Provide a reference reactive android app with coroutines

[Ghedeon]

There is a need for less synthetic and more practical examples of Channel/Actor usage. It's an understandable (because of experimental status) impression that this topic is being avoided but maybe having a reference app from JB (or Google?) will trigger a productive discussion about in-the-field use cases.

Current status:
KotlinConf 2018. @chrisbanes https://github.com/chrisbanes/tivi
Awesome talk about coroutines and channels. The app is rewritten to replace Single/Maybe/Completable with suspended functions. The reactive part didn't got converted to Channels tho, as one might expect, and still uses RxJava Flowable. Reactive (Rx), no channels.

Google Sample: todo-mvp-kotlin
https://github.com/googlesamples/android-architecture/tree/todo-mvp-kotlin/
Uses suspended functions and good ol' callbacks when it's not enough. That's even pre Rx era.
Not reactive, no channels.

Google Sample: todo-mvvm-live-kotlin
https://github.com/googlesamples/android-architecture/tree/todo-mvvm-live-kotlin/
Suspended functions and workarounds like:

override fun refreshTasks() {
        cacheIsDirty = true
}

in order to avoid reactiveness, therefore — not truly reactive, no channels.

A subjective idea of what kind of app would fill the gap:

• Could be the same todo-mvvm-kotlin

• Clean architecture with clear data/domain/ui(presentation) layers

• LiveData in the ui layer. Android LiveData / architecture components integration #258 in particular will benefit from it. Suspended functions and channels in domain and data.

• Valid reactive use case, that most of these apps are trying to dodge. For example: the same idea with cached request that todo already has, but with a small twist: you hit the cache, send it to channel, perform the request, update the cache, send the fresh data to channel. This way you can't avoid having a stream of data.

• CoroutineScope in ViewModel. Maybe using viewModelScope from viewmodel-ktx library: https://android.googlesource.com/platform/frameworks/support/+/androidx-master-dev/lifecycle/viewmodel/ktx/src/main/java/androidx/lifecycle/ViewModel.kt

• Idiomatic error handling

• Coroutines tests

[elizarov]

I'm adding flow label here since whatever "example reactive app" we'll have a "recommended to write code with coroutines" it will have to be using flows. Also adding up for grab, because that would be great place for external contributions and, most likely, the sampel app will have to live in its own repository. We also plan to remove existing example app from the report after converting them into tests for CI purposes.

[elizarov]

Closing this one as Android team is doing a great job on teaching coroutines use nowadays.