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Persist states after viewModelLifecyle till the app get closed

I'm a beginner android developer and I got stuck with an issue.
viewModel keeps the states change data related to a fragment and when the user navigate to some fragment then the viewModel no longer keeps the states.
What I want is that when the user change something in the fragment and after that navigate to other fragment and then come back to the fragment where he changes something, he should see the changes he made.
Solutions are there for storing ex. shared_preference & Room, but my requirement is that I only want to keep the state data alive until the use closed the App.
I've googling so far but still didn't understand the approach developers follow to do these things. So can you also provide some guidance on approaches I should follow that consider the best practices. My main motive is to learn.
<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto">
<data>
<variable
name="homeViewModel"
type="com.android.example.presentation.home.HomeViewModel" />
</data>
<FrameLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".presentation.home.HomeFragment">
<androidx.constraintlayout.widget.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:keepScreenOn="true">
<TextView
android:id="#+id/sampleText"
android:text="#{String.valueOf(homeViewModel.sampleText)}"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.5" />
<Button
android:id="#+id/sampleButton"
android:onClick="#{() -> homeViewModel.samplePressed()}"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="32dp"
android:text="Button"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="#+id/sampleText" />
</androidx.constraintlayout.widget.ConstraintLayout>
</FrameLayout>
</layout>
class HomeViewModel : ViewModel() {
private val _sampleText = MutableLiveData(0)
val sampleText : LiveData<Int>
get() = _sampleText
fun samplePressed() {
_sampleText.postValue(1)
}
}

Thanks to the comment from Vivek Gupta
This can be achieved by scoping the ViewModel to the activity's lifecycle
previous
class HomeFragment : Fragment() {
private val viewModel: HomeViewModel by viewModels()
}
Now change it this
class HomeFragment : Fragment() {
private val viewModel: HomeViewModel by activityViewModels()
}
Edit: another way to do this if we have viewModelFactory
class HomeFragment : Fragment() {
private lateinit var binding: FragmentHomeBinding
private lateinit var mainRepository: MainRepository
private lateinit var viewModel: HomeViewModel
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?) : View {
binding = DataBindingUtil.inflate(inflater, R.layout.fragment_home, container, false)
return binding.root
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
mainRepository = MainRepository(RetrofitInstance.api)
val viewModelFactory = HomeViewModelFactory(mainRepository)
viewModel = ViewModelProvider(requireActivity(), viewModelFactory).get(HomeViewModel::class.java)
binding.homeViewModel = viewModel
binding.lifecycleOwner = viewLifecycleOwner
}
}

How to pass multiple views as parameters in BindingAdapter in Data binding android?

I have one relative layout and one ImageView. I want to set visibility based on Image loading like if image loads successfully then imageview is visible and if some error occurs relative layout is visible. How can I manage this scenario in data binding using BindingAdapter ?

Your question is not clear so I don't know if it will help you.
These are steps to implement
1: Create parameters in BindingAdapter.kt
#BindingAdapter("showLoading")
fun View.showLoading(loading: Boolean) {
if (loading) {
visible()
} else {
gone()
}
}
#BindingAdapter("showError")
fun View.showError(error: Boolean) {
if (error) {
visible()
} else {
gone()
}
}
fun View.gone() {
this.visibility = View.GONE
}
fun View.visible() {
this.visibility = View.VISIBLE
}
2: Use parameters [app:showLoading="#{viewModel.showLoading}"] and [app:showError="#{viewModel.showError}"] created in file XML
<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto">
<data>
<variable
name="viewModel"
type="com.xxx.xxx.MainViewModel" />
</data>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#EAEAE2"
android:orientation="vertical">
<Button
android:id="#+id/btnAdd"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="ShowLoading" />
<ProgressBar
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:showLoading="#{viewModel.showLoading}" />
<RelativeLayout
android:id="#+id/viewError"
android:layout_width="100dp"
android:layout_height="100dp"
android:background="#color/colorPrimary"
app:showError="#{viewModel.showError}" />
</LinearLayout>
</layout>
3: Create showError and showLoading variables in Viewmodel.
And assign the viewModel variable to the binding.
class MainViewModel: ViewModel() {
val showError: MutableLiveData<Boolean> = MutableLiveData()
val showLoading: MutableLiveData<Boolean> = MutableLiveData()
init {
showError.postValue(false)
showLoading.postValue(true)
}
}
class MainActivity : AppCompatActivity() {
private val viewModel = MainViewModel()
private lateinit var binding: ActivityMainBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
binding.lifecycleOwner = this
binding.viewModel = viewModel
initViews()
}
private fun initViews() {
binding.btnAdd.setOnClickListener {
viewModel.showError.postValue(true)
viewModel.showLoading.postValue(false)
}
}
}

DataBinding doesn't update value when changing LiveData

I want the value to increase when I press the button and write it in the textview.
However, the code below shows the increasing value as Log
But, on screen, the value in textview does not change.
I'm not sure what's wrong. Please let me know if you know.
Fragment.kt
class SettingFragment : Fragment(), View.OnClickListener {
companion object {
fun newInstance() = SettingFragment()
private val TAG = "SettingFragment"
}
private lateinit var viewModel: SettingViewModel
private lateinit var binding: FragmentSettingBinding
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
binding = DataBindingUtil.inflate(inflater, R.layout.fragment_setting, container, false)
return binding.root
}
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
viewModel = ViewModelProvider(this).get(SettingViewModel::class.java)
binding.lifecycleOwner = this
binding.button.setOnClickListener(this)
}
override fun onClick(v: View?) {
viewModel.increase()
Log.d(TAG, "Log Data" + viewModel.testInt.value)
}
}
FragmentViewModel
class SettingViewModel : ViewModel() {
val testInt: MutableLiveData<Int> = MutableLiveData()
init {
testInt.value = 0
}
fun increase() {
testInt.value = testInt.value?.plus(1)
}
}
fragment.xml
<?xml version="1.0" encoding="utf-8"?>
<layout>
<data>
<variable
name="viewModel"
type="com.example.ui.setting.SettingViewModel" />
</data>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".ui.setting.SettingFragment">
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="#{String.valueOf(viewModel.testInt)}" />
<Button
android:id="#+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
</LinearLayout>
</layout>

<variable
name="viewModel"
type="com.example.ui.setting.SettingViewModel" />
This viewModel variable doesn't bind to a certain variable programmatically. So, when the value incremented, it won't reflect to the layout.
To fix this set the fragment's viewModel to this value:
viewModel = ViewModelProvider(this).get(SettingViewModel::class.java)
binding.viewModel = viewModel // <<<<< Here is the change
binding.lifecycleOwner = this
binding.button.setOnClickListener(this)
Side note:
You shouldn't use onActivityCreated() as it's is deprecated as of API level 28; you can normally its part of code to onCreateView(). And you can check here for other alternatives.

Button Text Not Changing with ViewModel Changes

I'm trying to change the text of a button based on user interaction. The text is bound to a boolean variable in the view model and is supposed to observe changes in that variable. When the variable changes, the text is supposed to switch. But it's not.
<?xml version="1.0" encoding="utf-8"?>
<layout 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"
xmlns:bind="http://schemas.android.com/apk/res-auto">
<data>
<variable
name="userDataViewModel"
type="com.mysite.myapp.viewModels.UserDataViewModel" />
</data>
<androidx.constraintlayout.widget.ConstraintLayout
android:id="#+id/change_group_location_root"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".fragments.GroupLocationFragment">
<Button
android:id="#+id/select_location_button"
android:layout_width="match_parent"
android:layout_height="64dp"
android:text='#{userDataViewModel.userData.groupChanged ? "changed" : "not changed"}'/>
</androidx.constraintlayout.widget.ConstraintLayout>
</layout>
Here is the part of the fragment where I bind the view model:
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?): View? {
binding = DataBindingUtil.inflate(inflater, R.layout.fragment_group_location, container, false)
binding.lifecycleOwner = this
userDataViewModel = activity?.run {
ViewModelProviders
.of(this, UserDataViewModelFactory(prefs, userDataFetcherService))
.get(UserDataViewModel::class.java)
} ?: throw Exception("Invalid Activity")
userDataViewModel
.getUserData()
.observe(this, Observer {
binding.userDataViewModel = userDataViewModel
})
return binding.root
}
I can see that the boolean variable in the view model toggles to 'true', but the button text still says 'not changed'. I just don't see what I am overlooking.
EDIT
Adding in the ViewModel class in case the issue is there.
class UserDataViewModel(private val prefs: SharedPreferences): ViewModel() {
val userData: MutableLiveData<UserData> by lazy {
MutableLiveData<UserData>().also {
val userDataString = prefs.getString(UserData.USER_DATA_SHARED_PREFERENCE_KEY, "")
it.value = Gson().fromJson(userDataString, UserData::class.java)
}
}
fun getUserData(): LiveData<UserData> {
return userData
}
}

Might be an issue with a button text which you set.
<androidx.constraintlayout.widget.ConstraintLayout
android:id="#+id/change_group_location_root"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".fragments.GroupLocationFragment">
<Button
android:id="#+id/select_location_button"
android:layout_width="match_parent"
android:layout_height="64dp"
android:text='#{userDataViewModel.userData.groupChanged ? `changed` : `not changed`}'/>
change it with back quote (`) sign.

You can use string from string.xml also like below
<Button
android:id="#+id/select_location_button"
android:layout_width="match_parent"
android:layout_height="64dp"
android:text="#{userDataViewModel.userData.groupChanged ? #string/txt_changed : #string/txt_not/-changed}"

Android kotlin data-binding with fragments error:cannot find symbol import com.example.***.databinding.FragmentUserBindingImpl;

I am relatively new to kotlin and trying to build a project with data binding in some fragments.I have a fragment named UserFragment with a Recyclerview in it like this:
class UserFragment : Fragment() {
private lateinit var binding: FragmentUserBinding
private lateinit var viewModel: UserListViewModel
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
binding=DataBindingUtil.inflate(inflater,R.layout.fragment_user, container, false)
binding.userRecycler.layoutManager = LinearLayoutManager(context, RecyclerView.VERTICAL, false)
viewModel = ViewModelProviders.of(this).get(UserListViewModel::class.java)
viewModel.errorMessage.observe(this, Observer {
errorMessage -> if(errorMessage != null) showError(errorMessage) else hideError()
})
binding.mViewModel=viewModel
return binding.root
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
//start add activity
val i= Intent(activity,AddUserActivity::class.java)
userFab.setOnClickListener(View.OnClickListener {
startActivity(i)
})
}
private fun showError(#StringRes errorMessage:Int){
errorSnackbar = Snackbar.make(binding.root, errorMessage, Snackbar.LENGTH_INDEFINITE)
errorSnackbar?.setAction(R.string.retry, viewModel.errorClickListener)
errorSnackbar?.show()
}
private fun hideError(){
errorSnackbar?.dismiss()
}
}
and the xml layout file fragment_user.xml looks like this:
<?xml version="1.0" encoding="utf-8"?>
<layout
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">
<data>
<variable
name="mViewModel"
type="com.example.***.ui.User.UserListViewModel" />
</data>
<androidx.constraintlayout.widget.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<EditText
android:id="#+id/userDateEditText"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:hint="#string/pick_date"
android:background="#drawable/roundededittext"
android:layout_marginEnd="8dp"
app:layout_constraintEnd_toEndOf="parent"
android:layout_marginStart="8dp"
app:layout_constraintStart_toStartOf="parent"
android:layout_marginTop="8dp"
app:layout_constraintTop_toTopOf="parent"/>
<androidx.recyclerview.widget.RecyclerView
android:id="#+id/userRecycler"
android:layout_width="293dp"
android:layout_height="475dp" android:layout_marginTop="8dp"
app:layout_constraintTop_toBottomOf="#+id/userDateEditText"
app:adapter="#{viewModel.getUserListAdapter()}"
app:layout_constraintEnd_toEndOf="parent"
android:layout_marginEnd="8dp" app:layout_constraintStart_toStartOf="parent"
android:layout_marginStart="8dp" android:layout_marginBottom="8dp"
app:layout_constraintBottom_toBottomOf="parent"/>
<com.google.android.material.floatingactionbutton.FloatingActionButton
android:id="#+id/userFab"
android:layout_width="wrap_content"
android:layout_height="wrap_content" android:layout_marginBottom="48dp"
app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintEnd_toEndOf="parent"
android:layout_marginEnd="8dp"/>
<ProgressBar
style="?android:attr/progressBarStyle"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:mutableVisibility="#{viewModel.getLoadingVisibility()}"
android:id="#+id/userProgressBar" app:layout_constraintEnd_toEndOf="parent"
android:layout_marginEnd="140dp" app:layout_constraintStart_toStartOf="parent"
android:layout_marginStart="8dp" android:layout_marginTop="8dp"
app:layout_constraintTop_toTopOf="parent" android:layout_marginBottom="8dp"
app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintHorizontal_bias="0.804"
app:layout_constraintVertical_bias="0.499"/>
</androidx.constraintlayout.widget.ConstraintLayout>
</layout>
there is also a similar adapter class and item_user.xml:
class UserListAdapter : RecyclerView.Adapter<UserListAdapter.ViewHolder>() {
private lateinit var userModelList:List<UserModel>
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): UserListAdapter.ViewHolder {
val binding: ItemUserBinding = DataBindingUtil.inflate(LayoutInflater.from(parent.context), R.layout.item_user, parent, false)
return ViewHolder(binding)
}
override fun onBindViewHolder(holder: UserListAdapter.ViewHolder, position: Int) {
holder.bind(userModelList[position])
}
override fun getItemCount(): Int {
return if(::userModelList.isInitialized) userModelList.size else 0
}
fun updateUserList(userModelList:List<UserModel>){
this.userModelList = userModelList
notifyDataSetChanged()
}
class ViewHolder(private val binding: ItemUserBinding):RecyclerView.ViewHolder(binding.root){
private val viewModel = UserViewModel()
fun bind(userModel: UserModel){
viewModel.bind(userModel)
binding.viewModel =viewModel
}
}
}
the item-user.xml:
<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto">
<data>
<variable
name="viewModel"
type="com.example.***.ui.MyUser.UserViewModel" />
</data>
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:paddingLeft="16dp"
android:paddingRight="16dp">
<TextView
android:id="#+id/user_title"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:textStyle="bold"
app:mutableText="#{viewModel.getUserTitle()}"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="#+id/user_description"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
app:mutableText="#{viewModel.getUserDesc()}"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="#+id/user_title" />
</android.support.constraint.ConstraintLayout>
</layout>
note that databinding has been enabled in the gradle
and the very important issue here is that in both my fragment and the adapter
this line:
binding.viewModel =viewModel
reports a type mismatch like this:
Type mismatch.
Required:MyUser.UserListViewModel?
Found:com.example.***.ui.MyUser.UserListViewModel
and when i build my project the error is as follows:
error: cannot find symbol
import com.example.***.databinding.FragmentUserBindingImpl;

The first error is pretty explicit: binding.viewModel expects a nullable UserListViewModel? and get a non-nullable UserListViewModel(see kotlin null safety doc).
You can try something like this to get rid of it:
Declare your viewModel as
private var viewModel: UserListViewModel? = null
and set your binding this way:
viewModel?.let{binding.viewModel = it}
Concerning the second error, your declarations seems fine, but sometimes the Android Studio's cache get corrupted, try Invalidate Caches/Restart, it may help.

Type mismatch.
Required:MyUser.UserListViewModel?
Found:com.example.***.ui.MyUser.UserListViewModel
Basically the error is telling that your
binding.viewModel //is a nullable type and there for it expects a nullable
//type to be assigned as well
So just turn your view model into a nullable type by addind the ? simbol after its delacarion (note late init types do not allow nullable types). try it like this
private var viewModel: UserListViewModel? = null
About the second error data binding library need to compile in order to autogenerate the binding class, just rebuild the project and this error will be gone.
Please feel free to use this template as a base to avoid all that boilerplate code
#Retention(AnnotationRetention.RUNTIME)
#Target(AnnotationTarget.CLASS)
annotation class ContentView(#LayoutRes val id: Int)
fun ViewGroup.inflate(#LayoutRes layoutId: Int,
addContainer: Boolean = false): View {
return LayoutInflater.from(context).inflate(layoutId,this,addContainer)
}
#Suppress("UNCHECKED_CAST")
abstract class BaseFragment<Model : ViewModel, Binding : ViewDataBinding> : Fragment() {
/**
* Data binding class variable all view elements declared on the
* Xml file will be available within this instance if a view model
* Is required for the xml to work we will need to bind it on #onBindViewModel
*/
protected lateinit var binding: WeakReference<Binding?>
/**
* Fragments view model according to MVVM android architecture
* Each fragment class should have one , in order to facilitate
* Live Data and Binding library features, tho we can skip it
*/
protected lateinit var viewModel: WeakReference<Model?>
/**
* Here is where most likely you will get not null data , both binding and
* view model references can be destroyed by garbage collector
* If this application reaches low memory levels
*
* This optional method is used to bind the required view model inside the
* Xml file, this is optional to use tho is recommended
* Bind them by calling the view model binding.customViewModel = viewModel
*/
protected open fun onBindViewModel(viewModel: Model?, binding: Binding?) {}
/**
* There will be the occasion where custom params will be needed on view model's
* Constructor in this case will want to override the default creation #see ViewModelFactory
*/
protected open fun onCreateViewModel(modelType: Class<Model>): Model? = if (modelType != ViewModel::class.java)
ViewModelProviders.of(requireActivity()).get(modelType) else null
/**
* Here we will inherit view model and binding values based on the class
* Parameters and store them in global variables so any class extending
* From Base activity has access to binding and view model instances by default
*/
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
val layout = this.javaClass.annotations.find { it.annotationClass == ContentView::class } as ContentView?
?: throw RuntimeException("You annotate this class with #ContentView and provide a layout resource")
container?.let { binding = WeakReference(DataBindingUtil.bind(it.inflate(layout.id))!!) }
?: run { binding = WeakReference(DataBindingUtil.bind(inflater.inflate(layout.id, null))) }
viewModel = WeakReference(
onCreateViewModel(
(this.javaClass.genericSuperclass
as ParameterizedType).actualTypeArguments[0] as Class<Model>
)
)
setHasOptionsMenu(true)
onBindViewModel(viewModel.get(), binding.get())
return binding.get()?.root
}
}
And just use it like this (see how much boiler plate code is gone)
#ContentView(R.layout.fragment_user)
class UserFragment: BaseFragment<UserListViewModel, FragmentUserBinding> {
override fun onBindViewModel(viewModel: UserListViewModel?, binding: FragmentUserBinding?) {
binding.viewModel = viewModel
}
}