MVVM Android

MVVM Android is the framework based on Android Architecture components, which gives you set of base classes to implement concise, testable and solid application. It combines built-in support for Dagger 2 dependency injection, View DataBinding, ViewModel and RxJava use cases. Architecture described here is used among wide variety of projects and it's production ready.

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dependencies {
    implementation("com.thefuntasty.mvvm:mvvm:LatestVersion")
    implementation("com.thefuntasty.mvvm:bindingadapters:LatestVersion")
    implementation("com.thefuntasty.mvvm:dagger:LatestVersion")
    implementation("com.thefuntasty.mvvm:cr-usecases:LatestVersion")
    implementation("com.thefuntasty.mvvm:rx-usecases:LatestVersion")
    
    // Testing
    testImplementation("com.thefuntasty.mvvm:mvvm-test:LatestVersion")
    testImplementation("com.thefuntasty.mvvm:rx-usecases-test:LatestVersion")
    testImplementation("com.thefuntasty.mvvm:cr-usecases-test:LatestVersion")
}    

Getting started - Minimal project file hierarchy

Minimal working project must contain files as presented in example-minimal
module. File hierarchy might looks like this:

example-minimal
`-- src/main
    |-- java/com/example
    |   |-- injection  
    |   |   |-- ActivityBuilderModule.kt
    |   |   |-- ApplicationComponent.kt
    |   |   `-- ApplicationModule.kt
    |   |-- ui 
    |   |   |-- base/BaseActivity.kt
    |   |   `-- main
    |   |       |-- MainActivity.kt
    |   |       |-- MainActivityModule.kt
    |   |       |-- MainView.kt
    |   |       |-- MainViewModel.kt
    |   |       |-- MainViewModelFactory.kt
    |   |       `-- MainViewState.kt
    |   `-- App.kt 
    `-- res/layout/activity_main.xml  

Keep in mind this description focuses on architecture .kt files. Android related files like an
AndroidManifest.xml are omitted. Let's describe individual files one by one:

ActivityBuilderModule.kt

File contains Dagger module class that takes responsibility of proper injection
into Activities. This is the place where every Activity and its ActivityModule
in project must be specified to make correct ViewModel injection work.

@Module
abstract class ActivityBuilderModule {

    @ContributesAndroidInjector(modules = [MainActivityModule::class])
    abstract fun mainActivity(): MainActivity
}

ApplicationComponent.kt

ApplicationComponent interface combines your singleton Dagger modules and defines
how DaggerApplicationComponent should be generated.

@Singleton
@Component(
    modules = [
        AndroidInjectionModule::class,
        AndroidSupportInjectionModule::class,
        ActivityBuilderModule::class,
        ApplicationModule::class
    ]
)
interface ApplicationComponent : AndroidInjector<App> {

    @Component.Builder
    interface Builder {

        @BindsInstance
        fun application(app: App): Builder

        fun build(): ApplicationComponent
    }
}

ApplicationModule.kt

Application module definition. Your singleton scoped objects might
be specified here and injected wherever needed. Example implementation:

@Module
class ApplicationModule {

    @Singleton
    @Provides
    fun moshi(): Moshi = Moshi.Builder().build()
}

BaseActivity.kt

All of Activities in the project should inherit from this class to make DataBinding work properly.
Be aware of fact BR class used in this class is generated when there is at least one layout file
with correctly defined data variables. Read more here.

abstract class BaseActivity<VM : BaseViewModel<VS>, VS : ViewState, B : ViewDataBinding> :
    BaseDaggerBindingActivity<VM, VS, B>() {

    override val brViewVariableId = BR.view
    override val brViewModelVariableId = BR.viewModel
    override val brViewStateVariableId = BR.viewState
}

MainActivity.kt

Example Activity implementation. viewModelFactory and layoutResId must be overridden in every
Activity in order to make ViewModel injection and DataBinding work. ActivityMainBinding used
in BaseActivity constructor is generated from related activity_main.xml layout file. Make sure this file
exists and have root tag <layout> before you try to build your code. ViewModel can be
accessed through derived viewModel field.

class MainActivity : BaseActivity<MainViewModel, MainViewState, ActivityMainBinding>(), MainView {

    @Inject override lateinit var viewModelFactory: MainViewModelFactory

    override val layoutResId = R.layout.activity_main
}

MainActivityModule.kt

MainActivity scoped module. It becomes useful when you want to provide specific
activity related configuration e.g.:

@Module
abstract class MainActivityModule {

    @Provides
    fun provideUser(activity: MainActivity): User = 
            activity.intent.getParcelableExtra("user")
}

MainView.kt

Interface representing actions executable on your Activity/Fragment. These actions
might be invoked directly from xml layout thanks to view data variable.

interface MainView : BaseView

MainViewModel.kt

Activity/Fragment specific ViewModel implementation. You can choose between extending
BaseViewModel or BaseRxViewModel with build-in support for RxJava based use cases.

class MainViewModel @Inject constructor() : BaseViewModel<MainViewState>() {

    override val viewState = MainViewState
}

MainViewModelFactory.kt

Factory responsible for ViewModel creation. It is injected in Activity/Fragment.

class MainViewModelFactory @Inject constructor(
    override val viewModelProvider: Provider<MainViewModel>
) : BaseViewModelFactory<MainViewModel>() {
    override val viewModelClass = MainViewModel::class
}

MainViewState.kt

State representation of an screen. Should contain set of LiveData fields observed
by Activity/Fragment. State is stored in ViewModel thus survives screen rotation.

object MainViewState : ViewState {
    val user = DefaultValueLiveData<User>(User.EMPTY)
}

activity_main.xml

Layout file containing proper DataBinding variables initialization. Make sure correct
types are defined.

<layout xmlns:android="http://schemas.android.com/apk/res/android">

    <data>
        <variable name="view" type="com.thefuntasty.mvvmsample.ui.main.MainView"/>
        <variable name="viewModel" type="com.thefuntasty.mvvmsample.ui.main.MainViewModel"/>
        <variable name="viewState" type="com.thefuntasty.mvvmsample.ui.main.MainViewState"/>
    </data>

    <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical"
            android:gravity="center">

    </LinearLayout>
</layout>

Use Cases

Modules cr-usecases and rx-usecases contains set of base classes useful for easy execution of
background tasks based on Coroutines or RxJava streams respectively. In terms of Coroutines
two base types are available - UseCase (single result use case) and FlowUseCase (multi result use case).
RxJava base use cases match base Rx "primitives": ObservableUseCase, SingleUseCase, FlowableUseCase, MaybeUseCase
and finally CompletableUseCase.

Following example describes how to make an API call and how to deal with
result of this call.

LoginUseCase.kt

class LoginUseCase @Inject constructor(
    private val apiManager: ApiManager // Retrofit Service
) : SinglerUseCase<LoginData, User>() {

    override fun prepare(args: LoginData): Single<User> {
        return apiManager.getUser(args)
    }
}

data class LoginData(val email: String, val password: String)

LoginViewState.kt

class LoginViewState : ViewState {
    // IN - values provided by UI
    val email = DefaultValueLiveData("")
    val password = DefaultValueLiveData("")

    // OUT - Values observed by UI
    val fullName = MutableLiveData<String>()
    val isLoading = MutableLiveData<Boolean>()
}

LoginViewModel.kt

class LoginViewModel @Inject constructor(
    private val loginUseCase: LoginUseCase // Inject UseCase
) : BaseRxViewModel<LoginViewState>() {
    override val viewState = LoginViewState()

    fun logIn() = with(viewState) {
        loginUseCase.execute(LoginData(email.value, email.password)) {
            onStart {
                isLoading.value = true
            }
            onSuccess {
                isLoading.value = false
                fullName.value = user.fullName // handle success & manipulate state
            }
            onError {
                isLoading.value = false
                // handle error
            }
        }
    }
}

Synchronous execution of cr-usecase

Module cr-usecases allows you to execute use cases synchronously.

fun onButtonClicked() = launchWithHandler {  
    ...
    val data = useCase.execute().getOrDefault("Default")  
    ...
}

execute method returns a Result that can be either successful Success or failed Error.

launchWithHandler launches a new coroutine encapsulated with a try-catch block. By default exception thrown in launchWithHandler is rethrown but it is possible to override this behavior with defaultErrorHandler or just log these exceptions in logUnhandledException.

UI changes flow

There are two main ways how to reflect data changes in UI. Through ViewState observation
or one-shot Events.

ViewState observation

You can observe state changes and reflect these changes in UI via DataBinding
observation directly in xml layout:

<layout xmlns:android="http://schemas.android.com/apk/res/android">

    <data>
        <variable name="view" type="com.thefuntasty.mvvmsample.ui.detail.DetailView"/>
        <variable name="viewModel" type="com.thefuntasty.mvvmsample.ui.detail.DetailViewModel"/>
        <variable name="viewState" type="com.thefuntasty.mvvmsample.ui.detail.DetailViewState"/>
    </data>
    
    <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{viewState.myTextLiveData}"/>
</layout>

Events

Events are one-shot messages sent from ViewModel to an Activity/Fragment. They
are based on LiveData bus. Events are guaranteed to be delivered only once even when
there is screen rotation in progress. Basic event communication might look like this:

MainEvents.kt

sealed class MainEvent : Event<MainViewState>()

object ShowDetailEvent : MainEvent()

MainViewModel.kt

class MainViewModel @Inject constructor() : BaseViewModel<MainViewState>() {

    override val viewState = MainViewState

    fun onDetail() {
        sendEvent(ShowDetailEvent)
    }
}

MainActivity.kt

class MainActivity : BaseActivity<MainViewModel, MainViewState, ActivityMainBinding>(), MainView {

    // ...

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        observeEvent(ShowDetailEvent::class) { 
            startActivity(DetailActivity.getStartIntent(this)) 
        }
    }
}

Stores (Repositories)

All our applications respect broadly known repository pattern. The main message this
pattern tells: Define Store (Repository) classes with single entity related business logic
eg. UserStore, OrderStore, DeviceStore etc. Let's see this principle on UserStore class
from sample app:

@Singleton
class UserStore @Inject constructor() {
    private val userRelay = BehaviorRelay.createDefault(User.EMPTY)

    fun setUser(user: User) {
        userRelay.accept(user)
        // ... optionally persist user
    }

    fun getUser(): Observable<User> {
        return userRelay.hide()
    }
}

With this approach only one class is responsible for User related data access. Besides
custom classes, Room library Daos or for example Retrofit API interfaces might be
perceived on the same domain level as stores. Thanks to use cases we can easily access,
manipulate and combine this kind of data on background threads.

class GetUserFullNameObservabler @Inject constructor(
    private val userStore: UserStore
) : ObservablerUseCase<String>() {

    override fun prepare(): Observable<String> {
        return userStore.getUser()
            .map { "${it.firstName} ${it.lastName}" }
    }
}

We strictly respect this injection hierarchy:

Testing

In order to create successful applications, it is highly encouraged to write tests for your application. But testing can be tricky sometimes so here are our best practices and utilities that will help you to achieve this goal with this library.

See these tests in example module for more detailed sample.

ViewModel testing

mvvm-test dependency contains utilities to help you with ViewModel testing.

ViewModelTest that should be used as a base class for view model tests since it contains JUnit rules for dealing with a live data and with RxJava in tests.

See these tests in example module for more detailed sample of view model testing.

Events testing

The spy object should be used for an easy way of testing that expected events were sent to the view.

viewModel = spyk(SampleViewModel(mockViewState, ...), recordPrivateCalls = true)
...
verify { viewModel.sendEvent(ExpectedEvent) }

Mocking of observeWithoutOwner

When you are using observeWithoutOwner extensions then everyObserveWithoutOwner will be helpful for mocking of these methods.

So if a method in the view model looks somehow like this:

viewState.counter.observeWithoutOwner { value ->
    viewState.counterText.value = value.toString() 
}

then it can be mocked with the following method:

val counterLambda = viewModel.everyObserveWithoutOwner { 
    viewState.counter
}
...
counterLambda.invoke(1) 

invoke(...) call will invoke a lambda argument passed to the observeWithoutOwner method in the tested method.

Mocking of Use Cases

Add rx-usecase-test or cr-usecase-test dependencies containing utilities to help you with mocking use cases in a view model.

Since all 'execute' methods for use cases are implemented as extension functions, we created testing methods that will help you to easily mock them.

So if a method in the view model looks somehow like this:

fun onLoginClicked(name: String, password: String) {
    loginUseCase.execture(LoginData(name, password)) {
        onSuccess = { ... }
    }
}

then it can be mocked with the following method:

mockLoginUseCase.mockExecute(args = ...) { Single.just(user) } // For RxJava Use Cases 
or
mockLoginUseCase.mockExecute(args = ...) { user } // For Coroutines Use Cases

In case that use case is using nullable arguments:

mockLoginUseCase.mockExecuteNullable(args = ...) { Single.just(user) } // For RxJava Use Cases
or
mockLoginUseCase.mockExecuteNullable(args = ...) { user } // For Coroutines Use Cases

Activity and Fragment tests

mvvm-test dependency contains utilities to help you with espresso testing.

If you want to test Activities or Fragments then you have few possibilities. You can test them with the mocked implementation of a view model and view state, or you can test them with the real implementation of a view model and view state and with mocked use cases.

Since Fragments and Activities from the dagger module are using AndroidInjection, we created utilities to deal with this.

In your tests, you can use doAfterActivityInjection and doAfterFragmentInjection to overwrite injected dependencies. These methods are called right after AndroidInjection and that allows overwriting of needed dependencies. In the following example, we are replacing the view model with the implementation that is using a view model with mocked dependencies and some random class with mocked implementation.

doAfterActivityInjection<SampleActivity> { activity ->  
    val provider = SampleViewModel(mockk(), SampleViewState()).asProvider()  
    activity.viewModelFactory = SampleViewModelFactory(viewModelProvider)  
    activity.someInjectedClass = mockk()  
}	

See these tests in example module for more detailed samples of espresso test that can be executed as local unit tests or connected android tests.

Templates

Our MVVM framework requires several files to be created with each new Activity or Fragment. To make the process of screen creation smooth, our framework is shipped with a template module. It contains templates for creating "MVVM Activity" and "MVVM Fragment" via Android Studio GUI.

To import templates into Android Studio follow these steps:

On Mac/Linux

1. Add plugin into your project Gradle file

plugins {
    id "com.thefuntasty.mvvm.android-templates" version "LatestVersion"
}

2. Sync Gradle dependencies
3. Run from your Android Studio terminal

./gradlew copyTemplates

4. Restart Android Studio

Note: You can also run this task from Gradle sidebar in Android Studio, you can find it under
Tasks -> thefuntasty -> copyTemplates

On Windows

1. Copy folders MVVMActivity and MVVMFragment from mvvm-android\templates\src\main\resources\templates in to C:\Program Files\Android\Android Studio\plugins\android\lib\templates\other

2. Restart/Start Android Studio

When the templates are in place you can use them directly from GUI

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