为什么在Android中需要viewmodel工厂? [英] Why a viewmodel factory is needed in Android?

问题描述

我们一直在讨论这个问题，但我们不知道创建视图模型工厂来创建视图模型而不是直接实例化视图模型的原因.创建仅创建视图模型的工厂有什么好处?

We have been discussing about this but we don't know the reason of creating a viewmodel factory to create a viewmodel instead of instantiate the viewmodel directly. What is the gain of creating a factory that just creates the viewmodel?

我只是举了一个简单的示例，说明了我如何在没有工厂的情况下做到这一点

I just put a simple example of how I did it without Factory

这是kodein模块:

here is the kodein module:

val heroesRepositoryModel = Kodein {
    bind<HeroesRepository>() with singleton {
        HeroesRepository()
    }

    bind<ApiDataSource>() with singleton {
        DataModule.create()
    }

    bind<MainViewModel>() with provider {
        MainViewModel()
    }
}

我没有使用工厂实例化viewmodel的Activity的一部分

The piece of the Activity where I instantiate the viewmodel without using the factory

class MainActivity : AppCompatActivity() {
    private lateinit var heroesAdapter: HeroAdapter
    private lateinit var viewModel: MainViewModel
    private val heroesList = mutableListOf<Heroes.MapHero>()
    private var page = 0
    private var progressBarUpdated = false

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        viewModel = ViewModelProviders.of(this)
                .get(MainViewModel::class.java)
        initAdapter()
        initObserver()
        findHeroes()
    }

我直接实例化用例的ViewModel，而无需将其放在构造函数中

The ViewModel where I instantiate the usecase directly without having it in the constructor

class MainViewModel : ViewModel(), CoroutineScope {

    private val heroesRepository: HeroesRepository = heroesRepositoryModel.instance()
    val data = MutableLiveData<List<Heroes.MapHero>>()

    private var job: Job = Job()
    override val coroutineContext: CoroutineContext
        get() = uiContext + job

    fun getHeroesFromRepository(page: Int) {
        launch {
            try {
                val response = heroesRepository.getHeroes(page).await()
                data.value = response.data.results.map { it.convertToMapHero() }
            } catch (e: HttpException) {
                data.value = null
            } catch (e: Throwable) {
                data.value = null
            }
        }
    }

    override fun onCleared() {
        super.onCleared()
        job.cancel()
    }
}

这里是使用工厂的示例

class ListFragment : Fragment(), KodeinAware, ContactsAdapter.OnContactListener {

    override val kodein by closestKodein()

    private lateinit var adapterContacts: ContactsAdapter

    private val mainViewModelFactory: MainViewModelFactory by instance()
    private val mainViewModel: MainViewModel by lazy {
        activity?.run {
            ViewModelProviders.of(this, mainViewModelFactory)
                .get(MainViewModel::class.java)
        } ?: throw Exception("Invalid Activity")
    }

    override fun onCreateView(
            inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fragment_list, container, false)
    }

viewmodelfactory:

The viewmodelfactory:

class MainViewModelFactory (private val getContacts: GetContacts) : ViewModelProvider.Factory {

    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(MainViewModel::class.java)) {
            return MainViewModel(getContacts) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

和视图模型:

class MainViewModel(private val getContacts: GetContacts) : BaseViewModel() {
    lateinit var gamesList: LiveData<PagedList<Contact>>
    var contactsSelectedData: MutableLiveData<List<Contact>> = MutableLiveData()
    var contactsSelected: ArrayList<Contact> = ArrayList()
    private val pagedListConfig by lazy {
        PagedList.Config.Builder()
                .setEnablePlaceholders(false)
                .setInitialLoadSizeHint(PAGES_CONTACTS_SIZE)
                .setPageSize(PAGES_CONTACTS_SIZE)
                .setPrefetchDistance(PAGES_CONTACTS_SIZE*2)
                .build()
    }

这是完整的第一个示例:

Here is the complete first example:

https://github.com/ibanarriolaIT/Marvel/tree/mvvm

还有完整的第二个示例:

And the complete second example:

https://github.com/AdrianMeizoso/Payment-App

推荐答案

我们不能自行创建ViewModel.我们需要Android提供的ViewModelProviders实用程序来创建ViewModel.

We can not create ViewModel on our own. We need ViewModelProviders utility provided by Android to create ViewModels.

但是ViewModelProviders只能实例化没有arg构造函数的ViewModel.

But ViewModelProviders can only instantiate ViewModels with no arg constructor.

因此，如果我有一个带有多个参数的ViewModel，则需要使用一个Factory，可以将其传递给ViewModelProviders，以便在需要MyViewModel实例时使用.

So if I have a ViewModel with multiple arguments, then I need to use a Factory that I can pass to ViewModelProviders to use when an instance of MyViewModel is required.

例如-

public class MyViewModel extends ViewModel {
    private final MyRepo myrepo;
    public MyViewModel(MyRepo myrepo) {
         this.myrepo = myrepo;
    }
}

要实例化此ViewModel，我需要有一个工厂，ViewModelProviders可以使用该工厂来创建其实例.

To instantiate this ViewModel, I need to have a factory which ViewModelProviders can use to create its instance.

ViewModelProviders实用工具无法创建带有参数构造函数的ViewModel实例，因为它不知道如何以及在构造函数中传递哪些对象.

ViewModelProviders Utility can not create instance of a ViewModel with argument constructor because it does not know how and what objects to pass in the constructor.

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