为什么在 Android 中需要一个视图模型工厂? [英] Why a viewmodel factory is needed in Android?

问题描述

我们一直在讨论这个，但我们不知道创建viewmodel factory来创建viewmodel而不是直接实例化viewmodel的原因.创建一个只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?

我只是举了一个没有工厂的简单例子

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

这里是 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()
    }
}

我在不使用工厂的情况下实例化视图模型的 Activity 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，而没有在构造函数中使用它

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()
    }
}

这里有一个使用工厂的例子

So here a example using factory

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)
    }

视图模型工厂:

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

完整的第二个例子:

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

推荐答案

我们不能自己创建 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 ViewModels.

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，那么我需要使用一个工厂，我可以将它传递给 ViewModelProviders 以在需要 MyViewModel 的实例时使用.

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;
    }
}

要实例化这个 ViewModel，我需要有一个工厂，ViewModelProviders 可以用它来创建它的实例.

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

ViewModelProviders Utility 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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