CODEWITHSUNDEEP
androidkotlinmvvmobservableviewmodel
Sean Goldfarb
Sean Goldfarb

Reputation: 400

How to make the viewmodel observe the repository without lifecycle object

I'm building an android application with MVVM design and I have multiple layers(view, ViewModel, repository and dataSource both local and remote). I want that my repository object will observe the dataSources, do all of his logic for how and when storing cache, map the data to the correct form for the above layer and only then notify the view model about the new data.

Similar to that I want the ViewModel to observe the repository for new data arrival, then doing all the business logic and only after that notify the view.

My problem is that LiveData require lifecycle object that the ViewModel and the repository doesn't have.

I read about using simple observable rather than LiveData but I also read that it is bad practice because the observables are alive forever and this could lead to wierd crashes. In addition, I have PageKeyedDataSource which only returns LiveData.

I also read about using Transformations.map but what if I don't want only to map the data but rather to do something more complex.

Is There a way to make one layer safely observe another layer without creating a chain of LiveData observables from the view layer to the DataSource?

note: my ViewModel is used in multiple fragments if that somehow relevant.

Upvotes: 3

Views: 3183

Answers (3)

Razvan S.
Razvan S.

Reputation: 803

You could use DataBinding to reflect the ViewModel data in you UI. https://developer.android.com/topic/libraries/data-binding

An added benefit is that you write less boilerplate code.

Upvotes: 0

dhabensky
dhabensky

Reputation: 1055

My problem is that LiveData require lifecycle object

Actually, lifecycle is optional. There is an observeForever(Observer) method in LiveData that does not require lifecycle. But that means you should also manually call removeObserver(Observer) when your repository finished work, otherwise it will be a leak.
This really is not much different to using Rx's observables. In both cases you should override onCleared() in your viewmodel and manually unsubscribe (or remove observer) from the repository.

observables are alive forever and this could lead to wierd crashes

Nope, they are alive until you dispose them, but you must do it manually as Rx does not provide lifecycle-aware subscription.

Is There a way to make one layer safely observe another layer without creating a chain of LiveData observables from the view layer to the DataSource?

As your wrote, you are trying to do a View-that-observes-ViewModel-that-observes-Repo-that-observes-DataSource. It is already a chain, you should deal with it.

It is possible to to safely observe LiveData from ViewModel, cause View has a lifecycle and LiveData has lifecycle-aware observe.
But Repo and DataSource do not have lifecycles thus you should manually manage subscriptions. It is possible both with LiveData and Rx observables - you can choose whichever you prefer.

Upvotes: 3

Ashok Kumar
Ashok Kumar

Reputation: 1271

1.First of all, if you need context in viewmodel use AndroidViewModel . 2.Don't do any business logic in Viewmodel class as it's just mediator rather do calculations in repository class 3.use rxjava/rxkotlin in repository and return observable object to Viewmodel from repository method, once Viewmodel gets notified update view.

Upvotes: 0

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