Here, we will introduce the functions and how Kotlin is used.
When we use ViewModel, we will find that sometimes we need to use ViewModelFactory, sometimes we don’t.
Here, we will introduce the functions and how Kotlin is used.
When we use ViewModel, we will find that sometimes we need to use ViewModelFactory, sometimes we don’t.
1 Example of ViewModelFactory not being used
In the following example, we did not use the ViewModelFactory:
class MainActivity : AppCompatActivity() {
lateinit var viewModel: ListViewModel
private val countriesAdapter = CountryListAdapter(arrayListOf())
override fun onCreate(savedInstanceState: Bundle?) {
(savedInstanceState)
setContentView(.activity_main)
viewModel = (this).get(ListViewModel::)
()
{
layoutManager = LinearLayoutManager(context)
adapter = countriesAdapter
}
observeViewModel()
}
...
}
：
class ListViewModel: ViewModel() {
private val countriesService = ()
var job: Job? = null
private val exceptionHandler = CoroutineExceptionHandler{ coroutineContext, throwable ->
onError("Exception: ${}")
}
// Lifecycle-aware component MutableLiveData can only call back the corresponding method when the component is in an activated state, thereby refreshing the corresponding UI. val countries = MutableLiveData<List<Country>>()
val countryLoadError = MutableLiveData<String?>()
val loading = MutableLiveData<Boolean>()
fun refresh() {
fetchCountries()
}
private fun fetchCountries() {
= true
// Start a Ctrip through launch and return a Job-type object instance. We can start Ctrip through() (if launch(start = ) // If this is set), you can cancel Ctrip by job = CoroutineScope( + exceptionHandler).launch {
val response : Response<List<Country>> = ()
// after we get the response, we hope that we could switch back to main thread and display on screen.
withContext() {
if (){
= ()
= null
= false
} else
{
onError("Error: ${()}")
}
}
}
}
private fun onError(message: String) {
= message
= false
}
override fun onCleared() {
()
job?.cancel()
}
}
Here, we do not worry about the details in the code, but only observe how viewModel is defined and used.
2 Examples of using ViewModelFactory
In the following example, we used ViewModelFactory:
class LoginViewModelFactory(
private val repository: RegisterRepository,
private val application: Application
): {
@Suppress("Unchecked_cast")
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
if((LoginViewModel::)) {
return LoginViewModel(repository, application) as T
}
throw IllegalArgumentException("Unknown View Model Class")
}
}
class LoginViewModel(private val repository: RegisterRepository, application: Application) :
AndroidViewModel(application), Observable {
val users =
@Bindable
val inputUsername = MutableLiveData<String>()
@Bindable
val inputPassword = MutableLiveData<String>()
private val viewModelJob = Job()
private val uiScope = CoroutineScope( + viewModelJob)
...
}
class LoginFragment : Fragment() {
private lateinit var loginViewModel: LoginViewModel
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
val binding: FragmentLoginBinding = (
inflater,
.fragment_login, container, false
)
val application = requireNotNull().application
val dao = (application).registerDatabaseDao
val repository = RegisterRepository(dao)
val factory = LoginViewModelFactory(repository, application)
loginViewModel = ViewModelProvider(this, factory).get(LoginViewModel::)
= loginViewModel
= this
(this, Observer { hasFinished->
if (hasFinished == true){
("MYTAG","insidi observe")
displayUsersList()
()
}
})
...
}
}
3 Analysis
We found that when we wereUsed inViewModelProvidersstatementviewModelWhen we did not call anythingviewModelconstructor of . This is becauseViewModelProvidersManage and call for us internallyViewModelThe primary constructor is createdViewModeland return the instance.
If we pass the parameter toviewModelWhen the constructor of the , the rest does not change. At this time, the system will report an error: RunTimeError. The reason for this error is that()Methods create default internallyImplementation to create our parametersViewModel(Again, the ViewModel here has no parameters). So when we add parameters in the constructor,The internal implementation cannot initialize ourViewModel,becauseCalled CreateViewModelThe main constructor of the instance.
So if you add parameters in the constructor, you must create your ownImplementation to create a ViewModel instance.
So, what is? It is also the second example just now. We copy the relevant code below:
class LoginViewModelFactory(
private val repository: RegisterRepository,
private val application: Application
): {
@Suppress("Unchecked_cast")
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
if((LoginViewModel::)) {
return LoginViewModel(repository, application) as T
}
throw IllegalArgumentException("Unknown View Model Class")
}
}
Here are a few points to note:
We can use constructors or any other pattern we like (Singleton, FactoryPattern, etc.) to translateViewModelParameters passed to. This is because we are initializingViewModelCan't be thereActivityorFragmentCalled inViewModelConstructor, and we want to setViewModelThe parameter value of the constructor, so we need to pass the parameter value to
, it will createViewModel。Is acreateInterface to the method.createMethods are responsible for creating ourVeiwModelExamples of .
We're inThis is how the ViewModel is instantiated:
val factory = LoginViewModelFactory(repository, application) loginViewModel = ViewModelProvider(this, factory).get(LoginViewModel::)
We pass our parameters or dependencies to ourso that it can create ViewModel for us.(context, factory)Methods to get ourExamples of .
4 Conclusion
Now we should be very clearThe function and usage method are already there. Here is a simple summary:
When to use？
If our ViewModel has dependencies or parameter passing, then we should pass this dependency through the constructor (this is the best way to pass dependencies). At this timeNeed to be used.
When not to use？
If our ViewModel has no dependencies or parameter passing, then we won't need to create our own. ViewModel will be automatically created for us by default.
This is the end of this article about the detailed explanation of Kotlin usage examples. For more related Kotlin content, please search for my previous articles or continue browsing the related articles below. I hope everyone will support me in the future!