse.data?.userName == "Something"
emit(UserAccountVerificationUseCase.Result.UserEmailVerifiedSuccessfully(canSignIn)
}
else -> {
emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail)
}
}
}
is ResultData.Error -> {
if (sendEmailLinkResponse.isNetworkError(404)) {
emit(UserAccountVerificationUseCase.Result.ErrorEmailDoesNotExist)
} else {
emit(UserAccountVerificationUseCase.Result.ErrorVerifyingUserEmail)
}
}
}
}
}
}
This looks much more clean and it works perfectly. I still wonder if there's any more room for improvement here.
The most obvious problem I see here is that you're using Flow for single values instead of suspend functions.
Coroutines makes the single-value use case much simpler by using suspend functions that return plain values or throw exceptions. You can of course also make them return Result-like classes to encapsulate errors instead of actually using exceptions, but the important part is that with suspend functions you are exposing a seemingly synchronous (thus convenient) API while still benefitting from asynchronous runtime.
In the provided examples you're not subscribing for updates anywhere, all flows actually just give a single element and complete, so there is no real reason to use flows and it complicates the code. It also makes it harder to read for people used to coroutines because it looks like multiple values are coming, and potentially collect being infinite, but it's not the case.
Each time you write flow { emit(x) } it should just be x.
Following the above, you're sometimes using flatMapMerge and in the lambda you create flows with a single element. Unless you're looking for parallelization of the computation, you should simply go for .map { ... } instead. So replace this:
val resultingFlow = sourceFlow.flatMapMerge {
if (something) {
flow { emit(x) }
} else {
flow { emit(y) }
}
}
With this:
val resultingFlow = sourceFlow.map { if (something) x else y }

How to implement loadstate with adapter, recyclerView?

I would like to implement a feature like loadStateFlow in Paging 3.
I do not use pagination in my implementation and it is not necessary in my case.
Could I make it another way?
I have found something like LoadingStateAdapter library
https://developer.android.com/reference/kotlin/androidx/paging/LoadStateAdapter
For now I get a list using method in the fragment:
private fun collectNotificationItems() {
vm.notificationData.collectWith(viewLifecycleOwner) {
notificationAdapter.items = it
}
}
This is implementation I would like to achieve, example is in paging3:
private fun collectItems() {
vm.items.collectWith(viewLifecycleOwner, adapter::submitData)
adapter.loadStateFlow.collectWith(viewLifecycleOwner) { loadState ->
vm.setLoadingState(loadState.refresh is LoadState.Loading)
val isEmpty =
loadState.source.refresh is LoadState.NotLoading && loadState.append.endOfPaginationReached && archiveAdapter.itemCount < 1
vm.setEmptyStateVisible(isEmpty)
}
}
Where methods are:
in ViewModel
fun setLoadingState(isLoading: Boolean) {
_areShimmersVisible.value = isLoading && !_isSwipingToRefresh.value
if (!isLoading) _isSwipingToRefresh.value = false
}
areShimmers and isSwiping are MutableStateFlow
Could you recommend any other options?
EDIT:
I have the whole implementation a little bit different.
I have use case to make it
class GetListItemDetailsUseCase #Inject constructor(private val dao: Dao): BaseFlowUseCase<Unit, List<ItemData>>() {
override fun create(params: Unit): Flow<List<ItemData>> {
return flow{
emit(dao.readAllData())
}
}
}
For now it looks like the code above.
How to use DateState in that case?
EDIT2:
class GetNotificationListItemDetailsUseCase #Inject constructor(private val notificationDao: NotificationDao): BaseFlowUseCase<Unit, DataState<List<NotificationItemsResponse.NotificationItemData>>>() {
override fun create(params: Unit): Flow<DataState<List<NotificationItemsResponse.NotificationItemData>>> {
return flow{
emit(DataState.Loading)
try {
emit(DataState.Success(notificationDao.readAllDataState()))
} catch(e: Exception) {
emit(DataState.Error(e)) // error, and send the exception
}
}
}
}
DAO
#Query("SELECT * FROM notification_list ORDER BY id ASC")
abstract suspend fun readAllDataState(): DataState<List<NotificationItemsResponse.NotificationItemData>>
/\ error beacause of it:
error: Not sure how to convert a Cursor to this method's return type
fragment
private suspend fun collectNotificationItems() {
vm.notificationData.collectLatest { dataState ->
when(dataState) {
is DataState.Error -> {
collectErrorState()
Log.d("collectNotificationItems", "Collect ErrorState")
}
DataState.Loading -> {
Log.d("collectNotificationItems", "Collect Loading")
}
is DataState.Success<*> -> {
vm.notificationData.collectWith(viewLifecycleOwner) {
notificationAdapter.items = it
notificationAdapter.notifyDataSetChanged()
Log.d("collectNotificationItems", "Collect Sucess")
}
}
}
}
You could use a utility class (usually called DataState or something like that).
sealed class DataState<out T> {
data class Success<out T>(val data: T) : DataState<T>()
data class Error(val exception: Exception) : DataState<Nothing>()
object Loading : DataState<Nothing>()
}
Then, you change your flow's return type from Flow<YourObject> to Flow<DataState<YourObject>> and emit the DataStates within a flow {} or channelFlow {} block.
val notificationsFlow: Flow<DataState<YourObject>> get() = flow {
emit(DataState.Loading) // when you collect, you will receive this DataState telling you that it's loading
try {
// networking/database stuff
emit(DataState.Success(yourResultObject))
} catch(e: Exception) {
emit(DataState.Error(e)) // error, and send the exception
}
}
Finally, just change your collect {} to be like:
notificationsFlow.collectLatest { dataState ->
when(dataState) {
is DataState.Error -> { } // error occurred, deal with it here
DataState.Loading -> { } // it's loading, show progress bar or something
is DataState.Success -> { } // data received from the flow, access it with dataState.data
}
}
For more information on this regard, check this out.

Analogue of Kotlin Iterable.map but for single object

I can transform a list with map in one line like this:
override suspend fun getAllTxtFile(): List<TxtFileModel> {
return someDao.getAllTxtFile().map { with(txtFileDataMapper) { it.fromEntityToDomain() } }
}
But I don't know how to do it for single class object:
override suspend fun getTxtFile(txtFileName: String?): TxtFileModel {
val txtFile = someDao.getTxtFile(txtFileName)
val txtFileModel = with(txtFileDataMapper) { txtFile.fromEntityToDomain() }
return txtFileModel
}
So, the difference is that for list I can return a value in one line but for a single object only in 3 lines.
That is not big deal but I wonder if exist some anologue for .map but for a single object. I want something like .mapSingleObject:
override suspend fun getTxtFile(txtFileName: String?): TxtFileModel {
return someDao.getTxtFile(txtFileName).mapSingleObject{ with(txtFileDataMapper) { it.fromEntityToDomain() } }
}
You can use some functions from Kotlin standard library, such as let, run, or with.
If function fromEntityToDomain() is a TxtFile extension function defined within the txtFileDataMapper's class, you can use functions mentioned above:
override suspend fun getTxtFile(txtFileName: String?): TxtFileModel? =
someDao.getTxtFile(txtFileName)?.let {
with(txtFileDataMapper) { it.fromEntityToDomain() }
}

Not able to understand viewmodel example on android dev site

class MyViewModel : ViewModel() {
private val users: MutableLiveData<List<User>> by lazy {
MutableLiveData().also {
loadUsers()
}
}
fun getUsers(): LiveData<List<User>> {
return users
}
private fun loadUsers() {
// Do an asynchronous operation to fetch users.
}
}
Am trying to implement this way and its not compiling :
class MyViewModel : ViewModel() {
private val users: MutableLiveData<List<String>> by lazy {
return MutableLiveData().also {
loadUsers()
}
}
fun getUsers(): LiveData<List<String>> {
return users
}
private fun loadUsers() {
users.postValue(listOf("Tarun", "Chawla"))
}
}
Mostly am not understanding the by lazy here. The example on android website seems wrong as loadUsers() is not returning anything which can be a delegate for
users. can you please help me understanding above piece of code.
=======================================================
This is how I implemented:
private val users : MutableLiveData<List<String>> by lazy {
MutableLiveData<List<String>>().also {
loadUsers(it)
}
}
init {
Log.e("Tarund", "View Model created")
}
override fun onCleared() {
super.onCleared()
Log.e("Tarund", "View Model deleted")
}
fun getUsers(): LiveData<List<String>> {
return users
}
private fun loadUsers(users : MutableLiveData<List<String>>) {
users.postValue(listOf("Tarun", "Chawla"))
}
}
But if anyone can confirm if first example code above which I copy pasted from : https://developer.android.com/topic/libraries/architecture/viewmodel#kotlin is wrong
The code in the Android documentation is wrong.
The lazy construction itself is fine: loadUsers() doesn't need to return anything because the function also is defined as:
inline fun <T> T.also(block: (T) -> Unit): T
that means that in here:
private val sources: String by lazy {
String().also {
loadSources()
}
}
the block also {} will return the empty String created with String() that can be assigned using lazy initialization to the val users
The error trying to compile the Android docs code is:
Type inference failed: Not enough information to infer parameter T in constructor MutableLiveData()
that means that the compiler is not able to infer the type of the MutableLiveData instance created using the constructor wihtout type.
Without the apply block the compiler will be able to compile it because it can easily infer the type from the val definition:
private val sources: MutableLiveData<List<User>> by lazy {
MutableLiveData()
}
but adding the apply block goes back to the generic type and the compiler cannot infer it. So the solution, as you did, is specifying the type hold in the MutableLiveData container:
private val sources: MutableLiveData<List<User>> by lazy {
MutableLiveData<List<User>>().also {
loadSources()
}
}

Kotlin how to achieve android setOnClickListener like syntax

In Android+Kotlin both of the following syntaxes are valid
view.setOnClickListener(this) and view.setOnClickListener {} I have been trying to achieve this syntax for my custom functional interface without any success. Here is what I have tried so far
interface OnClickListener {
fun onClick(str: String)
}
class Test {
fun setClickListener(listener: OnClickListener) {
listener.onClick("")
}
}
fun main() {
val test = Test()
test.setClickListener { str ->
}
}
in the above snippet code in main() fails to compile as it needs an anonymous object of OnClickListener class. How can I achieve the same syntax for client code as with the original OnClickListener?
This is not possible if you are using Kotlin only. You can achieve this by using #FunctionalInterface of JAVA. No matter how inconsistent it seems, its by design in language.
It seems that SAM conversion only works for Java interfaces. According to the forum discussion this is by design: Kotlin has functional types and everyone should just use them. While the argument is perfectly reasonable the restriction does feel a bit inconsistent.
Please check this link
What I like to do is:
inline fun View.onClick(crossinline clickListener: (View) -> Unit) {
this.setOnClickListener {
clickListener(this)
}
}
Now you can call
button.onClick {
// handle click
}
EDIT: ah, I misunderstood your question. The answer is
interface OnClickListener {
fun onClick(target: Any)
}
class Test {
private var onClickListener: OnClickListener? = null
fun setOnClickListener(listener: OnClickListener?) {
this.onClickListener = listener
}
fun doClick() {
onClickListener?.onClick(this)
}
}
inline fun Test.setOnClickListener(crossinline listener: (Test) -> Unit) {
this.setOnClickListener(object: OnClickListener {
override fun onClick(target: Any) {
listener.invoke(target as Test)
}
})
}
fun main() {
val test = Test()
test.setOnClickListener { t ->
println("hello")
}
test.doClick()
}
Use
fun setClickListener(listener: (String) -> Unit)
and then call listener("some string")

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