MVP Android - How many presenters?

Question

I have a quick question. I am trying (and struggling) to design my application with the MVP design pattern.

Can I ask, for each view (activity, fragment) should I have a separate presenter class?

There aren't very many resources I can see online, that clearly with samples illustrate the MVP. Could anyone share if they have some?

PS I am also using RecyclerViewAdapter in this app so any pointers on that would be appreciated

Thanks in advance

Answer 1

Though old, this is very interesting question. Since nowadays MVP/MVC/MVVM is kind of "buzz-words" in Android community, this question deserves a more complete answer (IMHO).

Short answer:

Single presenter can be used with multiple views

Long answer:

In general, there is no one single definition of MVP/MVC - there are many approaches to implementation of these architectural patterns. You did not provide the definition of "your" MVP, therefore I can only guess what you have in mind.

That said, there are some "best practices" in Object Oriented Programming which any implementation of any architectural pattern should (ideally) take into consideration.

What you ask is whether you can reuse one presenter implementation with different views, right? Let's look at this question through prism of SOLID principles.

"L" stands for Liskov Substitution Principle (LSP). It is one of the most misunderstood principles in SOLID, but the general idea behind that principle says the following:

LSP: if a piece of code works with object of class A, it should also work seamlessly with objects of any subclass of A (i.e. subclasses must be usable instead of A everywhere)

Example of LSP violation in Android is Context: sublasses of Context (e.g. Application and Activity) are not equivalent. Some code that requires Context can work seamlessly with Application, but if you pass Activity instead, a memory leak will occur (this is very widespread bug in Android applications, which is caused primarily by violation of LSP by Google's devs).

Back top your question. I'm assuming that your presenter looks like this (note the interface for views):

public class SomePresenter {

    /**
     * Views bound to this presenter must implement this interface
     */
    interface SomeView {
        void doSomething1();
        void doSomething2();
    }

    public void bindView(SomeView someView) {
        // view binding logic
    }

    // more presenter's methods

}

LSP states that any class which implements SomeView can be usable with SomePresenter. The presenter shouldn't care whether the implementation of SomeView passed to it is Activity, Fragment, or, maybe, just a mock for unit test.

So, the full answer to your question is: one presenter can be reused with different views, as long as the presenter doesn't depend on particular implementations of views, but only on their super-class.

Additional information:

I would guess that you asked your question because, on the one hand, you felt that a single presenter should be able to work with different views (just think about A/B testing of different UIs), but, on the other hand, the fact that views are Activity and Fragment made you feel uncomfortable with this thought.

My personal opinion is that in MVC/MVP neither Activity nor Fragment should be views. The reasoning behind this claim is summarized in this post: Why Activities in Android are not UI Elements.

I also suggest you to take a look at a different approach to implementation of MVP in Android - if you'd use this one, it would be evident to you that a presenter should be able to work with different views, and you wouldn't have this feeling of "something doesn't feel right".