Is it better to launch a coroutine in the ViewModel or to mark the ViewModel function with suspend modifier and launch the coroutine in the activity/fragment itself?
Launching in the ViewModel:
class MainViewModel: ViewModel() {
fun addNewItem(item: Item) {
viewModelScope.launch {
// Add the item to database
}
}
}
class ItemsFragment: Fragment() {
fun onButtonClick() {
viewModel.addNewItem(Item())
}
}
Launching in the LifeCycleOwner:
class MainViewModel: ViewModel() {
suspend fun addNewItem(item: Item) {
// Add the item to database
}
}
class ItemsFragment: Fragment() {
fun onButtonClick() {
lifecycleScope.launchWhenStarted {
viewModel.addNewItem(Item())
}
}
}
It really depends on your use-case and if you'd like to tie the task to the viewModelScope or the view's lifecycleScope.
For better understanding consider the following two examples:
The user triggers a refresh - probably you don't want to tie this to the view's lifecycleScope, since in case of an orientation-change your task will be killed and you'll have to restart the fetching of data again.
Animation or other View related task - suppose you have to do some calculations for an animation that's related to how the view is laid out. In this case after an orientation-change you might have to recalculate things, since the view changed.
Generally speaking you're more likely to bump into the first scenario I believe.
In your case adding an item goes into the 1st use-case, since suppose you're launching from the view directly and using lifecycleScope from Fragment/Actvity, in that case if the suspend saving is still running and an orientation-change happens, the task will get killed and your user will wonder why the item wasn't added.
With that said, db operations generally run so fast that the above scenario will be hard to reproduce, but throw in an api request or delay for testing purposes and you can check out the theory.
When the Dark mode is enabled using the AppCompatDelegate.MODE_NIGHT_YES in kotlin, the activity or the fragment is recreated. If we are calling an API in the same class it will be recalled. Is there a way to eliminate recalling the API.
The ideal way to handle this is by using ViewModel.
As the official documentation says:
The ViewModel class is designed to store and manage UI-related data in
a lifecycle conscious way. The ViewModel class allows data to survive
configuration changes such as screen rotations.
What it means, is that your ViewModel will survive your UI recreation and there will be no data loss or API double calls.
Create your ViewModel class.
class MyViewModel : ViewModel() {
fun callAPI() {
// call you're API here
}
}
Just obtain your ViewModel once in Fragment's onCreate() lifecycle callback.
val model = ViewModelProviders.of(this)[MyViewModel::class.java]
As you want that your API call will be done onle once per screen, make it in the ViewModel's construcor (for example).
init {
callAPI()
}
Please read more about this here.
Edit: This question is a bit out of date now that Google has given us the ability to scope ViewModel to navigation graphs. The better approach (rather than trying to clear activity-scoped models) would be to create specific navigation graphs for the right amount of screens, and scope to those.
With reference to the android.arch.lifecycle.ViewModel class.
ViewModel is scoped to the lifecycle of the UI component it relates to, so in a Fragment-based app, that will be the fragment lifecycle. This is a good thing.
In some cases one wants to share a ViewModel instance between multiple fragments. Specifically I am interested in the case where many screens relate to the same underlying data.
(The docs suggest similar approach when multiple related fragments are displayed on the same screen but this can be worked around by using a single host fragment as per answer below.)
This is discussed in the official ViewModel documentation:
ViewModels can also be used as a communication layer between different
Fragments of an Activity. Each Fragment can acquire the ViewModel
using the same key via their Activity. This allows communication
between Fragments in a de-coupled fashion such that they never need to
talk to the other Fragment directly.
In other words, to share information between fragments that represent different screens, the ViewModel should be scoped to the Activity lifecycle (and according to Android docs this can also be used in other shared instances).
Now in the new Jetpack Navigation pattern, it is recommended to use a "One Activity / Many Fragments" architecture. This means that the activity lives for the whole time the app is being used.
i.e. any shared ViewModel instances that are scoped to Activity lifecycle will never be cleared - the memory remains in constant use.
With a view to preserving memory and using as little as required at any point in time, it would be nice to be able to clear shared ViewModel instances when no longer required.
How can one manually clear a ViewModel from it's ViewModelStore or holder fragment?
Quick solution without having to use Navigation Component library:
getActivity().getViewModelStore().clear();
This will solve this problem without incorporating the Navigation Component library. It’s also a simple one line of code. It will clear out those ViewModels that are shared between Fragments via the Activity
If you check the code here you'll find out, that you can get the ViewModelStore from a ViewModelStoreOwner and Fragment, FragmentActivity for example implements, that interface.
Soo from there you could just call viewModelStore.clear(), which as the documentation says:
/**
* Clears internal storage and notifies ViewModels that they are no longer used.
*/
public final void clear() {
for (ViewModel vm : mMap.values()) {
vm.clear();
}
mMap.clear();
}
N.B.: This will clear all the available ViewModels for the specific LifeCycleOwner, this does not allow you to clear one specific ViewModel.
As OP and Archie said, Google has given us the ability to scope ViewModel to navigation graphs. I will add how to do it here if you are using the navigation component already.
You can select all the fragments that needs to be grouped together inside the nav graph and right-click->move to nested graph->new graph
now this will move the selected fragments to a nested graph inside the main nav graph like this:
<navigation app:startDestination="#id/homeFragment" ...>
<fragment android:id="#+id/homeFragment" .../>
<fragment android:id="#+id/productListFragment" .../>
<fragment android:id="#+id/productFragment" .../>
<fragment android:id="#+id/bargainFragment" .../>
<navigation
android:id="#+id/checkout_graph"
app:startDestination="#id/cartFragment">
<fragment android:id="#+id/orderSummaryFragment".../>
<fragment android:id="#+id/addressFragment" .../>
<fragment android:id="#+id/paymentFragment" .../>
<fragment android:id="#+id/cartFragment" .../>
</navigation>
</navigation>
Now, inside the fragments when you initialise the viewmodel do this
val viewModel: CheckoutViewModel by navGraphViewModels(R.id.checkout_graph)
if you need to pass the viewmodel factory(may be for injecting the viewmodel) you can do it like this:
val viewModel: CheckoutViewModel by navGraphViewModels(R.id.checkout_graph) { viewModelFactory }
Make sure its R.id.checkout_graph and not R.navigation.checkout_graph
For some reason creating the nav graph and using include to nest it inside the main nav graph was not working for me. Probably is a bug.
Source: https://medium.com/androiddevelopers/viewmodels-with-saved-state-jetpack-navigation-data-binding-and-coroutines-df476b78144e
Thanks, OP and #Archie for pointing me in the right direction.
I think I have a better solution.
As stated by #Nagy Robi, you could clear the ViewModel by call viewModelStore.clear(). The problem with this is that it will clear ALL the view model scoped within this ViewModelStore. In other words, you won't have control of which ViewModel to clear.
But according to #mikehc here. We could actually create our very own ViewModelStore instead. This will allow us granular control to what scope the ViewModel have to exist.
Note: I have not seen anyone do this approach but I hope this is a valid one. This will be a really good way to control scopes in a Single Activity Application.
Please give some feedbacks on this approach. Anything will be appreciated.
Update:
Since Navigation Component v2.1.0-alpha02, ViewModels could now be scoped to a flow. The downside to this is that you have to implement Navigation Component to your project and also you have no granualar control to the scope of your ViewModel. But this seems to be a better thing.
If you don't want the ViewModel to be scoped to the Activity lifecycle, you can scope it to the parent fragment's lifecycle. So if you want to share an instance of the ViewModel with multiple fragments in a screen, you can layout the fragments such that they all share a common parent fragment. That way when you instantiate the ViewModel you can just do this:
CommonViewModel viewModel = ViewModelProviders.of(getParentFragment()).class(CommonViewModel.class);
Hopefully this helps!
It seems like it has been already solved in the latest architecture components version.
ViewModelProvider has a following constructor:
/**
* Creates {#code ViewModelProvider}, which will create {#code ViewModels} via the given
* {#code Factory} and retain them in a store of the given {#code ViewModelStoreOwner}.
*
* #param owner a {#code ViewModelStoreOwner} whose {#link ViewModelStore} will be used to
* retain {#code ViewModels}
* #param factory a {#code Factory} which will be used to instantiate
* new {#code ViewModels}
*/
public ViewModelProvider(#NonNull ViewModelStoreOwner owner, #NonNull Factory factory) {
this(owner.getViewModelStore(), factory);
}
Which, in case of Fragment, would use scoped ViewModelStore.
androidx.fragment.app.Fragment#getViewModelStore
/**
* Returns the {#link ViewModelStore} associated with this Fragment
* <p>
* Overriding this method is no longer supported and this method will be made
* <code>final</code> in a future version of Fragment.
*
* #return a {#code ViewModelStore}
* #throws IllegalStateException if called before the Fragment is attached i.e., before
* onAttach().
*/
#NonNull
#Override
public ViewModelStore getViewModelStore() {
if (mFragmentManager == null) {
throw new IllegalStateException("Can't access ViewModels from detached fragment");
}
return mFragmentManager.getViewModelStore(this);
}
androidx.fragment.app.FragmentManagerViewModel#getViewModelStore
#NonNull
ViewModelStore getViewModelStore(#NonNull Fragment f) {
ViewModelStore viewModelStore = mViewModelStores.get(f.mWho);
if (viewModelStore == null) {
viewModelStore = new ViewModelStore();
mViewModelStores.put(f.mWho, viewModelStore);
}
return viewModelStore;
}
Im just writing library to address this problem: scoped-vm, feel free to check it out and I will highly appreciate any feedback.
Under the hood, it uses the approach #Archie mentioned - it maintains separate ViewModelStore per scope. But it goes one step further and clears ViewModelStore itself as soon as the last fragment that requested viewmodel from that scope destroys.
I should say that currently whole viewmodel management (and this lib particularly) is affected with a serious bug with the backstack, hopefully it will be fixed.
Summary:
If you care about ViewModel.onCleared() not being called, the best way (for now) is to clear it yourself. Because of that bug, you have no guaranty that viewmodel of a fragment will ever be cleared.
If you just worry about leaked ViewModel - do not worry, they will be garbage collected as any other non-referenced objects. Feel free to use my lib for fine-grained scoping, if it suits your needs.
As it was pointed out it is not possible to clear an individual ViewModel of a ViewModelStore using the architecture components API. One possible solution to this issue is having a per-ViewModel stores that can be safely cleared when necessary:
class MainActivity : AppCompatActivity() {
val individualModelStores = HashMap<KClass<out ViewModel>, ViewModelStore>()
inline fun <reified VIEWMODEL : ViewModel> getSharedViewModel(): VIEWMODEL {
val factory = object : ViewModelProvider.Factory {
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
//Put your existing ViewModel instantiation code here,
//e.g., dependency injection or a factory you're using
//For the simplicity of example let's assume
//that your ViewModel doesn't take any arguments
return modelClass.newInstance()
}
}
val viewModelStore = this#MainActivity.getIndividualViewModelStore<VIEWMODEL>()
return ViewModelProvider(this.getIndividualViewModelStore<VIEWMODEL>(), factory).get(VIEWMODEL::class.java)
}
val viewModelStore = this#MainActivity.getIndividualViewModelStore<VIEWMODEL>()
return ViewModelProvider(this.getIndividualViewModelStore<VIEWMODEL>(), factory).get(VIEWMODEL::class.java)
}
inline fun <reified VIEWMODEL : ViewModel> getIndividualViewModelStore(): ViewModelStore {
val viewModelKey = VIEWMODEL::class
var viewModelStore = individualModelStores[viewModelKey]
return if (viewModelStore != null) {
viewModelStore
} else {
viewModelStore = ViewModelStore()
individualModelStores[viewModelKey] = viewModelStore
return viewModelStore
}
}
inline fun <reified VIEWMODEL : ViewModel> clearIndividualViewModelStore() {
val viewModelKey = VIEWMODEL::class
individualModelStores[viewModelKey]?.clear()
individualModelStores.remove(viewModelKey)
}
}
Use getSharedViewModel() to obtain an instance of ViewModel which is bound to the Activity's lifecycle:
val yourViewModel : YourViewModel = (requireActivity() as MainActivity).getSharedViewModel(/*There could be some arguments in case of a more complex ViewModelProvider.Factory implementation*/)
Later, when it's the time to dispose the shared ViewModel, use clearIndividualViewModelStore<>():
(requireActivity() as MainActivity).clearIndividualViewModelStore<YourViewModel>()
In some cases you would want to clear the ViewModel as soon as possible if it's not needed anymore (e.g., in case of it containing some sensitive user data like username or password). Here's a way of logging the state of individualModelStores upon every fragment switching to help you keep track of shared ViewModels:
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
if (BuildConfig.DEBUG) {
navController.addOnDestinationChangedListener { _, _, _ ->
if (individualModelStores.isNotEmpty()) {
val tag = this#MainActivity.javaClass.simpleName
Log.w(
tag,
"Don't forget to clear the shared ViewModelStores if they are not needed anymore."
)
Log.w(
tag,
"Currently there are ${individualModelStores.keys.size} ViewModelStores bound to ${this#MainActivity.javaClass.simpleName}:"
)
for ((index, viewModelClass) in individualModelStores.keys.withIndex()) {
Log.w(
tag,
"${index + 1}) $viewModelClass\n"
)
}
}
}
}
}
I found a simple and fairly elegant way to deal with this issue. The trick is to use a DummyViewModel and model key.
The code works because AndroidX checks the class type of the model on get(). If it doesn't match it creates a new ViewModel using the current ViewModelProvider.Factory.
public class MyActivity extends AppCompatActivity {
private static final String KEY_MY_MODEL = "model";
void clearMyViewModel() {
new ViewModelProvider(this, new ViewModelProvider.NewInstanceFactory()).
.get(KEY_MY_MODEL, DummyViewModel.class);
}
MyViewModel getMyViewModel() {
return new ViewModelProvider(this, new ViewModelProvider.AndroidViewModelFactory(getApplication()).
.get(KEY_MY_MODEL, MyViewModel.class);
}
static class DummyViewModel extends ViewModel {
//Intentionally blank
}
}
In my case, most of the things I observe are related to the Views, so I don't need to clear it in case the View gets destroyed (but not the Fragment).
In the case I need things like a LiveData that takes me to another Fragment (or that does the thing only once), I create a "consuming observer".
It can be done by extending MutableLiveData<T>:
fun <T> MutableLiveData<T>.observeConsuming(viewLifecycleOwner: LifecycleOwner, function: (T) -> Unit) {
observe(viewLifecycleOwner, Observer<T> {
function(it ?: return#Observer)
value = null
})
}
and as soon as it's observed, it will clear from the LiveData.
Now you can call it like:
viewModel.navigation.observeConsuming(viewLifecycleOwner) {
startActivity(Intent(this, LoginActivity::class.java))
}
As I know you can't remove ViewModel object manually by program, but you can clear data that stored in that,for this case you should call onCleared() method manually
for doing this:
Override onCleared() method in that class that is extended from ViewModel class
In this method you can clean data by making null the field that you store data in it
Call this method when you want clear data completely.
Typically you don't clear the ViewModel manually, because it is handled automatically. If you feel the need to clear your ViewModel manually, you're probably doing too much in that ViewModel...
There's nothing wrong with using multiple viewmodels. First one could be scoped to the Activity while another one could be scoped to the fragment.
Try to use the Activity scoped Viewmodel only for things that need to be shared. And put as many things as possible in the Fragment Scoped Viewmodel. The Fragment scoped viewmodel will be cleared when the fragment is destroyed. Reducing the overall memory footprint.
For example i have a button, if user click the button, it just call finish(). Shall i tell the presenter the button is get clicked? :
//--------------HomeView.kt--------------
var presenter: HomePresenter? = null
override fun onCreate(...) {
btBack.setOnClickListener {
presenter.onBackPress()
}
}
private fun onBackPress() {
finish()
}
//--------------HomePresenter.kt--------------
var view : HomeView? = null
private fun onBackPress() {
view?.onBackPress()
}
or can i simplified it just call finish() in its onClickListener()? :
//--------------HomeView.kt--------------
override fun onCreate(...) {
btBack.setOnClickListener {
finish()
}
}
Unless you need some logic to be executed before finishing the activity, tell the presenter, otherwise I do not see much any advantage do this
As with almost all architectural questions, this comes down to personal preferences and how much you like your code to be clean.
Personally, I would suggest to always call the presenter for each interaction on the view (even if they are just one line calls).
this allows you to move all the logic out of the view and into the presenter (since all interactions on the view just pass the call directly to the presenter, without any attached logic)
it makes your whole app much more testable, since you can mock the view from the presenter and test the logic right there
it makes the architecture of your app cleaner, since you always know that calls get send to the presenter for every single interaction
it makes your code more future proof, since you can add new logic right there in the presenter call and do not have to move code around first (or worse: just be lazy and add the logic directly in the view)
Of course I have to admit that it adds quite a lot of boilerplate code, but I think this is worth it.
I have a ViewModel named SharedViewModel:
public class SharedViewModel<T> extends ViewModel {
private final MutableLiveData<T> selected = new MutableLiveData<>();
public void select(T item) {
selected.setValue(item);
}
public LiveData<T> getSelected() {
return selected;
}
}
I've implemented it based on SharedViewModel example on the Google's Arch ViewModel reference page:
https://developer.android.com/topic/libraries/architecture/viewmodel.html#sharing_data_between_fragments
It is very common that two or more fragments in an activity need to communicate with each other. This is never trivial as both
fragments need to define some interface description and the owner
activity must bind the two together. Moreover, both fragments must
handle the case where the other fragment is not yet created or not
visible.
I have two fragments, called ListFragment and DetailFragment.
Until now I used these two fragments inside an activity called MasterActivity, and everything worked well.
I got the ViewModel in ListFragment, selected the value to use it on DetailFragment.
mStepSelectorViewModel = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
However, now, in certain cases, I need that ListFragment (a layout to a different device configuration) will be added to a different activity, called DetailActivity. Is there a way to do that similarly to the above example?
A little late but you can accomplish this using a shared ViewModelStore. Fragments and activities implement the ViewModelStoreOwner interface. In those cases fragments have a store per instance and activities save it in a static member (I guess so it can survive configuration changes).
Getting back to the shared ViewModelStore, let say for example that you want it to be your Application instance. You need your application to implement ViewModelStoreOwner.
class MyApp: Application(), ViewModelStoreOwner {
private val appViewModelStore: ViewModelStore by lazy {
ViewModelStore()
}
override fun getViewModelStore(): ViewModelStore {
return appViewModelStore
}
}
Then in the cases when you know that you need to share ViewModels between activity boundaries you do something like this.
val viewModel = ViewModelProvider(myApp, viewModelFactory).get(CustomViewModel::class.java)
So now it will use the Store defined in your app. That way you can share ViewModels.
Very important. Because in this example the ViewModels live in your application instance they won't be destroyed when the fragment/activity that uses them gets destroyed. So you will have to link them to the lifecycle of the last fragment/activity that will use them, or manually destroy them.
Well, I created a library for this purpose named Vita, You can share ViewModels between activities and even fragments with different host activity:
val myViewModel = vita.with(VitaOwner.Multiple(this)).getViewModel<MyViewModel>()
The created ViewModel in this way stay alive until its last LifeCycleOwner is destroyed.
Also you can create ViewModels with application scope:
val myViewModel = vita.with(VitaOwner.None).getViewModel<MyViewModel>()
And this type of ViewModel will be cleared when user closes app
Give it a try and kindly let me know your feedback:
https://github.com/FarshadTahmasbi/Vita
you can use factory to make viewmodel and this factor will return single object of view model.. As:
class ViewModelFactory() : ViewModelProvider.Factory {
override fun create(modelClass: Class): T {
if (modelClass.isAssignableFrom(UserProfileViewModel::class.java)) {
val key = "UserProfileViewModel"
if(hashMapViewModel.containsKey(key)){
return getViewModel(key) as T
} else {
addViewModel(key, UserProfileViewModel())
return getViewModel(key) as T
}
}
throw IllegalArgumentException("Unknown ViewModel class")
}
companion object {
val hashMapViewModel = HashMap<String, ViewModel>()
fun addViewModel(key: String, viewModel: ViewModel){
hashMapViewModel.put(key, viewModel)
}
fun getViewModel(key: String): ViewModel? {
return hashMapViewModel[key]
}
}
}
In Activity:
viewModelFactory = Injection.provideViewModelFactory(this)
// Initialize Product View Model
userViewModel = ViewModelProviders.of(this, viewModelFactory).get(
UserProfileViewModel::class.java)`
This will provide only single object of UserProfileViewModel which you can share between Activities.
I think we still get confused with the MVVM framework on Android.
For another activity, do not get confused because it must necessarily be the same, why?
This makes sense if it has the same logic (even if the logic could still be abstract in other useful classes), or if the view in the XML is almost identical.
Let's take a quick example:
I create a ViewModel called vmA, and an activity called A and I need the user's data, I will go to insert the repository in vmA of the User.
Now, I need another activity that needs to read user data,
I create another ViewModel called vmB and in it I will call the user repository.
As described, the repository is always the same.
Another way already suggested is to create N instances of the same ViewModel with the implementation of the Factory.
If you want a ViewModel that is shared by all your activities (as opposed to some),
then why not store what you want stored in that ViewModel
inside your Application class?
The trend presented at the last Google I/O seems to be to abandon the concept of Activities in favor of single-activity apps that have a lot of Fragments.
ViewModels are the way to remove the great number of interfaces the activity of an interface formerly had to implement.
Thus this aproach no longer makes for giant and unmaintainable activities.
Here's a link
Hope it helps you. O(∩_∩)O~
In addition:
1) The inspiration for the code came from smart pointer in c++.
2) It will be auto cleared when no activities or fragments references ShareViewModel.
The ShareViewModel # onShareCleared() function will be called at the same time!
You don't need to destroy them manually!
3) If you use dagger2 to inject the ViewModelFactory for share the viewmodel
between two activities (maybe three), Here's sample