How to organize interaction between external interface and internal system logic?

In Android, interaction between the external interface (UI) and the internal system logic is usually organized through architectural patterns such as MVVM (Model-View-ViewModel) or MVP (Model-View-Presenter).

Main ideas:

• View (Activity/Fragment) is responsible for displaying data and handling user events.
• ViewModel/Presenter contains interaction logic, processes data, and updates the View.
• Model represents data and business logic.

For communication between View and ViewModel, LiveData or Flow are often used, which allow the UI to automatically update when data changes.

Example with LiveData:

class MyViewModel : ViewModel() {
    val data = MutableLiveData<String>()

    fun loadData() {
        // load data
        data.value = "Hello, world!"
    }
}

// In Activity or Fragment
viewModel.data.observe(this) { value ->
    textView.text = value
}

Thus, the UI reacts to data changes, and internal logic is separated and testable.