Android MVP - passing string resources from the presenter to the view

Issue

I've recently started developing an Android app with a model–view–presenter architecture. One issue that keeps coming up that I haven't been able to find a good solution for is passing strings from the presenter to be displayed in the view.

I am trying to keep Android system dependencies out of the presenter layer to make unit testing easier. This is simple when providing a string that comes from a server or some other external source. When I need to display a simple error message that is always the same, I can just have a method like showWhateverError(), where the view already knows which string resource to use and can handle loading the resource itself. And when I have business logic that determines which string resource to provide the view, I can just reference the string resource ID in the presenter (although that feels wrong too).

The case that I haven't come up with a good solution for is when I need to display a string that sometimes comes from the server and is sometimes a string resource, based on some business logic. Any ideas would be helpful!

Solution

For these situations I have additional helper class which I call a Formatter. I pass the current state from my Presenter to the View which then asks the Formatter for the appropriate strings based on that state. I think a small example will help:

You have an object which represents the data you wish to set on the View. Let's call it State for now:

public class State {

    private final boolean isServerString;
    private final String serverString;

    public State(boolean isServerString, String serverString) {
        this.isServerString = isServerString;
        this.serverString = serverString;
    }

    public boolean isServerString() {
        return isServerString;
    }

    public String getServerString() {
        return serverString;
    }
}

In you Presenter you would create this based on whatever logic you need and pass it to the View:

public class MessagePresenter {

    private void setMessage() {

        // logic here

        State state = new State(true, "Hello from the server");
        view().setMessage(state);
    }
}

Then in your Activity/Fragment/ViewGroup you'd have something along the lines of:

public class MyActivity extends Activity implements MessageView {

    private MessageFormatter formatter;
    private TextView messageTextView;

    @Override
    public setMessage(State state) {

        String message = formatter.getMessage(state);
        messageTextView.setText(message);
    }
}

As you can see the view asks the Formatter for the String to display in the TextView. The Formatter would look something like this:

public class MessageFormatter {

    private Context context;

    public MessageFormatter(Context context) {
        this.context = context;
    }

    public String getMessage(State state) {

        return state.isServerString()
                ? state.getServerString()
                : context.getString(R.string.default_message);
    }
}

It takes a Context as a constructor param and, yes it does have a little bit of logic in it. However the heavy lifting logic remains in the Presenter. Most of the time it should just be a simple boolean check.

The real power of using this method comes into play when you make a Formatter interface and then use your Presenter to decide which Formatter to instantiate. As an example you can then create a HoliidayMessageFormatter and a DefaultMessageFormatter which would allow you to give you app a different theme based on a small logic check in the Presenter.

Answered By - Jahnold