I'm trying to pass data from MainActivity to SecondActivity using a MutableLiveData in a ViewModel,, the following code gives me a NullPointerException when SecondActivity tries to access the LiveData
ViewModel:
class MyViewModel: ViewModel() {
val current=MutableLiveData<String>()
}
Main Activity:
class MainActivity : AppCompatActivity() {
private val viewModel: MyViewModel by lazy {
ViewModelProviders.of(this).get(MyViewModel::class.java)
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
textView.text="test"
viewModel.current.value="test"
textView.setOnClickListener {
startActivity(Intent(this,SecondActivity::class.java))
}
}}
SecondActivity:
class SecondActivity: AppCompatActivity() {
private val viewModel: MyViewModel by lazy {
ViewModelProviders.of(this).get(MyViewModel::class.java)
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_second)
textView2.text = viewModel.current.value!! // NullPointerException
}}
The viewmodel in your second Activity is not the same as that on the 1st Activity; there is a cache of ViewModels by owner (the activities in this case), and these owners are different in each case.
ViewModels can be used to share data between Fragments on the same Activity, but they don't work for sharing data between activities. For that, pass the data in the Intent to start the 2nd Activity (if lightweight), otherwise store it in some long lived object.
val intent = Intent(this,SecondActivity::class.java).apply {
putExtra("KEY", "<string to pass here>")
}
startActivity(intent)
Related
This question already has answers here:
Passing data between a fragment and its container activity
(16 answers)
Closed 1 year ago.
How to correctly transfer data from a fragment to an activity? I do as follows:
1- Create an interface
interface IProfileToActivity {
fun profileInfo(data: AllHeroes.Global)
}
2- Then I inheritance in the activity
class ProfileActivity : AppCompatActivity(), IProfileToActivity {
private lateinit var myBinding: ActivityProfileBinding
override fun profileInfo(data: AllHeroes.Global) {
myBinding.tvUsername.text = data.name
myBinding.tvDivision.text = data.rank.rankName
Log.i("Apex Info 3", data.toString())
}
}
3- sending from a fragment
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
(activity as? IProfileToActivity)?.profileInfo(allInfoApexResponse.global)
mHeroesAdapter.heroesList(allAdapterListHero)
}
but nothing happens, why? what did I do wrong?
You need not create an interface here. You can use requireActivity() to get a reference to the parent activity. Using it you can access public fields and functions of you activity.
class ProfileActivity : AppCompatActivity() {
private lateinit var myBinding: ActivityProfileBinding
fun profileInfo(data: AllHeroes.Global) {
myBinding.tvUsername.text = data.name
myBinding.tvDivision.text = data.rank.rankName
Log.i("Apex Info 3", data.toString())
}
}
And in your fragment:
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
(requireActivity as ProfileToActivity).profileInfo(allInfoApexResponse.global)
mHeroesAdapter.heroesList(allAdapterListHero)
}
There are many ways to pass data from fragment to activity:
Using shared ViewModel.
A ViewModel is used to manage and store UI related data in a
lifecycle conscious way.
~Read more
class SharedViewModel: ViewModel() {
private val currItems: MutableLiveData<List<Item>> =
MutableLiveData<List<Item>>(listOf())
fun getCurrItem(): LiveData<List<Item>> {
return currItems
}
fun sendCurrItems(items: MutableList<Item>) {
currItems.postValue(items)
}
}
class ItemFragment: Fragment() {
private val sharedModel: SharedViewModel by activityViewModels()
}
MainActivity: AppCompactActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val model = ViewModelProvider(this).get(SharedViewModel::class.java)
}
}
In the above class, data is being stored and updated using an MutableList. Thing to be noted here is, the above class is a singleton class, i.e. once it is created, it gets destroyed only when the activity is ended.
Let us assume that an item has to be shared from a ItemFragment to the MainActivity
One callback has to be implemented the MainActivity. For that, one can use an Interface
interface ItemListener{
fun sendItem(item : MutableList<Item>)
}
ItemFragment:
class ItemFragment: Fragment() {
override fun sendItems(items: MutableList<Item>?) {
// Send an Item from here as well as update it
}
// Or just simply call sendItem method.
}
MainActivity:
class MainActivity: AppCompactActivity(){
fun receiveItem(context : Context){
private var mCallback: ItemListener? = null
mCallback = context
}
}
On a button click i have to get some value from API call and then launch one screen. I have two options:
Call the observer each time when user will click on button.
Call the observer on fragment onActivityCreated() and store the value in variable and act accordingly on button click.
So which approach I should follow?
Actually it's up to you. But i always prefer to call it in Activity's onCreate() function, so activity only has 1 observer. If you call it in button click, it will give you multiple observers as much as button clicking
Here is some example :
class HomeProfileActivity: BaseActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
initObserver()
initView()
}
private fun initObserver() {
viewModel.profileWorkProccess.observe(this, {
swipeRefreshLayout.isRefreshing = it
})
viewModel.isLoadingJobs.observe(this, {
layoutProgressBarJobs.visibility = View.VISIBLE
recyclerViewJobs.visibility = View.GONE
dotsJobs.visibility = View.GONE
})
//other viewmodel observing ......
}
private fun initView() {
imageProfile.loadUrl(user.image, R.drawable.ic_user)
textName.text = identity.user?.fullName
textAddress.text = identity.user?.city
buttonGetData.setOnClickListener { viewModel.getData(this) }
}
}
If the button is placed on the Activity, and data is displayed in the Fragment, you need to store variable in Activity ViewModel and observe it in Fragment
You only need to call observe one time when fragment is created.
For example:
class MyActivity : AppCompatActivity() {
val viewModel: MyActViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
myButton.setOnClickListener { view ->
viewModel.getData()
}
}
}
class MyActViewModel: ViewModel {
val data: LiveData<String> = MutableLiveData()
fun getData() {}
}
class MyFragment: Fragment {
val actViewModel: MyActViewModel by activityViewModels()
override fun onActivityCreated(...) {
....
actViewModel.data.observe(viewLifecycleOwner, Observer { data ->
...
}
}
}
I was making a base class so that all bindings for child will be set in base
I have done till this
abstract class BaseActivity2<B : ViewBinding?, T : BaseViewModel?> : AppCompatActivity() {
private var viewBinding: B? = null
private var baseViewModel: T? = null
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
}
}
but am unable to get a way to bind view in oncreat()
generally we bind layout in view binding as
binding = ActivityLoginBinding.inflate(layoutInflater)
setContentView(binding.root)
but i am looking for generalized way in base activity
You can declare a lambda property in the constructor to create the binding object
abstract class BaseActivity<B : ViewBinding>(val bindingFactory: (LayoutInflater) -> B) : AppCompatActivity() {
private lateinit var binding: B
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = bindingFactory(layoutInflater)
setContentView(binding.root)
}
}
You can also define binding as lazy property
private val binding: B by lazy { bindingFactory(layoutInflater) }
Then you need to override nothing in your activities
class MainActivity : BaseActivity<ActivityMainBinding>(ActivityMainBinding::inflate)
Other answer will also solve problem but I would like to do in a clean way.
My Base Class
abstract class BaseVMActivity<VM : ViewModel, B : ViewBinding> : BaseActivity() {
lateinit var viewModel: VM
lateinit var binding: B
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewModel = ViewModelProvider(this, factory).get(getViewModelClass())
binding = getViewBinding()
setContentView(binding.root)
}
private fun getViewModelClass(): Class<VM> {
val type = (javaClass.genericSuperclass as ParameterizedType).actualTypeArguments[0]
return type as Class<VM>
}
abstract fun getViewBinding(): B
}
My activity:
class MainActivity : BaseVMActivity<AppViewModel, ActivityMainBinding>() {
override fun getViewBinding() = ActivityMainBinding.inflate(layoutInflater)
}
Now I can directly access viewModel or binding:
fun dummy(){
binding.bvReport.text = viewModel.getReportText()
}
It's cleaner to override binding object getter inside the child activity I think. So:
abstract class VBActivity<VB : ViewBinding> : AppCompatActivity() {
protected abstract val binding: VB
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(binding.root)
}
}
And lets say MainActivity will be something like:
class MainActivity : VBActivity<ActivityMainBinding>() {
override val binding get() = ActivityMainBinding.inflate(layoutInflater)
}
Basically I have a state management system using ViewModel that looks like this:
class ViewModelA: ViewModel() {
private val repository: RepositoryA by inject()
private val _stateLiveData = MutableLiveData<ViewState>()
val stateLiveData: LiveData<ViewState> get() = _stateLiveData
private val _eventLiveData = SingleLiveEvent<ViewEvent>()
val eventLiveData: LiveData<ViewEvent> get() = _eventLiveData
private val exceptionHandler = CoroutineExceptionHandler { _, _ ->
_stateLiveData.postValue(ViewState.Error)
}
fun loadList() {
if (_stateLiveData.value is ViewState.Loading) return
launch(exceptionHandler) {
_stateLiveData.run {
value = ViewState.Loading
value = repository.getDocumentList().let {
if (it.isEmpty()) ViewState.Error
else ViewState.Data(it)
}
}
}
}
}
But whenever I am sharing a ViewModel with several Fragments, it becomes bigger and bigger. I am looking for a solution for this, because I don't want to centralize all the logic for an entire application flow inside a ViewModel and I also don't want to pass arguments here and there all the time.
PS: Sorry about my bad english.
Edit: Clarify a bit the question.
I didn't quite understand your question. However, if your question was as follows:
How can I share the same ViewModel Object and use it inside multiple Fragments.
You can check the documentation of ViewModelProvider which is a utility class that provides ViewModels for a specific scope like Activity.
Following is an example code of the usage of ViewModelProvider within two Fragments that will be created and used in the same Activity object:
// An example ViewModel
class SharedViewModel : ViewModel() {
val intLiveData = MutableLiveData<Int>() // an example LiveData field
}
// the first fragment
class Fragment1 : Fragment() {
private lateinit var viewModel: SharedViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewModel = requireActivity().let { activity ->
ViewModelProvider(activity).get(SharedViewModel::class.java)
}
}
}
// the other fragment
class Fragment2 : Fragment() {
private lateinit var viewModel: SharedViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewModel = requireActivity().let { activity ->
ViewModelProvider(activity).get(SharedViewModel::class.java)
}
}
}
I have an activity containing an associated view model. The view model loads the data from an online repository. In the same activity I have a table layout containing a viewpager. In the view page are several fragments.
How can I access the activity view model in my fragments?
you can share viewmodel between fragments like this
class MasterFragment : Fragment() {
private lateinit var itemSelector: Selector
private lateinit var model: SharedViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
model = activity?.run {
ViewModelProviders.of(this).get(SharedViewModel::class.java)
} ?: throw Exception("Invalid Activity")
itemSelector.setOnClickListener { item ->
// Update the UI
}
}
}
class DetailFragment : Fragment() {
private lateinit var model: SharedViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
model = activity?.run {
ViewModelProviders.of(this).get(SharedViewModel::class.java)
} ?: throw Exception("Invalid Activity")
model.selected.observe(this, Observer<Item> { item ->
// Update the UI
})
}
}
https://developer.android.com/topic/libraries/architecture/viewmodel
or just make your viewmodel in your activity public then call it in your fragment like:
(activity as MainActivity).viewmodel // MainActivity is your activity