为什么为我的viewModel类实现ViewModelProvider.Factory很重要?

问题描述

我已经构建了一个片段，该片段基本上包含一个Recyclerviewer以及一个使用数据绑定和LiveData(这是我经常实现的代码)的针对我的数据类型的自定义ViewModel，但是这一次发生了运行时异常错误！

I have built a fragment that basically contains a Recyclerviewer with a custom ViewModel for my Data type using data binding and LiveData (which is a piece of code that I frequently implement) but in this time a runtime exception error has occurred!

/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.tdm_project, PID: 9460
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.tdm_project/com.example.tdm_project.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.tdm_project.viewmodel.ArticleViewModel
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2734)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2799)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1537)
        at android.os.Handler.dispatchMessage(Handler.java:110)
        at android.os.Looper.loop(Looper.java:203)
        at android.app.ActivityThread.main(ActivityThread.java:6269)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1063)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:924)
     Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.tdm_project.viewmodel.ArticleViewModel
        at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:154)
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:211)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:135)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:103)
        at com.example.tdm_project.HomeFragment.onCreateView(HomeFragment.kt:75)
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2439)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1460)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1784)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1852)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:802)
        at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManager.java:2625)
        at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411)
        at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2366)
        at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2273)
        at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3273)
        at androidx.fragment.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:3229)
        at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:201)
        at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:620)
        at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:178)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1248)
        at android.app.Activity.performStart(Activity.java:6683)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2697)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2799) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1537) 
        at android.os.Handler.dispatchMessage(Handler.java:110) 
        at android.os.Looper.loop(Looper.java:203) 
        at android.app.ActivityThread.main(ActivityThread.java:6269) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1063) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:924) 
     Caused by: java.lang.InstantiationException: java.lang.Class<com.example.tdm_project.viewmodel.ArticleViewModel> has no zero argument constructor
        at java.lang.Class.newInstance(Native Method)

在搜索了我的问题之后，我发现的唯一建议是即使使用的是ViewModel类而不是AndroidViewModel，也要实现ViewModelFactory.

After googling my problem, the only suggestion I found is to implement a ViewModelFactory even though am using a ViewModel class and not AndroidViewModel.

请参见此答案

ArticleViewModel类

ArticleViewModel class

class ArticleViewModel : ViewModel {

    //lists
    private var articleMList = MutableLiveData<ArrayList<ArticleViewModel>>()
    private var articleInnerList = ArrayList<ArticleViewModel>()

    constructor(
     article: Article
    ) : super() {
        //const with parameters
    }

    //to observe my list
    fun getArticles() : MutableLiveData<ArrayList<ArticleViewModel>>{

        articleMList.value = articleInnerList

        return articleMList
    }

    //retrieve data from backend
    fun getData() {
            //some code
        }
}

片段类

class HomeFragment : Fragment() {


    //viewmodel
    private lateinit var vmodel : ArticleViewModel



    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {

        //set the view
        rootView = inflater.inflate(R.layout.home_fragment, container, false)

         //a function to initialize my recyclerview
        intialiserHorizontally()

          //creating the instance of viewmodel
        vmodel = ViewModelProviders.of(activity!!).get(ArticleViewModel::class.java)

        vmodel.getArticles().observe(this, Observer {
             customHAdapter.swapData(it)
        })

//getting the data from my db
        vmodel.getData()




        return rootView
    }

我的问题是为什么在这种情况下必须实现ViewModelProvider.Factory?

My question is why it is necessary to implement a ViewModelProvider.Factory in this case?

推荐答案

ViewModel系统仅知道如何在ViewModel子类上使用零参数构造函数(或用于单参数构造函数). AndroidViewModel子类).因此，要么:

On its own, the ViewModel system only knows how to use a zero-argument constructor on your ViewModel subclasses (or a single-parameter constructor for AndroidViewModel subclasses). So, either:

• 根本没有构造函数，或者

• Have no constructors at all, or

具有一个显式的零参数构造函数(但请考虑摆脱另一个变量，因为什么都不要使用它)，或者

Have an explicit zero-argument constructor (but consider getting rid of your other one, as nothing should use it), or

实现工厂，因此您可以调用所需的自定义构造函数

Implement a factory, so you can call your desired custom constructor

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