How to test reading from/updating file in MVP?

Question

I'm trying to change my app from having no design pattern to using MVP.

Originally I had the following code:

override fun onCreateInputView(): View {

    //favoritesData is an instance variable, same with "Gson", "parser", "favorites", and "stringArrayListType"
    favoritesData = File(filesDir, "favorites_data.json")

    if (favoritesData.exists()) {
        favorites = Gson.fromJson(parser.parse(FileReader(favoritesData)), stringArrayListType)
    }
}

and

fun updateFavoritesFile() {
    favoritesData.writeText(Gson.toJson(favorites))
}

After trying to use MVP I changed the code to:

class AnimeFaceKeyboardPresenter(val view : AnimeFaceKeyboardView, private val model : KeyboardModel = KeyboardModel()) : Presenter {

    override fun onCreateInputView() {
        model.favorites = view.loadFavoritesFile()

        //At some point, call view.updateFavoritesFile(arrayListOf("test","test2"))

    }

    override fun onStartInputView() {
    }

}

and the code in the activity itself to:

override fun loadFavoritesFile() : ArrayList<String> {
    val favoritesData = File(filesDir, favoritesFileName)
    var favorites = ArrayList<String>()

    //"favorites" is no longer an instance variable     

    if (favoritesData.exists()) {
        favorites = Gson.fromJson(parser.parse(FileReader(favoritesData)), stringArrayListType)
    }

    return favorites
}


override fun updateFavoritesFile(favorites: ArrayList<String>) {
    File(filesDir, favoritesFileName).writeText(Gson.toJson(favorites))
}

override fun onCreateInputView(): View {
       super.onCreateInputView()

       presenter = AnimeFaceKeyboardPresenter(this)
       presenter.onCreateInputView()

}

I'm not sure if I'm using MVP correctly, but if I am, how would I go about testing this code. For example - writing a test that calls updateFavoritesFile(arrayListOf("test1","test2")) and uses loadFavoritesFile() to check if the contents is as expected.

Answer 1

Well, you might want to relocate your file read and write to your model (they are associated with data which doesn't really belong in your view).

Then your test consists of instantiating your model object, and testing the methods which can be done without the view and presenter (simplifying the tests).

I would be very tempted to abstract your file as some form of "Repository" object that knows how to read and write strings (but you don't care where or how). You would pass the repository object to your model as a construction property. The advantage of this is that you can create a mock or fake Repository object which you can use to "feed" test data and examine written data, making testing that part of your model a little easier.

Don't forget, your view shouldn't have direct access to your model under MVP .. that would me more like MVC (one of the very few differences between MVP and MVC).