Store new position of RecyclerView items in SQLite after being dragged and dropped

问题

I have a two dimensional ArrayList of type String which is the input to the adapter class of the RecyclerView. The data in the list is taken from an SQLite database everytime when onResume() is called. I have implemented a drag and drop feature and an onMove() function that swaps the list elements successfully. However, I need to store the modified list before onResume() is called which will rewrite the list and fill it with old positions. My guess is to implement the rewriting functionality within the onStop() event. This is my main activity:

RecyclerView recyclerView;
RecyclerViewAdapter adapter;
RecyclerView.LayoutManager layoutManager;

ArrayList<ArrayList<String>> data;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    assert fab != null;
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent i = new Intent(MainActivity.this, ViewDbActivity.class);
            startActivity(i);
        }
    });

    recyclerView = (RecyclerView) findViewById(R.id.myRecyclerView);
    layoutManager = new LinearLayoutManager(this);
    recyclerView.setLayoutManager(layoutManager);
    RecyclerView.ItemAnimator itemAnimator = new DefaultItemAnimator();

    ItemTouchHelper item = new ItemTouchHelper(new ItemTouchHelper.Callback() {

        @Override
        public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
            int dragFlags = ItemTouchHelper.UP | ItemTouchHelper.DOWN;
            int swipeFlags = ItemTouchHelper.RIGHT;
            return makeMovementFlags(dragFlags, swipeFlags);
        }

        @Override
        public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
            Collections.swap(data, viewHolder.getAdapterPosition(), target.getAdapterPosition());
            adapter.notifyItemMoved(viewHolder.getAdapterPosition(), target.getAdapterPosition());
            return true;
        }

        @Override
        public void onMoved(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, int fromPos, RecyclerView.ViewHolder target, int toPos, int x, int y) {
            super.onMoved(recyclerView, viewHolder, fromPos, target, toPos, x, y);
            Toast.makeText(MainActivity.this, "Moved", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {
            data.remove(viewHolder.getAdapterPosition());
            adapter.notifyItemRemoved(viewHolder.getAdapterPosition());
        }
    });

    item.attachToRecyclerView(recyclerView);
    recyclerView.setItemAnimator(itemAnimator);
}

@Override
protected void onResume() {
    super.onResume();

    initialize();

    if(adapter == null) {
        Log.v(TAG, "Adapter = NULL. Initialized");
        setAdapter();
    }
    else {
        Log.v(TAG, "notify is called");
        adapter.updateData(data);
    }
}

public void initialize() {
    Database ourDB = new Database(this);
    ourDB.openDB();
    data = ourDB.getData();
    ourDB.closeDB();
}

public void setAdapter() {
    adapter = new RecyclerViewAdapter(data, this);
    recyclerView.setAdapter(adapter);
}

It is important to update the database as long as I know when to store the data. How can I solve this problem? Any suggestions?

!!!

ANSWER:

Final solution I have implemented:

@Override
public void onMoved(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, int fromPos, RecyclerView.ViewHolder target, int toPos, int x, int y) {
    super.onMoved(recyclerView, viewHolder, fromPos, target, toPos, x, y);

    //preparing new position values
    long movedItem = Long.parseLong(data.get(fromPos).get(0));     // .get(fromPos).get(0) returns PRIMARY KEY from the list
    long draggedItem = Long.parseLong(data.get(toPos).get(0));

    // updating affected rows with new orders
    Database db = new Database(MainActivity.this);
        db.open();
        db.updateOrder(draggedItem, toPos);
        db.updateOrder(movedItem, fromPos);
        db.close();
}

And inside of DB class:

public void updateOrder(long rowID, int newPos) {
    ContentValues cv = new ContentValues();
    cv.put(KEY_ORDER, newPos);
    ourDatabase.update(DATABASE_TABLE, cv, KEY_ROWID + "=" + rowID, null);
}

回答1:

You need a field in each DB row for storing the order. Then you need to implements those features:

• On new row insert (when you insert a new object in database) you need to set the order field to the next int. You can get the current max value (with sql function MAX) and then simply do +1

• When user move an item in RecyclerView, in method onMovedyou must update all other rows. You can use the fromPos and toPos for that. More on that below

• When you fill your RecyclerView with data you need to order them by order field

---

Explanation of 2nd feature to be implemented: basically you need to update all rows with order between fromPos and toPos:

• if user moved the item up (for example from position 4 to 2), you need to:

  1. get primary key field of current item (using position 4)
  2. change all rows between order 2 and order 4: so change 2 -> 3 and 3 -> 4
  3. Change current item order (using primary key of first point) to toPos: in this example change current item order to 2

• if user moved the item down (for example from position 2 to 4) you need to:

  1. get primary key field of current item (using position 2)
  2. change all rows between order 2 and order 4: so change 4 -> 3 and 3 -> 2
  3. change current item order (using primary key of first point) to toPos: in this example change current item order to 4

Hope it helps a little

来源：https://stackoverflow.com/questions/38223107/store-new-position-of-recyclerview-items-in-sqlite-after-being-dragged-and-dropp