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ViewsAndroid MVVM - Update ViewModel when data changes
I'm working on an app using the MVVM pattern with RxJava. The architecture is the following:
It's the first time i use this pattern and i'm not sure about the best way to update a ViewModel (and consequently the corresponding View) when a change occurs in the data, made by another component of the application.
For example: suppose we have an Activity showing the list of users i follow (like a social app), from this list i select a user and open his profile in another Activity. Now, from this second Activity i decide to unfollow the user and when i press the back button to return to the first Activity i would like the list to be updated automatically (deleting the corresponding user, obviously without having to re-download all the data).
The problem is that the two Activity have two different ViewModel. How can i make the changes made by the second Activity affect the ViewModel of the first one? Is it the responsibility of the Repository to inform the first Activity of the changes?
- I'd rather not use
startActivityForResult - I'd rather not to inject the
ViewModelof the firstActivityin the second one
Thanks a lot!
1 answers
Answer question0
i decide to unfollow the user and when i press the back button to return to the first Activity i would like the list to be updated automatically (deleting the corresponding user, obviously without having to re-download all the data).
The problem is that the two Activity have two different ViewModel.
I thought you have a Repository that wraps a "Model" (local datasource) that is able to expose LiveData<*>, no?
In which case all you need to do is this:
@Dao
public interface ItemDao {
@Query("SELECT * FROM ITEMS")
LiveData<List<Item>> getItemsWithChanges();
@Query("SELECT * FROM ITEMS WHERE ID = :id")
LiveData<List<Item>> getItemWithChanges(String id);
}
Now your repository can return LiveData from the DAO:
public class MyRepository {
public LiveData<List<Item>> getItems() {
// either handle "fetch if needed" here, or with NetworkBoundResource
return itemDao.getItemsWithChanges();
}
}
Which you get in your ViewModel:
public class MyViewModel extends ViewModel {
private final LiveData<List<Item>> items;
public MyViewModel(MyRepository repository) {
this.items = repository.getItems();
}
public LiveData<List<Item>> getItems() {
return items;
}
}
And if you observe this, then when you modify the item in Room, then it'll automatically update this LiveData in onStart (when you start observing again).