Home » Java » In MVP structure which class responsible for keep list items and how to notify data change in this

In MVP structure which class responsible for keep list items and how to notify data change in this

Posted by: admin December 28, 2021 Leave a comment

Questions:

I trying to refactor one of my activity class to implement mvp(using mvp mosby library) . I have a RecyclerView and in this view there is some items that some changes apply to them during the run time. for example I do some I/O operation and change one row.

I think it’s better to keep my items in presenter class; what is the best practice for this? keep this in 1)presenter or 2)activity or 3)only keep view related item in adapter and all other item in presenter.

the activity now keep items directly and change item row in activity and then notify adapter. isn’t better to move all this line in adapter and notify adapter in the adapter class? for example i want change icon of some row.where and which class is responsible for that? adapter? activity? now I want to implement it like this in adapter:

changeItemIcon(int position, int iconRes){
    mImages.get(position).setICon(iconRes);
    notifyItemChanged(position);
}

I invoke this method on activity and invoke activity method from presenter.

is it good? what is the best practice to do this?

UPDATE

also I find this question ( Best way to update data with a RecyclerView adapter ) that using adapter method for changing items. but what about modify? Need I keep reference to items in my activity?

Answers:

for example i want change icon of some row.where and which class is responsible for that? adapter? activity?

I know it sounds a little bit strange, but changing an element is always the responsibility of your “business logic”, even just for “icons”.

The workflow should be as follows (unidirectional data flow):

  1. View appeares, tells presenter to load a list of items
  2. Presenter loads items form “business logic” and registers himself as an
    observer / listener / callback (whatever you want to call it)
  3. Presenter receives result and tells the view to display the list of
    items (through RecyclerView and corresponding adapter).

so far is what you have implemented I guess, now it comes to the point where you want to change an item.

  1. User clicks on an item in your RecyclerView which then should trigger to change the icon of this item. Therefore View should call: presenter.changeItem()
  2. Presenter is just the man in the middle in this case and will invoke the “business logic layer” to tell that the item should be changed to new state (icon has changed).
  3. “Business logic layer” will change the models state (change the items icon) and then will notify its observer / listeners that the model has been changed.
  4. Since Presenter is still observing / listening to the business logic layer (see point 2.) the Presenter will be notified (see point 6.) with a new (updated) list of items containing the updated item which icon has been changed.
  5. Similar to point 3. Presenter will tell the view to display the new (updated) list of items (through RecyclerView and corresponding adapter).

Do you see the unidirectional data flow? That is very important. Immutability FTW.

###

MVP has two different variants: Passive View and Supervising Controller. Depending on your taste, you can stick to one or mix both of them in your app.

If you choose Passive View, you need to hide Model from View and let Presenter format data then set to the View. In this case, you need to keep Model reference in Presenter. View should only hold view-data (adapter) for its displaying purpose.

If you stick to Supervising Controller, you can allow View to directly bind data from Model and ask Model to perform some simple logic. Presenter should only care complex logic, i.e some operations which need to involve Services. In this case, you can give Model (your items) to View (activity) and let it interact with Model in some simple manner.

PS: Please also check out our new MVP framework: Robo MVP at http://robo-creative.github.io/mvp.

###

I’ve never used mosby, but I’ve just read their docs (good reading btw) and here’s my understanding:

A recycler view usually consists of the view (android term) and an adapter. Both are connected inside a fragment or activity. In terms of MVP/mosby this is all view layer. The presenter should only retrieve and pass the to-be-shown data from your service (model layer in mosby, “service layer” or “business logic” in other concepts), which in turn gets it from a DAO or repository (model layer).

The docs say that the presenter only handles the view state, not the actual contents. Your state is “showing list”.