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Q&A

Proper way of sending large amount of data from View to ViewModel

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The problem

I am working on a simple drawing app for Android. I have a custom view on which the user can draw. This means that a lot of data (a list of points) is generated in this View.

I want to store this data in the ViewModel.

If I need to update a View after something changes in the ViewModel, I can just use LiveData in the ViewModel and put an observer in the View. However, what I need is the other direction: changes in the View should lead to an update of the data in the ViewModel.

What I've learnt so far

I can use MutableLiveData for this, however, it is unclear to me what the right way is to give the View access to the MutableLiveData.

I can access a ViewModel from an Activity, by calling:

viewModel = new ViewModelProvider(this).get(MyViewModel.class);

But, I cannot access the ViewModel from the View. ViewModelProvider takes a ViewModelStoreOwner as its parameter. An Activity is a ViewModelStoreOwner, but a View is not.
I believe this is intentional, otherwise the View would be a ViewModelStoreOwner itself. I could pass a reference to the Activity or the ViewModel itself to the View, but this feels wrong.

It seems that I need to use MutableLiveData. I could then use setValue or postValue on the MutableLiveData whenever the View has more data to send.
However, that just shifts the problem: how should the View get access to the MutableLiveData?

What I've tried

For now, I have a workaround.
I have created an interface ListOfPoints that is implemented by the Activity. The View has a method setPointsOwner(ListOfPoints) . So, effectively, the View still gets a reference to the Activity, but can only use it to manipulate the data. The implementation of the interface, inside the Activity, then updates the MutableLiveData it gets from the ViewModel.

This works, but still feels wrong.

The question

How can I send a lot of data from a View to a ViewModel?

Is it proper coding style to give the View access to MutableLiveData from the owning Activity?
Or is there a way in which I can make the ViewModel observe the View, without letting it have a direct reference to the View?

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3 comments

I have also added [binding] tag as the concept seems to be used (bind view to viewmodel). Very good question btw. Alexei‭ 5 months ago

@Alexei‭ Thanks! "Binding" seems a rather broad tag. But I'll let the community decide about that. FractionalRadix‭ 5 months ago

@FractionalRadix Yes, [binding] is very general, but it is what I heard being used for frameworks such a Windows Forms. WPF, Angular and Xamarin. Maybe something like [android-viewbinding] is more appropriate, but I am not sure if this is what you are using (never programmed directly with Java in Android). Alexei‭ 5 months ago

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