Advanced ViewModels (part I): Dependencies and Passing parameters.

ViewModel as the bridge between the View and the Model.

TL;DR: We can pass parameters to our ViewModel, use it as a data holder, also to share data between Fragments, and to persist its state across process recreation.

Update 9/15/2020: I decided to create a multi-part series about this topic since there will be more about this journey on MVVM architecture and ViewModels for sure 😉

This is part of a multi-part series regarding Advanced ViewModels on Android. This post focuses on which are the dependencies we will need and how to pass parameters to our ViewModel.

Other posts in this series:

Advanced ViewModels (part II): Data Holder.

Advanced ViewModels (part III): Share a ViewModel between Fragments.

Advanced ViewModels (part IV): Persisting state across process recreation.

Declaring dependencies

We will be using the following dependencies from lifecycle:

// ViewModel
implementation “androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version”
// LiveData
implementation “androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version”
// Saved state module for ViewModel
implementation “androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version”

And for fragment-ktx:

// Fragment KTX
implementation “androidx.fragment:fragment-ktx:$fragment_version”

Passing parameters

We can pass parameters to our ViewModel’s constructor as we are used to do for any class. However, to be able to handle that param we will need a Factory within our ViewModel:

import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider
import com.thenewsapp.data.NewsService
class NewsViewModel(private val newsService: NewsService) : ViewModel() {
// My awesome stuff
class Factory(private val newsService: NewsService) : ViewModelProvider.Factory {
@Suppress("UNCHECKED_CAST")
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
           return NewsViewModel(newsService) as T
       }
   }
}

Then, in our View (Fragment/Activity) we will be creating our ViewModel this way:

private val viewModel: NewsViewModel by viewModels { NewsViewModel.Factory(newsService) }

Per documentation, by viewModels is a property delegate to get a reference to the ViewModel scoped to this Fragment.

Let me share with you all the repo:

lucaslabs/TheNewsApp

And my tech talk about Advanced ViewModels on Android for GDG Córdoba, Argentina (in Spanish):

Please, continue reading the next posts in this multi-part series for more tips regarding advanced ViewModels 🙌