Why a viewmodel factory is needed in Android?

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?

Because Android will only give you a new instance if it's not yet created for that specific given ViewModelStoreOwner.

Let's also not forget that ViewModels are kept alive across configuration changes, so if you rotate the phone, you're not supposed to create a new ViewModel.

If you are going back to a previous Activity and you re-open this Activity, then the previous ViewModel should receive onCleared() and the new Activity should have a new ViewModel.

Unless you're doing that yourself, you should probably just trust the ViewModelProviders.Factory to do its job.

(And you need the factory because you typically don't just have a no-arg constructor, your ViewModel has constructor arguments, and the ViewModelProvider must know how to fill out the constructor arguments when you're using a non-default constructor).

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

But ViewModelProviders can only instantiate ViewModels with no arg constructor.

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.

For example -

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

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

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.