I'm implementing the viewModel and for communicate between the viewModel and fragment I'm doing this :
public class SplashViewModel extends AndroidViewModel {
private LiveData<Boolean> actions;
public SplashViewModel(#NonNull Application application) {
super(application);
actions= new MutableLiveData<>();
}
public void aViewModelMethod() {
//doing some stuff
if (stuff == X){
//I need to hide a view for exemple, I'm doing this
actions.postValue(true);
}
}
Now inside my Fragment I have an observable who is trigger when actions.postValue(true) is reached
viewModel.actions.observe(getViewLifecycleOwner(), new Observer<Boolean>() {
#Override
public void onChanged(Boolean action) {
if (action){
databinding.myView.setVisibility(View.VISIBLE);
}
}
});
This work fine but if I have a lot of communication I need to implement each time a new variable, and observe it ?
It's ok when they are 4 or 5 but what I am suppose to do when they are hundreds ?
I try to change boolean by an integer with a switch and a list of actions, but when the viewModel is initialize it's possible that several postValue are trigger and when I created the observable I'm only get the last one, that make sense.
Usually, I have two observable live data in my view model. First is represent the state of the whole screen. Second I use for "single-shot" events like toasts, navigation, showing dialogs.
My view model:
class PinCreateViewModel(...) : ViewModel() {
val event = MutableLiveData<BaseEvent<String>>()
val state = MutableLiveData<PinCreateViewState>()
}
I have a single state object for the whole screen:
sealed class PinCreateViewState {
object FirstInput : PinCreateViewState()
data class SecondInput(val firstEnteredPin: String) : PinCreateViewState()
object Error : PinCreateViewState()
object Loading : PinCreateViewState()
}
I think with this approach it's easy to think about my screen states, easy to design my screen as a finite state machine, and easy to debug. Especially, I like this approach to very complex screens. In this case, I have a single source of truth for my whole screen state.
But sometimes I want to show dialogs, toast or open new screens. These things are not part of my screen state. And this is why I want to handle them separately. And in this case, I use Events:
sealed class BaseEvent(private val content: String) {
var hasBeenHandled = false
private set
fun getContentIfNotHandled(): String? {
return if (hasBeenHandled) {
null
} else {
hasBeenHandled = true
content
}
}
fun peekContent(): String = content
}
class ErrorEvent(content: String) : BaseEvent(content)
class MessageEvent(content: String) : BaseEvent(content)
And my Fragment interaction with ViewModel looks like this:
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
observe(viewModel.event, this::onEvent)
observe(viewModel.state, this::render)
}
private fun render(state: PinCreateViewState) {
when (state) {
PinCreateViewState.FirstInput -> setFirstInputState()
is PinCreateViewState.SecondInput -> setSecondInputState()
PinCreateViewState.Error -> setErrorState()
PinCreateViewState.Loading -> setLoadingState()
}
}
fun onEvent(event: BaseEvent<String>) {
event.getContentIfNotHandled()?.let { text ->
when (event) {
is MessageEvent -> showMessage(text)
is ErrorEvent -> showError(text)
}
}
}
I really like Kotlin Sealed classes because it forces me to handle all possible cases. And I can find unhandled states even before compilation.
PostValue method Posts a task to the main thread to set the given value. If you called this method multiple times before a main thread executed a posted task, only the last value would be dispatched.
If you will have hundreds of communication between your fragment and viewModel so you just have to deduce your fragment logic like if you have to show view on some conditions then just observe one non mutable live data in your fragment and use two live data's mutable and another non mutable .... use non mutable to set that boolean on every sort of stuff and checks in your viewModel and in the beginning assign that live data to your non mutable one.
private val _liveData = MutableLiveData<Boolean>()
internal val liveData: LiveData<Boolean> = _liveData
It's the better approach ,i hope i understand you question better if not please elaborate it more so that i can help .
Related
In my application I want update data with SharedFlow and my application architecture is MVI .
I write below code, but just update one of data!
I have 2 spinners and this spinners data fill in viewmodel.
ViewModel code :
class MyViewModel #Inject constructor(private val repository: DetailRepository) : ViewModel() {
private val _state = MutableStateFlow<MyState>(MyState.Idle)
val state: StateFlow<MyState> get() = _state
fun handleIntent(intent: MyIntent) {
when (intent) {
is MyIntent.CategoriesList -> fetchingCategoriesList()
is MyIntent.PriorityList -> fetchingPrioritiesList()
}
}
private fun fetchingCategoriesList() {
val data = mutableListOf(Car, Animal, Color, Food)
_state.value = DetailState.CategoriesData(data)
}
private fun fetchingPrioritiesList() {
val data = mutableListOf(Low, Normal, High)
_state.value = DetailState.PriorityData(data)
}
}
With below codes I filled spinners in fragment :
lifecycleScope.launch {
//Send
viewModel.handleIntent(MyIntent.CategoriesList)
viewModel.handleIntent(MyIntent.PriorityList)
//Get
viewModel.state.collect { state ->
when (state) {
is DetailState.Idle -> {}
is DetailState.CategoriesData -> {
categoriesList.addAll(state.categoriesData)
categorySpinner.setupListWithAdapter(state.categoriesData) { itItem ->
category = itItem
}
Log.e("DetailLog","1")
}
is DetailState.PriorityData -> {
prioritiesList.addAll(state.prioritiesData)
prioritySpinner.setupListWithAdapter(state.prioritiesData) { itItem ->
priority = itItem
}
Log.e("DetailLog","2")
}
}
When run application not show me number 1 in logcat, just show number 2.
Not call this line : is DetailState.CategoriesData
But when comment this line viewModel.handleIntent(MyIntent.PriorityList) show me number 1 in logcat!
Why when use this code viewModel.handleIntent(MyIntent.CategoriesList) viewModel.handleIntent(MyIntent.PriorityList) not show number 1 and 2 in logcat ?
The problem is that a StateFlow is conflated, meaning if you rapidly change its value faster than collectors can collect it, old values are dropped without ever being collected. Therefore, StateFlow is not suited for an event-like system like this. After all, it’s in the name that it is for states rather than events.
It’s hard to suggest an alternative because your current code looks like you shouldn’t be using Flows at all. You could simply call a function that synchronously returns data that you use synchronously. I don’t know if your current code is a stepping stone towards something more complicated that really would be suitable for flows.
I'm using kotlin with MVVM pattern on my project.
In my viewModel I need to generate an error message and toast while input a text.
The error messages are like:
<string name="error_message">%s: is too long</string>
<string name="error_message1">name can't be empty</string>
viewModel.kt
func checkInput(context: Context, text: String) {
if (text.isEmpty()) {
Toast.make(context.getString(R.string.error_message1).show()
}
if (text.length > 10) {
Toast.make(String.format(context.getString(R.string.error_message1), maxNumber).show()
}
}
In the way above it works. But in my knowledge viewModel shouldn't hold any Android UI stuff that is context shouldn't be there. (Correct me if I'm wrong)
If there are no combine strings with variable, then I can return Int, that is return R.string.xxx directly. But now I don't know the proper way to do it.
I know subclass AndroidViewModel then I don't needs to care about context. I don't want to use this way. Because I needs localized my app with Activity's context, I think application context can't achieve it.
How to deal with this situation? thanks!
You have to generate an event type which you need to listen to your ui component (Activity/Fragment) from there you need to display error msg or toast.
only logic part will be remain in ViewModel and display related work goes to ui component.
To achieve this try using Sealed class for this.
sealed class ScreenState {
object Loading : ScreenState()
object Success: ScreenState()
object showToast(var type: String) : ScreenState()
}
wrap this inside live data
private val _screenState = MutableLiveData<ScreenState?>()
val screenStateLiveData: LiveData<ScreenState?> = _screenState
inside your ViewModel
func checkInput(text: String) {
if (text.isEmpty()) {
_screenState.postvalue(ScreenState.showToast("1"))
}
if (text.length > 10) {
_screenState.postvalue(ScreenState.showToast("2"))
}
}
observe the liveData screenStateLiveData in your ui component and display error based on your showToast() type.
private fun observeScreenStateData() {
mViewModel.screenStateLiveData.observe(mContext as FragmentActivity, Observer {
it?.let {
when (it) {
is LiveTvData.Loading -> {}
is LiveTvData.Success ->. {}
is LiveTvData.showToast -> displayErrorMsg(it.type)
}
})
}
First of all why do you need to place checkInput() in ViewModel? You get the final string from the view which is the Activity/Fragment class validate the string and then store it ViewModel.
You can just move your checkInput() to the Activity/Fragment class and validate from there. You can also do the other way around (Store in viewmodel -> Observe from the Activity/Fragment-> then )
But in my knowledge viewModel shouldn't hold any Android UI stuff
that is context shouldn't be there. (Correct me if I'm wrong)
You are right here! For more understanding of why refer here and here
Thanks #Ankit 's answer, it give me the hint, sealed interface is the workaround, that is very like enum in swift, that can pass parameter.
sealed interface ValidateError {
data class NormalError(val resId: Int): ValidateError
data class ParameterError(val resId: Int, val parameter: Any): ValidateError
}
Then in view:
when (it) {
is ValidateError.NormalError -> Toast.make(it.resourceId).show()
is ValidateError.ParameterError -> {
showToast(String.format(getString(it.resourceId), it.parameter))
}
}
it is a known issue that ListAdapter (actually the AsyncListDiffer from its implementation) does not update the list if the new list only has modified items but has the same instance. The updates do not work on new instance list either if you use the same objects inside.
For all of this to work, you have to create a hard copy of the entire list and objects inside.
Easiest way to achieve this:
items.toMutableList().map { it.copy() }
But I am facing a rather weird issue. I have a parse function in my ViewModel that finally posts the items.toMutableList().map { it.copy() } to the LiveData and gets observes in the fragment. Even with the hard copy, DiffUtil does not work. If I move the hard copy inside the fragment, then it works.
To get this easier, if I do this:
IN VIEW MODEL:
[ ... ] parse stuff here
items.toMutableList().map { it.copy() }
restaurants.postValue(items)
IN FRAGMENT:
restaurants.observe(viewLifecycleOwner, Observer { items ->
adapter.submitList(items)
... then, it doesn't work. But if I do this:
IN VIEW MODEL:
[ ... ] parse stuff here
restaurants.postValue(items)
IN FRAGMENT:
restaurants.observe(viewLifecycleOwner, Observer { items ->
adapter.submitList(items.toMutableList().map { it.copy() })
... then it works.
Can anybody explain why this doesn't work?
In the mean time, I have opened an issue on the Google Issue Tracker because maybe they will fix the AsyncListDiffer not updating same instance lists or items. It defeats the purpose of the new adapter. The AsyncListDiffer SHOULD ALWAYS accept same instance lists or items, and fully update using the diff logic that the user customises in the adapter.
I made a quick sample using DiffUtil.Callback and ListAdapter<T, K> (so I called submitList(...) on the adapter), and had no issues.
Then I modified the adapter to be a normal RecyclerView.Adapter and constructed an AsyncDiffUtil inside of it (using the same DiffUtil.Callback from above).
The architecture is:
Activity -> Fragment (contains RecyclerView).
Adapter
ViewModel
"Fake Repository" that simply holds a val source: MutableList<Thing> = mutableListOf()
Model
I've created a Thing object: data class Thing(val name: String = "", val age: Int = 0).
For readability I added typealias Things = List<Thing> (less typing). ;)
Repository
It's fake in the sense that items are created like:
private fun makeThings(total: Int = 20): List<Thing> {
val things: MutableList<Thing> = mutableListOf()
for (i in 1..total) {
things.add(Thing("Name: $i", age = i + 18))
}
return things
}
But the "source" is a mutableList of (the typealias).
The other thing the repo can do is "simulate" a modification on a random item. I simply create a new data class instance, since it's obviously all immutable data types (as they should be). Remember this is just simulating a real change that may have come from an API or DB.
fun modifyItemAt(pos: Int = 0) {
if (source.isEmpty() || source.size <= pos) return
val thing = source[pos]
val newAge = thing.age + 1
val newThing = Thing("Name: $newAge", newAge)
source.removeAt(pos)
source.add(pos, newThing)
}
ViewModel
Nothing fancy here, it talks and holds the reference to the ThingsRepository, and exposes a LiveData:
private val _state = MutableLiveData<ThingsState>(ThingsState.Empty)
val state: LiveData<ThingsState> = _state
And the "state" is:
sealed class ThingsState {
object Empty : ThingsState()
object Loading : ThingsState()
data class Loaded(val things: Things) : ThingsState()
}
The viewModel has two public methods (Aside from the val state):
fun fetchData() {
viewModelScope.launch(Dispatchers.IO) {
_state.postValue(ThingsState.Loaded(repository.fetchAllTheThings()))
}
}
fun modifyData(atPosition: Int) {
repository.modifyItemAt(atPosition)
fetchData()
}
Nothing special, just a way to modify a random item by position (remember this is just a quick hack to test it).
So FetchData, launches the async code in IO to "fetch" (in reality, if the list is there, the cached list is returned, only the 1st time the data is "made" in the repo).
Modify data is simpler, calls modify on the repo and fetch data to post the new value.
Adapter
Lots of boilerplate... but as discussed, it's just an Adapter:
class ThingAdapter(private val itemClickCallback: ThingClickCallback) :
RecyclerView.Adapter<RecyclerView.ViewHolder>() {
The ThingClickCallback is just:
interface ThingClickCallback {
fun onThingClicked(atPosition: Int)
}
This Adapter now has an AsyncDiffer...
private val differ = AsyncListDiffer(this, DiffUtilCallback())
this in this context is the actual adapter (needed by the differ) and DiffUtilCallback is just a DiffUtil.Callback implementation:
internal class DiffUtilCallback : DiffUtil.ItemCallback<Thing>() {
override fun areItemsTheSame(oldItem: Thing, newItem: Thing): Boolean {
return oldItem.name == newItem.name
}
override fun areContentsTheSame(oldItem: Thing, newItem: Thing): Boolean {
return oldItem.age == newItem.age && oldItem.name == oldItem.name
}
nothing special here.
The only special methods in the adapter (aside from onCreateViewHolder and onBindViewHolder) are these:
fun submitList(list: Things) {
differ.submitList(list)
}
override fun getItemCount(): Int = differ.currentList.size
private fun getItem(position: Int) = differ.currentList[position]
So we ask the differ to do these for us and expose the public method submitList to emulate a listAdapter#submitList(...), except we delegate to the differ.
Because you may be wondering, here's the ViewHolder:
internal class ViewHolder(itemView: View, private val callback: ThingClickCallback) :
RecyclerView.ViewHolder(itemView) {
private val title: TextView = itemView.findViewById(R.id.thingName)
private val age: TextView = itemView.findViewById(R.id.thingAge)
fun bind(data: Thing) {
title.text = data.name
age.text = data.age.toString()
itemView.setOnClickListener { callback.onThingClicked(adapterPosition) }
}
}
Don't be too harsh, I know i passed the click listener directly, I only had about 1 hour to do all this, but nothing special, the layout it's just two text views (age and name) and we set the whole row clickable to pass the position to the callback. Nothing special here either.
Last but not least, the Fragment.
Fragment
class ThingListFragment : Fragment() {
private lateinit var viewModel: ThingsViewModel
private var binding: ThingsListFragmentBinding? = null
private val adapter = ThingAdapter(object : ThingClickCallback {
override fun onThingClicked(atPosition: Int) {
viewModel.modifyData(atPosition)
}
})
...
It has 3 member variables. The ViewModel, the Binding (I used ViewBinding why not it's just 1 liner in gradle), and the Adapter (which takes the Click listener in the ctor for convenience).
In this impl., I simply call the viewmodel with "modify item at position (X)" where X = the position of the item clicked in the adapter. (I know this could be better abstracted but this is irrelevant here).
there's only two other implemented methods in this fragment...
onDestroy:
override fun onDestroy() {
super.onDestroy()
binding = null
}
(I wonder if Google will ever accept their mistake with Fragment's lifecycle that we still have to care for this).
Anyway, the other is unsurprisingly, onCreateView.
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
val root = inflater.inflate(R.layout.things_list_fragment, container, false)
binding = ThingsListFragmentBinding.bind(root)
viewModel = ViewModelProvider(this).get(ThingsViewModel::class.java)
viewModel.state.observe(viewLifecycleOwner) { state ->
when (state) {
is ThingsState.Empty -> adapter.submitList(emptyList())
is ThingsState.Loaded -> adapter.submitList(state.things)
is ThingsState.Loading -> doNothing // Show Loading? :)
}
}
binding?.thingsRecyclerView?.adapter = adapter
viewModel.fetchData()
return root
}
Bind the thing (root/binding), get the viewModel, observe the "state", set the adapter in the recyclerView, and call the viewModel to start fetching data.
That's all.
How does it work then?
The app starts, the fragment is created, subscribes to the VM state LiveData, and triggers the Fetch of data.
The ViewModel calls the repo, which is empty (new), so makeItems is called the list now has items and cached in the repo's "source" list. The viewModel receives this list asynchronously (in a coroutine) and posts the LiveData state.
The fragment receives the state and posts (submit) to the Adapter to finally show something.
When you "click" on an Item, ViewHolder (which has a click listener) triggers the "call back" towards the fragment which receives a position, this is then passed onto the Viewmodel and here the data is mutated in the Repo, which again, pushes the same list, but with a different reference on the clicked item that was modified. This causes the ViewModel to push a new LIveData state with the same list reference as before, towards the fragment, which -again- receives this, and does adapter.submitList(...).
The Adapter asynchronously calculates this and the UI updates.
It works, I can put all this in GitHub if you want to have fun, but my point is, while the concerns about the AsyncDiffer are valid (and may be or been true), this doesn't seem to be my (super limited) experience.
Are you using this differently?
When I tap on any row, the change is propagated from the Repository
UPDATE: forgot to include the doNothing function:
val doNothing: Unit
get() = Unit
I've used this for a while, I normally use it because it reads better than XXX -> {} to me. :)
While doing
items.toMutableList().map { it.copy() }
restaurants.postValue(items)
you are creating a new list but items remains the same. You have to store that new list into a variable or passing that operation directly as a param to postItem.
I created live data which emits a single event as in this example.
My question is next:
How to notify only last subscribed observer when the value in the LiveData changes?
What comes to my mind is to store observers in the linked list in SingleLiveData class and then to call super.observe only if a passed observer is the same as the last element of the list.
I'm not sure if this is the best approach.
I want to use this mechanism to propagate FAB click events from activity to the fragments which are shown inside of the ViewPager. Fragments are dynamically added to view pager adapter, so let's say that we know the order of the fragments.
In the end, I found a workaround for this problem. I had to move away from the live data that emits a single event since it couldn't behave the way I needed it to behave.
Instead of this, I used simple mutable live data which emits an event object which wraps a data as in the last paragraph of this article by Jose Alcérreca.
I'm showing fragments in a view pager so I have only one visible fragment at the time.
So my view model looks like this:
class ActionViewModel : ViewModel() {
private val onCreateLiveData: MutableLiveData<Event<String>> = MutableLiveData()
fun observeOnCreateEvent(): LiveData<Event<String>> = onCreateLiveData
fun onCreateCollectionClick(message: String) {
this.onCreateLiveData.value = Event(message)
}
}
Event wrapper class implementation looks like this:
/*Used as a wrapper for data that is exposed via a LiveData that represents an
event.*/
open class Event<out T>(private val content: T) {
var hasBeenHandled = false
private set // Allow external read but not write
/**
* Returns the content and prevents its use again.
*/
fun getContentIfNotHandled(): T? {
return if (hasBeenHandled) {
null
} else {
hasBeenHandled = true
content
}
}
/**
* Returns the content, even if it's already been handled.
*/
fun peekContent(): T = content
}
In fragments now we can observe events like this:
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
actionViewModel = ViewModelProviders.of(requireActivity()).get(ActionViewModel::class.java)
actionViewModel.observeOnCreateEvent()
.observe(this, Observer {
it?.takeIf { userVisibleHint }?.getContentIfNotHandled()?.let {
//DO what ever is needed
}
})
}
Fragment userVisibleHint property will return true if the fragment is currently visible to the user. Since we are only showing one fragment at the time this works for us. This means that the fragment will only access the event data if it is visible.
Also, implementation of the Event wrapper allows only one read of the value, so that every next time Observer gets this event, its value will be null and we'll ignore it.
Conclusion: This way we are simulating a single event live data which notifies only last subscribed observer.
If you're using Kotlin, you can replace LiveData with Flow. StateFlow can be used to replace regular LiveData, while SharedFlow can be used for stateless events. It will also provide you null safety and all the operators and configurations that come with Flow.
The migration is described here among other places. Here's a basic example:
ViewModel:
interface MyViewModel {
val myData: StateFlow<MyData>
val myEvents: SharedFlow<MyEvent>
}
class MyViewModelImpl: MyViewModel {
override val myData = MutableStateFlow(MyData())
override val myEvents = MutableSharedFlow<MyEvent>(replay = 0, extraBufferCapacity = 1, BufferOverflow.DROP_OLDEST)
/*
* Do stuff
*/
}
Activity:
lifecycleScope.launch {
myData.collect {
// handle stateful data
}
}
lifecycleScope.launch {
myEvents.collect {
// handle stateless events
}
}
Note that lifecycleScope requires the appropriate ktx dependency.
Herer's some more reading about Flow in Android.
I found solution for me in LD extension:
fun <T> LiveData<T>.observeAsEvent(owner: LifecycleOwner, observer: Observer<in T>) {
var previousKey: Any? = value?: NULL
observe(owner) { value ->
if (previousKey == NULL || previousKey != value) {
previousKey = value
observer.onChanged(value)
}
}
}
private const val NULL = "NULL"
Usage for this:
viewModel.resultLiveData.observeAsEvent(viewLifecycleOwner) {
...
}
I crafted a solution, feel free to take a look
https://github.com/ueen/LiveEvent
I've created a library to handle the most common cases that we might encounter while working with event-driven data scenarios
https://github.com/javaherisaber/LiveX
It contains the following types of classes:
LiveData
LiveEvent
OneShotLiveEvent
SingleLiveEvent
Multiple observers can register, all of them receive the event based on lifecycle
Multiple observers can register, each one receive the event only once
Only one observer can register and receive the event only once
Multiple observers can register, only the first one receive the event
I am working on an Android App using the MVVM pattern along LiveData (possibly Transformations) and DataBinding between View and ViewModel. Since the app is "growing", now ViewModels contain lots of data, and most of the latter are kept as LiveData to have Views subscribe to them (of course, this data is needed for the UI, be it a Two-Way Binding as per EditTexts or a One-Way Binding). I heard (and googled) about keeping data that represents the UI state in the ViewModel. However, the results I found were just simple and generic. I would like to know if anyone has hints or could share some knowledge on best practices for this case. In simple words, What could be the best way to store the state of an UI (View) in a ViewModel considering LiveData and DataBinding available? Thanks in advance for any answer!
I struggled with the same problem at work and can share what is working for us. We're developing 100% in Kotlin so the following code samples will be as well.
UI state
To prevent the ViewModel from getting bloated with lots of LiveData properties, expose a single ViewState for views (Activity or Fragment) to observe. It may contain the data previously exposed by the multiple LiveData and any other info the view might need to display correctly:
data class LoginViewState (
val user: String = "",
val password: String = "",
val checking: Boolean = false
)
Note, that I'm using a Data class with immutable properties for the state and deliberately don't use any Android resources. This is not something specific to MVVM, but an immutable view state prevents UI inconsistencies and threading problems.
Inside the ViewModel create a LiveData property to expose the state and initialize it:
class LoginViewModel : ViewModel() {
private val _state = MutableLiveData<LoginViewState>()
val state : LiveData<LoginViewState> get() = _state
init {
_state.value = LoginViewState()
}
}
To then emit a new state, use the copy function provided by Kotlin's Data class from anywhere inside the ViewModel:
_state.value = _state.value!!.copy(checking = true)
In the view, observe the state as you would any other LiveData and update the layout accordingly. In the View layer you can translate the state's properties to actual view visibilities and use resources with full access to the Context:
viewModel.state.observe(this, Observer {
it?.let {
userTextView.text = it.user
passwordTextView.text = it.password
checkingImageView.setImageResource(
if (it.checking) R.drawable.checking else R.drawable.waiting
)
}
})
Conflating multiple data sources
Since you probably previously exposed results and data from database or network calls in the ViewModel, you may use a MediatorLiveData to conflate these into the single state:
private val _state = MediatorLiveData<LoginViewState>()
val state : LiveData<LoginViewState> get() = _state
_state.addSource(databaseUserLiveData, { name ->
_state.value = _state.value!!.copy(user = name)
})
...
Data binding
Since a unified, immutable ViewState essentially breaks the notification mechanism of the Data binding library, we're using a mutable BindingState that extends BaseObservable to selectively notify the layout of changes. It provides a refresh function that receives the corresponding ViewState:
Update: Removed the if statements checking for changed values since the Data binding library already takes care of only rendering actually changed values. Thanks to #CarsonHolzheimer
class LoginBindingState : BaseObservable() {
#get:Bindable
var user = ""
private set(value) {
field = value
notifyPropertyChanged(BR.user)
}
#get:Bindable
var password = ""
private set(value) {
field = value
notifyPropertyChanged(BR.password)
}
#get:Bindable
var checkingResId = R.drawable.waiting
private set(value) {
field = value
notifyPropertyChanged(BR.checking)
}
fun refresh(state: AngryCatViewState) {
user = state.user
password = state.password
checking = if (it.checking) R.drawable.checking else R.drawable.waiting
}
}
Create a property in the observing view for the BindingState and call refresh from the Observer:
private val state = LoginBindingState()
...
viewModel.state.observe(this, Observer { it?.let { state.refresh(it) } })
binding.state = state
Then, use the state as any other variable in your layout:
<layout ...>
<data>
<variable name="state" type=".LoginBindingState"/>
</data>
...
<TextView
...
android:text="#{state.user}"/>
<TextView
...
android:text="#{state.password}"/>
<ImageView
...
app:imageResource="#{state.checkingResId}"/>
...
</layout>
Advanced info
Some of the boilerplate would definitely benefit from extension functions and Delegated properties like updating the ViewState and notifying changes in the BindingState.
If you want more info on state and status handling with Architecture Components using a "clean" architecture you may checkout Eiffel on GitHub.
It's a library I created specifically for handling immutable view states and data binding with ViewModel and LiveData as well as glueing it together with Android system operations and business use cases.
The documentation goes more in depth than what I'm able to provide here.
Android Unidirectional Data Flow (UDF) 2.0
Update 12/18/2019: Android Unidirectional Data Flow with LiveData — 2.0
I've designed a pattern based on the Unidirectional Data Flow using Kotlin with LiveData.
UDF 1.0
Check out the full Medium post or YouTube talk for an in-depth explanation.
Medium - Android Unidirectional Data Flow with LiveData
YouTube - Unidirectional Data Flow - Adam Hurwitz - Medellín Android Meetup
Code Overview
Step 1 of 6 — Define Models
ViewState.kt
// Immutable ViewState attributes.
data class ViewState(val contentList:LiveData<PagedList<Content>>, ...)
// View sends to business logic.
sealed class ViewEvent {
data class ScreenLoad(...) : ViewEvent()
...
}
// Business logic sends to UI.
sealed class ViewEffect {
class UpdateAds : ViewEffect()
...
}
Step 2 of 6 — Pass events to ViewModel
Fragment.kt
private val viewEvent: LiveData<Event<ViewEvent>> get() = _viewEvent
private val _viewEvent = MutableLiveData<Event<ViewEvent>>()
override fun onCreate(savedInstanceState: Bundle?) {
...
if (savedInstanceState == null)
_viewEvent.value = Event(ScreenLoad(...))
}
override fun onResume() {
super.onResume()
viewEvent.observe(viewLifecycleOwner, EventObserver { event ->
contentViewModel.processEvent(event)
})
}
Step 3 of 6 — Process events
ViewModel.kt
val viewState: LiveData<ViewState> get() = _viewState
val viewEffect: LiveData<Event<ViewEffect>> get() = _viewEffect
private val _viewState = MutableLiveData<ViewState>()
private val _viewEffect = MutableLiveData<Event<ViewEffect>>()
fun processEvent(event: ViewEvent) {
when (event) {
is ViewEvent.ScreenLoad -> {
// Populate view state based on network request response.
_viewState.value = ContentViewState(getMainFeed(...),...)
_viewEffect.value = Event(UpdateAds())
}
...
}
Step 4 of 6 — Manage Network Requests with LCE Pattern
LCE.kt
sealed class Lce<T> {
class Loading<T> : Lce<T>()
data class Content<T>(val packet: T) : Lce<T>()
data class Error<T>(val packet: T) : Lce<T>()
}
Result.kt
sealed class Result {
data class PagedListResult(
val pagedList: LiveData<PagedList<Content>>?,
val errorMessage: String): ContentResult()
...
}
Repository.kt
fun getMainFeed(...)= MutableLiveData<Lce<Result.PagedListResult>>().also { lce ->
lce.value = Lce.Loading()
/* Firestore request here. */.addOnCompleteListener {
// Save data.
lce.value = Lce.Content(ContentResult.PagedListResult(...))
}.addOnFailureListener {
lce.value = Lce.Error(ContentResult.PagedListResult(...))
}
}
Step 5 of 6 — Handle LCE States
ViewModel.kt
private fun getMainFeed(...) = Transformations.switchMap(repository.getFeed(...)) {
lce -> when (lce) {
// SwitchMap must be observed for data to be emitted in ViewModel.
is Lce.Loading -> Transformations.switchMap(/*Get data from Room Db.*/) {
pagedList -> MutableLiveData<PagedList<Content>>().apply {
this.value = pagedList
}
}
is Lce.Content -> Transformations.switchMap(lce.packet.pagedList!!) {
pagedList -> MutableLiveData<PagedList<Content>>().apply {
this.value = pagedList
}
}
is Lce.Error -> {
_viewEffect.value = Event(SnackBar(...))
Transformations.switchMap(/*Get data from Room Db.*/) {
pagedList -> MutableLiveData<PagedList<Content>>().apply {
this.value = pagedList
}
}
}
Step 6 of 6 — Observe State Change!
Fragment.kt
contentViewModel.viewState.observe(viewLifecycleOwner, Observer { viewState ->
viewState.contentList.observe(viewLifecycleOwner, Observer { contentList ->
adapter.submitList(contentList)
})
...
}