java - 如何在 Android Studio 中添加/删除项目时计算总价

Question

我是 Android 的新手。我正在尝试创建一个电子商务应用程序。目前，我正在创建添加到购物车的 activity。因此，我设法将商品添加到购物车并显示在 CartActivity 中。但问题是，我不知道如何获取价格并计算所有项目的总价。当我删除项目时(我使用 onSwiped)，价格没有下降。

CartActivity

  public class CartActivity extends AppCompatActivity implements 
  RecyclerItemTouchHelper.RecyclerItemTouchHelperListener {
    private float totalcost=0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    Log.d(TAG, "onDataChange: found a post id: babyyywhuu " + book_id);

    new ItemTouchHelper(new ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT) {
        @Override
        public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
            return false;
        }

        // Called when a user swipes left or right on a ViewHolder
        @Override
        public void onSwiped(RecyclerView.ViewHolder viewHolder, int swipeDir) {

            int position = viewHolder.getAdapterPosition(); // this is how you can get the position
            Book book= mAdapter.getItem(position); // You will have your own class ofcourse.

            DatabaseReference reference = FirebaseDatabase.getInstance().getReference();
            reference.child("carts")
                    .child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                    .child(book.getBook_id())
                    .removeValue();

        }
    }).attachToRecyclerView(recyclerView);
    init();

}


private void init(){
    Log.d(TAG, "init: initializing.");
    cartList = new ArrayList<>();
    setupPostsList();

    //reference for listening when items are added or removed from the watch list
     mDatabaseRef = FirebaseDatabase.getInstance().getReference()
            .child(getString(R.string.node_carts))
            .child(FirebaseAuth.getInstance().getCurrentUser().getUid());

    //set the listener to the reference
    mDatabaseRef.addValueEventListener(mLisenter);

}

private void getWatchListIds(){
    Log.d(TAG, "getWatchListIds: getting users watch list.");
    if(cartList != null){
        cartList.clear();
    }
    if(mPostsIds != null){
        mPostsIds.clear();
    }

    mPostsIds = new ArrayList<>();
    DatabaseReference reference = FirebaseDatabase.getInstance().getReference();

    Query query = reference.child(getString(R.string.node_carts))
            .orderByKey()
            .equalTo(FirebaseAuth.getInstance().getCurrentUser().getUid());

    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if(dataSnapshot.getChildren().iterator().hasNext()){
                DataSnapshot singleSnapshot = dataSnapshot.getChildren().iterator().next();
                for(DataSnapshot snapshot: singleSnapshot.getChildren()){
                    String id = snapshot.child(getString(R.string.field_book_id)).getValue().toString();
                    Log.d(TAG, "onDataChange: found a post id: " + id);
                    mPostsIds.add(id);

                }
                getPosts();
            }else{
                getPosts();
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}

private void getPosts(){
    if(mPostsIds.size() > 0){
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference();

        for(int i  = 0; i < mPostsIds.size(); i++){
            Log.d(TAG, "getPosts: getting post information for: " + mPostsIds.get(i));

            Query query = reference.child(getString(R.string.node_books))
                    .orderByKey()
                    .equalTo(mPostsIds.get(i));

            query.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    DataSnapshot singleSnapshot = dataSnapshot.getChildren().iterator().next();
                    Book book = singleSnapshot.getValue(Book.class);
                    Log.d(TAG, "onDataChange: found a post: " + book.getTitle());
                    cartList.add(book);
                    mAdapter = new CartListAdapter (CartActivity.this, cartList);
                    recyclerView.setAdapter(mAdapter);
here---->           totalcost += Float.parseFloat(book.getPrice());
                    String totPrice = Float.toString(totalcost);
                    TextView totPrice = findViewById(R.id.totalPrice);
                    totPrice.setText(totPrice);
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });
        }
    }
}
@Override
public void onResume() {
    super.onResume();
    cartList = new ArrayList<>();


}

/**
 * callback when recycler view is swiped
 * item will be removed on swiped
 * undo option will be provided in snackbar to restore the item
 */
@Override
public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction, int position) {
    if (viewHolder instanceof CartListAdapter.MyViewHolder) {
        // get the removed item name to display it in snack bar
        String name = cartList.get(viewHolder.getAdapterPosition()).getTitle();

        // backup of removed item for undo purpose
        final Book deletedItem = cartList.get(viewHolder.getAdapterPosition());
        final int deletedIndex = viewHolder.getAdapterPosition();

        // remove the item from recycler view
        mAdapter.removeItem(viewHolder.getAdapterPosition());

        // showing snack bar with Undo option
        Snackbar snackbar = Snackbar
                .make(linearlayour, name + " removed from cart!", Snackbar.LENGTH_LONG);
        snackbar.setAction("UNDO", new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                // undo is selected, restore the deleted item
                mAdapter.restoreItem(deletedItem, deletedIndex);

            }
        });
        snackbar.setActionTextColor(Color.YELLOW);
        snackbar.show();
    }
}

我尝试在 getPosts() 中计算它，但我知道这是完全错误的，因为价格一直在上涨。我是否必须创建方法来增加/减少方法？你对此有什么解决办法吗？感谢您的宝贵时间。

Answer 1

有 4 种情况您必须更新价格:-

1. 添加一个新项目(在你的案例中可能没有使用只是因为你在购物车页面上)
2. 删除项目
3. 更新项目数量(增加或减少数量)
4. 应用任何优惠券代码

只需创建一个新函数并重新计算总价，设置为 TextView 并在适配器通知或更新适配器后立即调用。