为什么我可以使用实例创建 ViewModel

Question

I create ViewModel with ViewModelProvider.Factory in my fragment. Everthing work fine.

var reportViewModel = ViewModelProviders.of(this,ViewModelProvide(RemoteDataProviderImpl(
        WeakReference(context!!))))
       .get(ReportViewModel::class.java)

But after i create viewmodel directly with instance and then work everthing fine

var reportViewModel = ReportViewModel(RemoteDataProviderImpl(WeakReference(context!!)))

It is simple ViewModel

class ReportViewModel(private var provider:RemoteDataProvider) : ViewModel(){
var posts = MutableLiveData<List<Report>>()
fun getPost(){
    provider.getComments().enqueue(object : Callback<ArrayList<Report>> {
        override fun onFailure(call: Call<ArrayList<Report>>, t: Throwable) {
            t.message
        }
        override fun onResponse(
            call: Call<ArrayList<Report>>,
            response: Response<ArrayList<Report>>
        ) {
            if (response.isSuccessful){
                val reports = response.body()
                posts.value = reports
                }
            }

        })
    }
}

What is difference this two code. If i can create viewModel with directly why i need ViewModelProviders?

Answer 1

Using the ViewModelProviders will allow Android to attach the ViewModel to the internal ViewModelStore . This will allow few niceties, like getting the onCleared function called when the activity or the fragment are destroyed. Another one is the viewmodel's retantion upon configuration changes (eg. orientation).

Answer 2

 ReportViewModel(RemoteDataProviderImpl(WeakReference(context!!)))

will

• create a new reference of the viewmodel without any scope of the activity or fragment.

• A new reference of the viewmodel will be created in onCreate of the activity. ( even if the activity is rotated or configuration is changed )

• It would not observe the scope of the activity,ie, it would exist even when the activity is destroyed !

While

 ViewModelProvider.of(this).get(...)

will

• Observe the scope of the activity, ie, when the activity is finished , onCleared(..) of the viewmodel would be called ! ( Viewmodel would be destroyed )

• Same instance of the viewmodel would persist even during the configuration change .

Answer 3

Because when you change config(rotate the phone), if you create viewModel with directly, the viewModel will destroy and lose all the data in viewModel.

But if you use ViewModelProvider to create viewModel, ViewModelProvider will save the viewModel instance and data, so you won't lose any data when config change.