Test

NewPresenter.kt

class NewPresenter(private val todoDao: TodoDao) {

    private var description: CharSequence? = null

    fun bind(view: TodoView) {
        todoDao.all()
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe { view.updateTodos(it) }
    }

    fun onSaveClicked() {
        todoDao.add(Todo().apply { text = description.toString() })
        description = null
    }

    fun onDescriptionChanged(text: CharSequence) {
        description = text
    }

    fun onTodoStateChanged(todo: Todo) = todoDao.update(todo)
}

OldPresenter.kt

class OldPresenter(private val todoDao: TodoDao) {

    private val description = BehaviorSubject.create<CharSequence>()

    fun bind(view: TodoView, disposables: CompositeDisposable) {
        disposables += view.addClicks
                .withLatestFrom(description, BiFunction<Unit, CharSequence, CharSequence> { _, description -> description })
                .observeOn(Schedulers.io())
                .subscribe {
                    todoDao.add(Todo().apply { text = it.toString() })
                    description.onNext("")
                }

        disposables += view.description
                .subscribe { description.onNext(it) }

        disposables += view.checkedTodo
                .observeOn(Schedulers.io())
                .map {
                    it.done = !it.done
                    it
                }
                .subscribe {
                    todoDao.update(it)
                }

        disposables += todoDao.all()
                .switchMap { list ->
                    description.first("").toFlowable()
                            .map { ViewState(list, it) }
                }
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(view.render)
    }
    
}

1. Hard to understand which event's view deliver to OldPresenter, in case of NewPresenter we have function with clear name onSaveClicked
2. withLatestFrom would become huge if you have more instance variables like description
3. Whole view is redrawn whenever changes happens (bad for performance)
4. With more complex screen activity and presenter will contains huge number of Observables, Subjects with event's floating around and eventually ends up in:
   https://github.com/kickstarter/android-oss/blob/master/app/src/main/java/com/kickstarter/viewmodels/ActivityFeedViewModel.java#L170
   and
   https://github.com/kickstarter/android-oss/blob/master/app/src/main/java/com/kickstarter/viewmodels/ActivityFeedViewModel.java#L51
5. Overall Presenter become more complex, hard to understand and maintain, because of rx transformations, passing event's here and there, etc.