Communication between BaseAdapter and Fragment

Question

First, I know this has been answered here : Android communication between fragment and baseadapter and I tried implementing it, but as it is my first time using interfaces and do not get the logic behind it, it did not work. My problem is I would like to set the value of the TextView of this fragment

Fragment.java

public class FragmentCart extends Fragment {
    private TextView totalTxt;
    private TextView totalItems;
    private ListView itemList;

    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_cart, container, false);
        totalTxt = (TextView)view.findViewById(R.id.totalTxt);
        totalItems = (TextView)view.findViewById(R.id.totalItems);
        itemList = (ListView)view.findViewById(R.id.itemList);
        itemList.setAdapter(MainActivity.cart);
        return view;
    }
}

with the value (double totalCost) from this adapter

Adapter.java

public class Cart extends BaseAdapter {
    private double totalCost = 0;

    public getView() {
        Item item.....
        totalCost += item.getPrice();
    }
}

The logic is that, as I add items to the cart (adapter) the TextView from the fragment updates as well. How do I do this with interface?

score 0 · Answer 1 · answered Aug 01 '18 at 15:50

To answer your question directly :

In Adapter.java declare a new interface that we'll call TotalCostUpdater:

public interface TotalCostUpdater {
    void onTotalCostUpdated(int totalCost);
}

Then, still in your adapter, you need a field implementing this interface, initialised through a constructor. From there you can call onTotalCostUpdated(totalCost) after you updated the value.

Your Adapter.java class will then look like this :

public class Cart extends BaseAdapter 
{

    public interface TotalCostUpdater {
        void onTotalCostUpdated(int totalCost);
    }

    private double totalCost = 0;

    private TotalCostUpdater costUpdater;

    public Cart(TotalCostUpdater costUpdater) {
        this.costUpdater = costUpdater;
    }

    public getView() 
    {
        Item item.....
        totalCost += item.getPrice();
        costUpdater.onTotalCostUpdated(totalCost);
    }
}

You then need to implement TotalCostUpdater, and override its onTotalCostUpdated method in FragmentCart.java :

public class FragmentCart extends Fragment implements Cart.TotalCostUpdater 
{
    private TextView totalTxt;
    private TextView totalItems;
    private ListView itemList;

    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) 
    {
        View view = inflater.inflate(R.layout.fragment_cart, container, false);
        totalTxt = (TextView)view.findViewById(R.id.totalTxt);
        totalItems = (TextView)view.findViewById(R.id.totalItems);
        itemList = (ListView)view.findViewById(R.id.itemList);
        itemList.setAdapter(MainActivity.cart);
        return view;
    }

    @Override
    public void onTotalCostUpdated(int totalCost) {
        totalTxt.setText(totalCost);
    }
}

When instantiating your adapter (at some point your must have written a new Cart()), you need to pass in your FragmentCart, which is now implementing the interface expected in your Cart's constructor :

new Cart(fragmentCart);

This should do the trick. Here are a few other things you should consider :

The adapter should not be the one holding the data, you should rather have a CartData class holding the actual cart values.
The adapter seems to be global from your MainActivity.cart. You should instantiate it directly within the Fragment class
You should use ButterKnife lib to get rid of all those findViewByIdcalls.

Communication between BaseAdapter and Fragment

1 Answers1