binding is a data binding, I set the lifecycle owner to the lifecycle of the view.
Is it necessary ?
It seems that the app can work well if I remove binding.lifecycleOwner = this.viewLifecycleOwner.
Code
class FragmentHome : Fragment() {
private lateinit var binding: LayoutHomeBinding
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
binding = DataBindingUtil.inflate(
inflater, R.layout.layout_home, container, false
)
// Set the lifecycle owner to the lifecycle of the view
binding.lifecycleOwner = this.viewLifecycleOwner //Must I set lifecycleOwner for a data binding?
...
}
}
If you are using a LiveData object with your binding class, it is required to set a lifecycle owner to define the scope of the LiveData object.
So if you have a LiveData object like:
private val _name = MutableLiveData("John")
val name: LiveData<String> = _name
And you use it for binding like:
android:text="#{vm.name}"
Then you need to specify a lifecycle owner.
Related
From a Google Codelab (can't remember which one), they adviced doing the following for fragments:
class MyFragment : Fragment() {
private var _binding: MyFragmentBinding? = null
private val binding get() = _binding!!
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?
): View {
_binding = MyFragmentBinding.inflate(inflater, container, false)
return binding.root
}
override fun onDestroyView() {
super.onDestroyView()
_binding = null
}
}
And then accessing the views with e.g. binding.button1.
Is there a specific reason for doing it like this, with _binding and binding? Are there better methods? Perhaps an extension for Fragments - like a BaseFragment - to avoid code duplication.
It's not recommended to use BaseFragment or BaseActivity or BaseViewModel... it will just add boilerplate code to your project.
For binding you can just use it like this:
Declaration:
private var binding: MyFragmentBinding? = null
onCreateView:
binding = MyFragmentBinding.inflate(inflater, container, false)
binding?.root
Usage:
binding?.button...
binding?.text...
binding?.cardView...
onDestroyView:
binding = null
And everything is going to work just fine but we use the null check a lot (?) and it's making the code messy and we need to get a lot of null checks if we need something from a certain view, so we are sure that between onCreateView and onDestroyView, the binding is not null so we have _binding and binding:
private var _binding: MyFragmentBinding? = null
private val binding get() = _binding!!
We make _binding mutable with var so we can give it a value, and we make it nullable so we can clear it later.
And we have binding that have a custom getter so that means that each time we call binding it's going to return the latest value from _binding and force that it's not null with !!.
Now we seperate our variables, we have _binding to initialize and clear our binding, and we have binding that is immutable and not nullable to use it only for accessing views without null check ?
See this question for some answers about the reason why binding needs to be nullable in a fragment.
See this answer of mine where I linked some articles about the problems with BaseFragments. You can usually achieve the code reuse without the drawbacks of inheritance by using extension properties and functions.
Here is an example of a property delegate that takes care of releasing the ViewBinding reference when necessary and rebuilding it when necessary. If you use this, all you need is a single binding property. Example is from the article about this tool.
class FirstFragment: Fragment(R.layout.first_fragment) {
private val binding by viewBinding(FirstFragmentBinding::bind)
override fun onViewCreated(view: View, bundle: Bundle?) {
super.onViewCreated(view, bundle)
binding.buttonPressMe.onClick {
showToast("Hello binding!")
}
}
I just saw that CommonsWare has adressed this issue in this post.
Here is the parent class:
abstract class ViewBindingFragment<Binding : ViewBinding>(
private val bindingInflater: (LayoutInflater, ViewGroup?, Boolean) -> Binding
) : Fragment() {
private var binding: Binding? = null
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?
): View {
return bindingInflater(inflater, container, false).apply { binding = this }.root
}
override fun onDestroyView() {
binding = null
super.onDestroyView()
}
protected fun requireBinding(): Binding = binding
?: throw IllegalStateException("You used the binding before onCreateView() or after onDestroyView()")
protected fun useBinding(bindingUse: (Binding) -> Unit) {
bindingUse(requireBinding())
}
}
He then subclasses ViewBindingFragment like so:
class ListFragment :
ViewBindingFragment<TodoRosterBinding>(TodoRosterBinding::inflate) {
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
useBinding { binding ->
binding.items.layoutManager = LinearLayoutManager(context)
}
}
}
Though I am not sure it will eventually lead to less code, if useBinding { binding -> } needs to be called in several functions.
I am learning mvvm. I want to pass frgment bundle argument in ViewModel savedstatehandle .I am using Hilt-Dagger. Please help me. Everytime savedstatehandle is null.
here is my code.
FIRST FRAGMENT:
#AndroidEntryPoint
class CustomerOrderSubmit : Fragment() {
private lateinit var binding: FragmentCustomerOrderSubmitBinding
val customerViewModel: CustomerViewModel by viewModels()
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
binding = FragmentCustomerOrderSubmitBinding.inflate(inflater, container, false)
binding.customerModel = customerViewModel
binding.lifecycleOwner = viewLifecycleOwner
customerViewModel.btnClick.observe(viewLifecycleOwner) {
when (it) {
true -> {
val args = Bundle()
args.putParcelable("customer",customerViewModel.args.value)
val orderReview = OrderReview()
orderReview.arguments = args
Log.d("PARC",args.toString())
val fragmentManager = requireActivity().supportFragmentManager.beginTransaction()
fragmentManager.replace(R.id.main_frame_layout,orderReview)
.addToBackStack(null).commit()
}
else -> {}
}
}
SECOND FRAGMENT: HERE I AM GETTING BUNDLE VALUE. BUT I WANT IT IN ViewModel
#AndroidEntryPoint
class OrderReview : Fragment() {
private lateinit var binding: FragmentOrderReviewBinding
private val customerViewModel: CustomerViewModel by viewModels()
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
binding = FragmentOrderReviewBinding.inflate(inflater, container, false)
binding.argsModel = customerViewModel
binding.lifecycleOwner = viewLifecycleOwner
val args : CustomerModel? = arguments?.getParcelable("customer")
Log.d("PARCF",args.toString())
return binding.root
}
}
VIEW MODEL: But getArgs.value is always null.
#HiltViewModel
class CustomerViewModel #Inject constructor(
private val cusomerRepository: CusomerRepository,
private val savedStateHandle: SavedStateHandle
) : ViewModel() {
val customerList: LiveData<List<CustomerModel>>
//get arguments
val getArgs = MutableLiveData<CustomerModel>(savedStateHandle["customer"])
I am getting Bundle data from fragments one to two but can't get the same data from Fragment one to ViewmodelSavedstatehandle but null every time. What have I done wrong? I need data in ViewModel.
Please help I am a new learner in MVVM.
I don't think you can pass data in this way from fragment to ViewModel.Pass this bundle data by accessing the method where it is needed.
ViewModel method
fun aMethodName(customr:String){
//your code here
//access customer
//Data type String can be changed to customer DataType
}
Fragment
from onCreateView or onViewCreated you can send the data to viewmodel
customerViewModel.aMethodName(customer)
I will try to explain my issue step by step.
I created data variable within xml file
<data>
<variable
name="SLC90RViewModel"
type="com.example.poy.ui.questionnaires.tools.SCL90RViewModel"/>
</data>
In my ViewModel i need to retrieve string array from resources (which is stored in xml file), so i passed app context into ViewModel constructor. Now it looks like this:
class SCL90RViewModel(val context: Context) : ViewModel() {
val item = context.getResources().getStringArray(R.array.firstArray)
}
Finally, in my Fragment i want to assign LifecycleOwner and ViewModel to the binding:
class QuestionnaireFragment : Fragment(R.layout.fragment_questionnaire) {
private var binding: FragmentQuestionnaireBinding? = null
private val scl90rViewModel: SCL90RViewModel by viewModels()
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
val fragmentBinding = FragmentQuestionnaireBinding.inflate(inflater, container, false)
binding = fragmentBinding
return fragmentBinding.root
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
binding?.apply {
lifecycleOwner = viewLifecycleOwner
SCL90RViewModel = scl90rViewModel
}
}
}
However, SCL90RViewModel in binding?.apply{} is outlined with red and message "Classifier 'SCL90RViewModel' does not have a companion object, and thus must be initialized here" pops up. How can i fix it?
SCL90RViewModel is ambiguous here, it can be the binding variable or the ViewModel class. So use this inside the apply block to refer to the binding variable.
binding?.apply {
lifecycleOwner = viewLifecycleOwner
this.SCL90RViewModel = scl90rViewModel
}
In my mind, I should add binding.lifecycleOwner=this when I use viewModel.
I find many projects such as Code A doesn't add binding.lifecycleOwner=this, why?
The Code A is from the project https://github.com/enpassio/Databinding
Code A
class AddToyFragment : androidx.fragment.app.Fragment() {
private lateinit var binding: AddToyBinding
...
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
binding = DataBindingUtil.inflate(
inflater, R.layout.fragment_add_toy, container, false
)
setHasOptionsMenu(true)
return binding.root
}
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
(requireActivity() as AppCompatActivity).supportActionBar?.setDisplayHomeAsUpEnabled(true)
//If there is no id specified in the arguments, then it should be a new toy
val chosenToy : ToyEntry? = arguments?.getParcelable(CHOSEN_TOY)
//Get the view model instance and pass it to the binding implementation
val factory = AddToyViewModelFactory(provideRepository(requireContext()), chosenToy)
mViewModel = ViewModelProviders.of(this, factory).get(AddToyViewModel::class.java)
binding.viewModel = mViewModel
binding.fab.setOnClickListener {
saveToy()
}
binding.lifecycleOwner=this //I think it should add
}
binding.lifecycleOwner used for observing LiveData with data binding.
Kind of android:text=#{viewModel.text} where val text:LiveData<String>. View will observe text changes at runtime.
as far as I understand it,
binding.lifecycleOwner= this
used in one side to make a subscription to receive messages when liveData is changed (so information in view to be consistent), and in another side to delete observer from list when view and fragment is destroyed (to prevent memory leaks). Fragment's viewLifecycleOwner is more suitable to use as lifecycleOwner for binding in this way, isn't it?
I have assigned value for a lateinit variable in Kotlin when creating instance for fragment, Then i used the variable in fragment's oncreateview method. It works perfectly, but sometime it return kotlin.UninitializedPropertyAccessException and my application was closed. I don't know when it occurred, i have checked multiple scenarios. but it never works.
Here i added my code,
private lateinit var mViewModel: SearchViewModel
companion object {
fun getInstance(activity: Activity?): SearchFragment {
val fragment = SearchFragment()
fragment.mViewModel = SearchViewModel(activity)
return fragment
}
}
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
mBinding = DataBindingUtil.inflate(inflater, R.layout.search_fragment, container, false)
mBinding.viewModel = mViewModel
}
Fragment.onCreateView is called by the framework on an instance it creates by using the default constructor, not by calling your getInstance method which it doesn't know about.
Maybe you should initialize the variable in onAttach instead?