Develop Reference

How to use the keyword also in kotlin android

Am learning android kotlin follow this:
https://developer.android.com/topic/libraries/architecture/viewmodel#kotlin
class MyViewModel : ViewModel() {
private val users: MutableLiveData<List<User>> by lazy {
MutableLiveData<List<User>>().also {
loadUsers(it)
}
}
fun getUsers(): LiveData<List<User>> {
return users
}
private fun loadUsers() {
// Do an asynchronous operation to fetch users.
}
}
Dont know how to write the fun loadUsers()
Here is my User:
class User {
constructor(name: String?) {
this.name = name
}
var name:String? = null
}
If dont use the keyword 'also' , i know how to do it.
But if use 'also' , it seems not work.
Here is how i try to write the fun loadUsers:
private fun loadUsers( it: MutableLiveData<List<User>>){
val users: MutableList<User> = ArrayList()
for (i in 0..9) {
users.add(User("name$i"))
}
it = MutableLiveData<List<User>>(users)
}
Error tips near it : Val cant be ressigned

Part 1: According to the Kotlin documentation, also provides the object in question to the function block as a this parameter. So, every function call and property object you access is implied to refer to your MutableLiveData<List<User>>() object. also returns this from the function block when you are done.
Thus, another way of writing your MutableLiveData<> would be like this:
val users = MutableLiveData<List<User>>()
users.loadUsers()
Part 2: As far as how to implement loadUsers(), that is a separate issue (your question is not clear). You can use Retrofit + RxJava to load the data asynchronously, and that operation is totally outside of the realm of ViewModel or also.
Part 3: With your approach, you have conflicting things going on. Instead of doing a loadUsers() from your lazy {} operation, I would remove your lazy {} operation and create a MutableLiveData<> directly. Then, you can load users later on and update the users property any time new data is loaded. Here is a similar example I worked on a while ago. It uses state flows, but the idea is similar. Also use a data class to model the User instead of a regular class. Another example.

It is solved change to code:
private fun loadUsers( it: MutableLiveData<List<User>>){
val users: MutableList<User> = ArrayList()
for (i in 0..9) {
users.add(User("name$i"))
}
it.value = users
}
it can't be reassigned , but it.value could .

How to call different api resource from paging source or remote mediator Kotlin

Hey I want to call two different api for my Paging Library 3. I want to ask what is best suit for me to use Paging Source or Remote Mediator?. What is the use case of both? Can someone please explain me.
For 1st api call only for single time
#GET("/movie?min=20")
Above api call returns this response
data class movie(
var id: Int?,
var name: String?,
var items : List<Genre>?
}
Now for 2nd api call its loop to call again and again
#GET("/movie?count=20&&before={time}")
Above api call retrun this
data class movie(
var items : List<Genre>?
}
Genre
data class Genre(
var type: String?,
var date: String?,
var cast: String?
}
Genre have data in both api call. I tried to google this and found this Example. But inside this both api return same data. But in my case both returns little bit different. Also id, name is only used in UI component else list will go to adapter. But I didn't understand how to achieved this. I am new in Flow, it too difficult to understand, to be honest I am trying to learning CodeLab. Another important thing when 1st time api call, in which the last item contains date will send to 2nd api call in time parameter and then 2nd api last item date call again 2nd api, this will go in loop. So how can I track this again in loop condition. Third I want to update data at top of list, can we store data in memory than we can update value on that list? Thanks for advance. Sorry for my wrong english.
UPDATE
After #dlam suggestion, I tried to practice some code
MainActivity
class MainActivity : AppCompatActivity() {
private lateinit var binding: ActivityMainBinding
private val viewModel by viewModels<ActivityViewModel>()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
lifecycleScope.launchWhenCreated {
viewModel.getMovie().collectLatest {
// setupAdapter()
}
}
}
}
ActivityViewModel
class ActivityViewModel(app: Application) : AndroidViewModel(app) {
fun getMovie(): Flow<PagingData<Genre>> {
return Pager(
config = PagingConfig(
pageSize = 20
),
pagingSourceFactory = {
MultiRequestPagingSource(DataSource())
}
).flow
}
}
MultiRequestPagingSource
class MultiRequestPagingSource(private val dataSource: DataSource) : PagingSource<String, Genre>() {
override fun getRefreshKey(state: PagingState<String, Genre>): String? {
return state.anchorPosition?.let { anchorPosition ->
state.closestPageToPosition(anchorPosition)?.nextKey
}
}
override suspend fun load(params: LoadParams<String>): LoadResult<String, Genre> {
val key = params.key ?: ""
return try {
val data = when (params) {
is LoadParams.Refresh -> {
dataSource.fetchInitialMovie()
}
is LoadParams.Append -> {
dataSource.fetchMovieBefore(key)
}
is LoadParams.Prepend -> null
}
LoadResult.Page(
data = data.result,
prevKey = null,
nextKey = data?.nextKey,
)
} catch (exception: IOException) {
LoadResult.Error(exception)
}
}
}
I am getting error on data = data.result
Type mismatch.
Required:
List<TypeVariable(Value)>
Found:
ArrayDeque<Genre>?
DataSource
package com.example.multirequestpaging
class DataSource {
data class MovieResult(
val result: ArrayDeque<Genre>?,
val nextKey: String?
)
fun fetchInitialMovie(): MovieResult {
val response = ApiInterface.create().getMovieResponse(20)
return MovieResult(
addInArrayDeque(response),
response.items?.last()?.date
)
}
fun fetchMovieBefore(key: String): MovieResult {
val response = ApiInterface.create().getMovieResponseBefore(20, key)
return MovieResult(
addInArrayDeque(response),
response.items?.last()?.date
)
}
private fun addInArrayDeque(response: MovieResponse): ArrayDeque<Genre> {
val result: ArrayDeque<Genre> = ArrayDeque()
response.items?.forEach {
result.add(it)
}
return result
}
}
For Full code Project Link
1. I want to add an item to the top of the list. How can I use invalidate function? Sorry I didn't understand where I can use.
2. I want to use id,name in other place so how can i get those variable value in my activity class.
3. Is my code structure is good?. Do I need to improved, please give an example. It will also help beginner, who is learning Paging Library.
Thanks

PagingSource is the main driver for Paging, it's responsible for loading items that get displayed and represents the single source of truth of data.
RemoteMediator is for layered sources, it is essentially a callback which triggers when PagingSource runs out of data, so you can fetch from a secondary source. This is primarily useful in cases where you fetching from both DB + Network, where you want locally cached data to power Paging, and then use RemoteMediator as a callback to fetch more items into the cache from network.
In this scenario you have two APIs, but they both fetch from the same Network source, so you only need PagingSource here. If I'm understanding correctly, you essentially want to call the first API on initial load and the second API on subsequent prepend / append page loads, which you can check / switch on by the type of LoadParams you get. See the subtypes here: https://developer.android.com/reference/kotlin/androidx/paging/PagingSource.LoadParams

Race Condition with LiveData

TL;DR of this Question:
Is it possible that a LiveData with a backing property (MutableLiveData) inside the ViewModel that is used to Observe and Add to an
ArrayList can have a race condition and using Synchronized or a Lock is required?
Provided that the ArrayList will get its value from a callback
I am trying to setup a group video call using the Agora Android SDK. I followed the documentation here. The problem is with the callbacks (onUserJoined, onUserOffline) from IRtcEngineEventHandler.
OnUserJoined Callback
mRtcEngine = RtcEngine.create(baseContext, APP_ID, object : IRtcEngineEventHandler() {
override fun onUserJoined(uid: Int, elapsed: Int) {
// onUserJoined callback is called anytime a new remote user joins the channel
super.onUserJoined(uid, elapsed)
// We mute the stream by default so that it doesn't consume unnecessary bandwidth
mRtcEngine?.muteRemoteVideoStream(uid, true)
// We are using a lock since uidList is shared and there can be race conditions
lock.lock()
try {
// We are using uidList to keep track of the UIDs of the remote users
uidList.add(uid)
} finally {
lock.unlock()
}
onUserOffline Callback
override fun onUserOffline(uid: Int, reason: Int) {
// onUserOffline is called whenever a remote user leaves the channel
super.onUserOffline(uid, reason)
// We use toRemove to inform the RecyclerView of the index of item we are removing
val toRemove: Int
// We are using a lock since uidList is shared and there can be race conditions
lock.lock()
try {
// We are fetching the index of the item we are about to remove and then remove the item
toRemove = uidList.indexOf(uid)
uidList.remove(uid)
} finally {
lock.unlock()
}
Here Lock is being used to access the uidlist in a thread safe way to prevent race condition. It was working for me when I followed the docs exactly, but when I tried to use a LiveData with a backing property (MutableLiveData) in a ViewModel for saving the uidlist, the observer on uidlist always returned an empty list.
My ViewModel
class MainViewModel: ViewModel() {
private val _uidList: MutableLiveData<ArrayList<Int>> = MutableLiveData()
val uidList: LiveData<ArrayList<Int>> get() = _uidList
init {
_uidList.value = ArrayList<Int>()
}
fun addToUserList(uid: Int) {
_uidList.value?.add(uid)
Log.d("adding user ","$uid")
}
fun removeFromUserList(uid: Int) {
_uidList.value?.remove(_uidList.value!!.indexOf(uid))
}
}
I am calling addToUserList() inside onUserJoined() and removeFromUserList() inside the onUserOffline()
Please guide me to the solution to this problem,
Thank you

You should not be mutating the value stored in LiveData, you'll get very strange behaviour. You have to swap the value out completely.
I'm feeling somewhat lazy so I will just give you the answer.
class MainViewModel: ViewModel() {
private val _uidList: MutableLiveData<List<Int>> = MutableLiveData()
val uidList: LiveData<List<Int>> get() = _uidList
init {
_uidList.value = emptyList<Int>()
}
fun addToUserList(uid: Int) {
_uidList.value = (_uidList.value ?: emptyList()) + uid
Log.d("adding user ","$uid")
}
fun removeFromUserList(uid: Int) {
val value = _uidList.value?.toMutableList()
if (value == null) return
value.remove(value.indexOf(uid))
_uidList.value = value
}

Firestore and Unicode

The documentation describes that Firestore supports Unicode. You just need to insert already formatted text into Firestore. But when unloading, the following are not taken into account:
Line break;
Unicode characters inserted directly into the text (eg \u000a).
The code is below.
Repository
suspend fun getData(): Response<List<Model>> =
suspendCoroutine { cont ->
val collection =
firestore
.collection(COLLECTION_NAME)
.whereEqualTo(DEFAULT_CONDITION_FIELD, DEFAULT_CONDITION_VALUE)
.orderBy(SORT_FIELD, SORT_DIRECTION)
.get()
collection
.addOnSuccessListener { query ->
val data = arrayListOf<Model>()
query.toObjects(ModelDomain::class.java).forEach { data.add(it.toModel()) }
cont.resume(Response.Success(data))
}
.addOnFailureListener { cont.resume(Response.Error(it)) }
}
ViewModel
private val _data: LiveData<Response<List<Model>>> = loadData()
val data get() = _data
private fun loadData(): LiveData<Response<List<Model>>> =
liveData(Dispatchers.IO) {
emit(Response.Loading)
try {
emit(repository.getData())
} catch (e: Exception) {
emit(Response.Error(e))
}
}
Model
data class ModelDomain(
var description: String = ""
) : KoinComponent {
fun toModel() =
Model(
description = description
)
}
data class Model(
val description: String
)
Part of the code has been omitted.
UPDATE
Just wrote in Notepad ++:
Copied this to Firestore:
Result:

Firestore does not, in any way, modify data that you write to it. If you write something to a document, then read the document, you will get exactly the same data that you put into it.
If you're looking at the document in the Firebase console, you will not see all carriage returns and whitespace. Those are collapsed to save space on screen when rendering large amounts of data. But if you read the data programmatically, it will definitely be exactly as you wrote it.

How insert image in room persistence library?

I am using room persistence library for my android application, Now I have to insert image in my db. I successfully define #Entity for the primitive data type. and also through converter class, i stored all object, date, time. Now I have to store Image. I am not able to understand how we define Column info and entity and how we insert that data as well as read data from the table.
What is the maximum size of data which inserted into the single row? What is max and min size of data in one field in Android SQLite?

It is usually not recommended to store image data into the database.
But however if it is required for your project then you can do so.
Image data are usually stored into db using BLOB data type, Room also provide support for BLOB data type Documentation
You can declare your entity class as mentioned below to store Image data.
#Entity(tableName = "test")
public class Test{
#PrimaryKey
#ColumnInfo(name = "_id")
private int id;
#ColumnInfo(typeAffinity = ColumnInfo.BLOB)
private byte[] image;
}

As Pinakin mentioned, it is not recommended to store an image into database and file path would be better but if it is required to store image I would suggest compress the image to below 2 MB (here is an example) to avoid breaking app. Room supports BLOB for image.
Entity class in kotlin:
ImageTest.kt
#Entity
class ImageTest {
#PrimaryKey(autoGenerate = true)
var id: Int = 1
#ColumnInfo(typeAffinity = ColumnInfo.BLOB)
var data: ByteArray? = null
}
ImageDao.kt
#Dao
interface ImageTestDao {
#Insert(onConflict = OnConflictStrategy.REPLACE)
fun upsertByReplacement(image: List<ImageTest>)
#Query("SELECT * FROM image")
fun getAll(): List<ImageTest>
#Query("SELECT * FROM image WHERE id IN (:arg0)")
fun findByIds(imageTestIds: List<Int>): List<ImageTest>
#Delete
fun delete(imageTest: ImageTest)
}
Databse.kt
import android.arch.persistence.room.Database
import android.arch.persistence.room.RoomDatabase
import android.arch.persistence.room.TypeConverters
#Database(entities = arrayOf(ImageTest::class), version = 1)
#TypeConverters(DataConverters::class)
abstract class Database : RoomDatabase() {
abstract fun getImageTestDao(): ImageTestDao
}
In DatabaseHelper something like
class DatabaseHelper(context: Context) {
init {
DatabaseHelper.context = WeakReference(context)
}
companion object {
private var context: WeakReference<Context>? = null
private const val DATABASE_NAME: String = "image_test_db"
private var singleton: Database? = null
private fun createDatabase(): Database {
return Room.databaseBuilder(context?.get() ?:
throw IllegalStateException("initialize by calling
constructor before calling DatabaseHelper.instance"),
Database::class.java,
DATABASE_NAME)
.build()
}
val instance: Database
#Synchronized get() {
if (null == singleton)
singleton = createDatabase()
return singleton as Database
}
fun setImage(img: Bitmap){
val dao = DatabaseHelper.instance.getImageTestDao()
val imageTest = ImageTest()
imageTest.data = getBytesFromImageMethod(image)//TODO
dao.updsertByReplacement(imageTest)
fun getImage():Bitmap?{
val dao = DatabaseHelper.instance.getImageTestDao()
val imageByteArray = dao.getAll()
return loadImageFromBytes(imageByteArray[0].data)
//change accordingly
}
Correct me if I am wrong. Hope this helps someone out there

Save the image as a file and save the file path Uri to Room
As seen in CameraX's image capture use case, when a photo is successfully taken, the File path reference Uri, savedUri, can be retrieved safely.
Then, the Uri can be converted to a string with savedUri.toString(), and saved to Room.
It's important to ensure the Room file reference is also updated if the file is moved or deleted.
The image String saved in Room may need to be converted back into a Uri to be displayed with an image library such as Glide with Uri.parse(someString).
In the CameraX sample, an image path's Uri can safely be obtained in onImageSaved.
It would then be saved into Room off of the main thread using Kotlin Coroutines or RxJava, preferably in a ViewModel or somewhere that handles the business logic separate from the view logic.
Getting Started with CameraX > 5. Implement ImageCapture use case
private fun takePhoto() {
// Get a stable reference of the modifiable image capture use case
val imageCapture = imageCapture ?: return
// Create time-stamped output file to hold the image
val photoFile = File(
outputDirectory,
SimpleDateFormat(FILENAME_FORMAT, Locale.US
).format(System.currentTimeMillis()) + ".jpg")
// Create output options object which contains file + metadata
val outputOptions = ImageCapture.OutputFileOptions.Builder(photoFile).build()
// Set up image capture listener, which is triggered after photo has
// been taken
imageCapture.takePicture(
outputOptions, ContextCompat.getMainExecutor(this), object : ImageCapture.OnImageSavedCallback {
override fun onError(exc: ImageCaptureException) {
Log.e(TAG, "Photo capture failed: ${exc.message}", exc)
}
override fun onImageSaved(output: ImageCapture.OutputFileResults) {
val savedUri = Uri.fromFile(photoFile)
val msg = "Photo capture succeeded: $savedUri"
Toast.makeText(baseContext, msg, Toast.LENGTH_SHORT).show()
Log.d(TAG, msg)
}
})
}
This strategy is outlined in Saving image in Room database on Reddit.
Cloud Storage
Creating a file for the image and saving the file path in Room covers local storage. In order to ensure the images are saved across multiple devices or when if data cache and data are cleared, a form of Cloud Storage is needed to upload the files to and to download and sync with the local storage.