如何实例化扩展AndroidViewModel的ViewModel? [英] How to instantiate ViewModel that extends AndroidViewModel?

问题描述

我正在学习一个教程，其中ViewModel扩展了一个抽象类以使用协程，这是扩展的类:

I'm following a tutorial where a ViewModel extends an abstract class in order to use coroutines, this is the class that extends:

abstract class BaseViewModel(application: Application) : AndroidViewModel(application), CoroutineScope {
private val job =  Job()

override val coroutineContext: CoroutineContext
    get() = job + Dispatchers.Main

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

这是ViewModel:

And this is the ViewModel:

class ViewModel(application: Application) : BaseViewModel(application) {}

所以在MainActivity中，我试图像这样实例化该类:

So in MainActivity I'm trying to instantiate the class like this:

    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    viewModel = ViewModelProvider(this)[ViewModel::class.java]}

在教程中，这个家伙完美地做到了这一点，但是当我尝试运行该应用程序时，它抛出了一个异常:

In the tutorial the guy perfectly does this but when I try to run the application it throws me an exception:

 Caused by: java.lang.RuntimeException: Cannot create an instance of class com.name.nameapp.main.viewmodel.ViewModel

我觉得我想念一些东西，你们可以指出它是什么，如果您需要更多信息，我会很乐意将其张贴

I feel like I'm missing something can you guys point what it is, if you need more information I'll gladly post it

推荐答案

您的 ViewModel 是 AndroidViewModel 的子级，需要一个 Application 对象.因此，您必须提供 Factory 类才能实例化 ViewModel .像这样:

Your ViewModel is child of AndroidViewModel which require an Application object. So you will have to provide the Factory class in order to instantiate the ViewModel. Like so:

val viewModelProvider = ViewModelProvider(
     this, 
     ViewModelProvider.AndroidViewModelFactory(application)
)
viewModel = viewModelProvider[MainViewModel::class.java]

如果您正在使用Jetpack的片段库

If you are using the fragment library from Jetpack

implementation "androidx.fragment:fragment-ktx:1.2.5"

您可以像这样使用属性委派:

You can use property delegation like so:

val viewModel: ViewModel by viewModels()

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