where to write callback function for ItemTouchHelper which is used with RecyclerView

Question

I want to write a piece of code into where item drag-drop is over. I got this answer https://stackoverflow.com/a/36275415/1684778 but I am confused about how to achieve this(where i have to implement that answer)

I want to write onDrop instead of onItemMove, how to achieve it.

DragItemTouchHelper

   public class DragItemTouchHelper extends ItemTouchHelper.Callback {
    public static final float ALPHA_FULL = 1.0f;

    private final MoveHelperAdapter mAdapter;

    public DragItemTouchHelper(MoveHelperAdapter adapter) {
        mAdapter = adapter;
    }

    @Override
    public boolean isLongPressDragEnabled() {
        return true;
    }

    @Override
    public boolean isItemViewSwipeEnabled() {
        return false;
    }

    @Override
    public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
        // Set movement flags based on the layout manager
        if (recyclerView.getLayoutManager() instanceof GridLayoutManager) {
            final int dragFlags = ItemTouchHelper.UP | ItemTouchHelper.DOWN | ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT;
            final int swipeFlags = 0;
            return makeMovementFlags(dragFlags, swipeFlags);
        } else {
            final int dragFlags = ItemTouchHelper.UP | ItemTouchHelper.DOWN;
            final int swipeFlags = ItemTouchHelper.START | ItemTouchHelper.END;
            return makeMovementFlags(dragFlags, swipeFlags);
        }
    }

    @Override
    public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder source, RecyclerView.ViewHolder target) {
        if (source.getItemViewType() != target.getItemViewType()) {
            return false;
        }

        // Notify the adapter of the move
        mAdapter.onItemMove(source.getAdapterPosition(), target.getAdapterPosition());
        return true;
    }

    @Override
    public void onSwiped(RecyclerView.ViewHolder viewHolder, int i) {
    }

    @Override
    public void onChildDraw(Canvas c, RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState, boolean isCurrentlyActive) {
        if (actionState == ItemTouchHelper.ACTION_STATE_SWIPE) {
            // Fade out the view as it is swiped out of the parent's bounds
            final float alpha = ALPHA_FULL - Math.abs(dX) / (float) viewHolder.itemView.getWidth();
            viewHolder.itemView.setAlpha(alpha);
            viewHolder.itemView.setTranslationX(dX);
        } else {
            super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive);
        }
    }

    @Override
    public void onSelectedChanged(RecyclerView.ViewHolder viewHolder, int actionState) {
        // We only want the active item to change
        if (actionState != ItemTouchHelper.ACTION_STATE_IDLE) {
            if (viewHolder instanceof TouchViewHolder) {
                // Let the view holder know that this item is being moved or dragged
                TouchViewHolder itemViewHolder = (TouchViewHolder) viewHolder;
                itemViewHolder.onItemSelected();
            }
        }

        super.onSelectedChanged(viewHolder, actionState);
    }

    @Override
    public void clearView(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
        super.clearView(recyclerView, viewHolder);

        viewHolder.itemView.setAlpha(ALPHA_FULL);

        if (viewHolder instanceof TouchViewHolder) {
            // Tell the view holder it's time to restore the idle state
            TouchViewHolder itemViewHolder = (TouchViewHolder) viewHolder;
            itemViewHolder.onItemClear();
        }
    }

    public interface MoveHelperAdapter {

        boolean onItemMove(int fromPosition, int toPosition);
    }

    public interface TouchViewHolder {
        void onItemSelected();

        void onItemClear();
    }
}

Answer 1

You can put your class that extends ItemTouchHelper callback in the end of the activity class where your RecyclerView is and attach it to RecyclerView defined in activity or fragment as below

ItemTouchHelper touchHelper = new ItemTouchHelper(your_custom_callback);
touchHelper.attachToRecyclerView(recyclerView);