Android application architecture - MVVM or MVC?

Question

I got an android project I\'m beginning to work on, and I want its structure to be as robust as possible.

I\'m coming from a WPF MVVM background and I\'ve been reading a

Answer 1

I think it would be more helpful to explain MVVM in android through an example. The complete article together with the GitHub repo info is here for more info.

Let’s suppose the same benchmark movie app example introduced in the first part of this series. User enters a search term for a movie and presses the ‘FIND’ button, based on which the app searches for the list of movies including that search term and shows them. Clicking on each movie on the list then shows its details.

I will now explain how this app is implemented in MVVM followed by the complete android app, which is available on my GitHub page.

When the user clicks on the ‘FIND’ button on the View, a method is called from the ViewModel with the search term as its argument:

    main_activity_button.setOnClickListener({
        showProgressBar()
        mMainViewModel.findAddress(main_activity_editText.text.toString())
    })

The ViewModel then calls the findAddress method from the Model to search for the movie name:

fun findAddress(address: String) {
    val disposable: Disposable = mainModel.fetchAddress(address)!!.subscribeOn(schedulersWrapper.io()).observeOn(schedulersWrapper.main()).subscribeWith(object : DisposableSingleObserver?>() {
        override fun onSuccess(t: List) {
            entityList = t
            resultListObservable.onNext(fetchItemTextFrom(t))
        }

        override fun onError(e: Throwable) {
            resultListErrorObservable.onNext(e as HttpException)
        }
    })
    compositeDisposable.add(disposable)
}

When the response comes from the Model, the onSuccess method of the RxJava observer carries the successful result, but as the ViewModel is View agnostic, it does not have or use any View instance to pass the result for showing. It instead triggers an event in the resultListObservable by calling resultListObservable.onNext(fetchItemTextFrom(t)) , which is observed by the View:

mMainViewModel.resultListObservable.subscribe({
    hideProgressBar()
    updateMovieList(it)
})

So the observable plays a mediator role between the View and ViewModel:

• ViewModel triggers an event in its observable
• View updates the UI by subscribing to ViewModel’s observable

Here’s the full code for the View. In this example, View is an Activity class, but Fragment can also be equally used:

class MainActivity : AppCompatActivity() {

    private lateinit var mMainViewModel: MainViewModel
    private lateinit var addressAdapter: AddressAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        mMainViewModel = MainViewModel(MainModel())
        loadView()
        respondToClicks()
        listenToObservables()
    }

    private fun listenToObservables() {
        mMainViewModel.itemObservable.subscribe(Consumer { goToDetailActivity(it) })
        mMainViewModel.resultListObservable.subscribe(Consumer {
            hideProgressBar()
            updateMovieList(it)
        })
        mMainViewModel.resultListErrorObservable.subscribe(Consumer {
            hideProgressBar()
            showErrorMessage(it.message())
        })
    }

    private fun loadView() {
        setContentView(R.layout.activity_main)
        addressAdapter = AddressAdapter()
        main_activity_recyclerView.adapter = addressAdapter
    }

    private fun respondToClicks() {
        main_activity_button.setOnClickListener({
            showProgressBar()
            mMainViewModel.findAddress(main_activity_editText.text.toString())
        })
        addressAdapter setItemClickMethod {
            mMainViewModel.doOnItemClick(it)
        }
    }

    fun showProgressBar() {
        main_activity_progress_bar.visibility = View.VISIBLE
    }

    fun hideProgressBar() {
        main_activity_progress_bar.visibility = View.GONE
    }

    fun showErrorMessage(errorMsg: String) {
        Toast.makeText(this, "Error retrieving data: $errorMsg", Toast.LENGTH_SHORT).show()
    }

    override fun onStop() {
        super.onStop()
        mMainViewModel.cancelNetworkConnections()
    }

    fun updateMovieList(t: List) {
        addressAdapter.updateList(t)
        addressAdapter.notifyDataSetChanged()
    }

    fun goToDetailActivity(item: MainModel.ResultEntity) {
        var bundle = Bundle()
        bundle.putString(DetailActivity.Constants.RATING, item.rating)
        bundle.putString(DetailActivity.Constants.TITLE, item.title)
        bundle.putString(DetailActivity.Constants.YEAR, item.year)
        bundle.putString(DetailActivity.Constants.DATE, item.date)
        var intent = Intent(this, DetailActivity::class.java)
        intent.putExtras(bundle)
        startActivity(intent)
    }

    class AddressAdapter : RecyclerView.Adapter() {
        var mList: List = arrayListOf()
        private lateinit var mOnClick: (position: Int) -> Unit

        override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): Holder {
            val view = LayoutInflater.from(parent!!.context).inflate(R.layout.item, parent, false)
            return Holder(view)
        }

        override fun onBindViewHolder(holder: Holder, position: Int) {
            holder.itemView.item_textView.text = mList[position]
            holder.itemView.setOnClickListener { mOnClick(position) }
        }

        override fun getItemCount(): Int {
            return mList.size
        }

        infix fun setItemClickMethod(onClick: (position: Int) -> Unit) {
            this.mOnClick = onClick
        }

        fun updateList(list: List) {
            mList = list
        }

        class Holder(itemView: View?) : RecyclerView.ViewHolder(itemView)
    }

}

Here is the ViewModel:

class MainViewModel() {
    lateinit var resultListObservable: PublishSubject>
    lateinit var resultListErrorObservable: PublishSubject
    lateinit var itemObservable: PublishSubject
    private lateinit var entityList: List
    private val compositeDisposable: CompositeDisposable = CompositeDisposable()
    private lateinit var mainModel: MainModel
    private val schedulersWrapper = SchedulersWrapper()

    constructor(mMainModel: MainModel) : this() {
        mainModel = mMainModel
        resultListObservable = PublishSubject.create()
        resultListErrorObservable = PublishSubject.create()
        itemObservable = PublishSubject.create()
    }

    fun findAddress(address: String) {
        val disposable: Disposable = mainModel.fetchAddress(address)!!.subscribeOn(schedulersWrapper.io()).observeOn(schedulersWrapper.main()).subscribeWith(object : DisposableSingleObserver?>() {
            override fun onSuccess(t: List) {
                entityList = t
                resultListObservable.onNext(fetchItemTextFrom(t))
            }

            override fun onError(e: Throwable) {
                resultListErrorObservable.onNext(e as HttpException)
            }
        })
        compositeDisposable.add(disposable)
    }

    fun cancelNetworkConnections() {
        compositeDisposable.clear()
    }

    private fun fetchItemTextFrom(it: List): ArrayList {
        val li = arrayListOf()
        for (resultEntity in it) {
            li.add("${resultEntity.year}: ${resultEntity.title}")
        }
        return li
    }

    fun doOnItemClick(position: Int) {
        itemObservable.onNext(entityList[position])
    }
}

and finally the Model:

class MainModel {
    private var mRetrofit: Retrofit? = null

    fun fetchAddress(address: String): Single>? {
        return getRetrofit()?.create(MainModel.AddressService::class.java)?.fetchLocationFromServer(address)
    }

    private fun getRetrofit(): Retrofit? {
        if (mRetrofit == null) {
            val loggingInterceptor = HttpLoggingInterceptor()
            loggingInterceptor.level = HttpLoggingInterceptor.Level.BODY
            val client = OkHttpClient.Builder().addInterceptor(loggingInterceptor).build()
            mRetrofit = Retrofit.Builder().baseUrl("http://bechdeltest.com/api/v1/").addConverterFactory(GsonConverterFactory.create()).addCallAdapterFactory(RxJava2CallAdapterFactory.create()).client(client).build()
        }
        return mRetrofit
    }

    class ResultEntity(val title: String, val rating: String, val date: String, val year: String)
    interface AddressService {
        @GET("getMoviesByTitle")
        fun fetchLocationFromServer(@Query("title") title: String): Single>
    }

}

Full article here