Reactive Apps with Model-View-Intent - Part 8: Navigation

In my previous blog post we discussed how the Coordinator pattern can be applied on Android. This time I would like to show how this can be used in Model-View-Intent.

If you don’t know yet what the Coordinator pattern is I highly recommend to go back and read the introdcution.

Applying this pattern in MVI is not much different from MVVM or MVP: we pass a lambda as navigation callback into our MviBasePresenter. The interesting part is how do we trigger this callbacks in a state driven architecture? Let’s take a look at a concrete example:

class FooPresenter(
  private var navigationCallback: ( () -> Unit )?
) : MviBasePresenter<FooView> {  
  lateinit var disposable : Disposable
  override fun bindIntents(){
    val intent1 = ...
    val intent2 = ...
    val intents = Observable.merge(intent1, intent2)

    val state = intents.switchMap { ... }

    // Here stars the interesting part
    val sharedState = state.share()
    disposable = sharedState.filter{ state ->
      state is State.Foo
    }.subscribe { navigationCallback!!() }
  
    subscribeViewState(sharedState, FooView::render)
  }

  override fun unbindIntents(){
    disposable.dispose() // Navigation disposable
    navigationCallback = null // Avoid memory leaks
  }
}

The idea is to reuse the same state observable that we usually use to render the state in our View by using RxJava’s share() operator. This plus the combination of .filter() allows us to listen for a certain state and then trigger the navigation once we reached that state. The Coordinator Pattern then just works as described in my previous blog post.