Firebase ValueEventListener 导致 RecyclerView 中出现重复条目

Question

由于ValueEventListener每次更新数据时都会触发，因此在显示Firebase Database时完成新上传时也会调用它。RecyclerView当发生这种情况时，我现在的方式会导致重复条目，因为Upload对象已经在mUploads List. 当上传完成后，整个数据库会再次查询，所有项目都会添加到已经存在的数据库中，ArrayList我不知道如何解决这个问题。我应该Arraylist在每次触发 ValueEventListener 时创建一个新的，还是需要一个完全不同的回调？它ValueEventListener还会干扰我的mAdapter.notifyItemRemoved通话，因为ValueEventListener一旦我删除某些内容，它就会被触发。

public class ImagesActivity extends AppCompatActivity implements ImageAdapter.OnItemClickListener {
private RecyclerView mRecyclerView;
private ImageAdapter mAdapter;

private ProgressBar mProgressCircle;

private DatabaseReference mDatabaseRef;
private FirebaseStorage mStorage;

private List<Upload> mUploads;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_show_images);

    mUploads = new ArrayList<>();

    mRecyclerView = findViewById(R.id.recycler_view);
    mRecyclerView.setHasFixedSize(true);
    mRecyclerView.setLayoutManager(new LinearLayoutManager(this));

    mProgressCircle = findViewById(R.id.progress_circle);

    mDatabaseRef = FirebaseDatabase.getInstance().getReference("uploads");
    mStorage = FirebaseStorage.getInstance();

    mDatabaseRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                Upload upload = postSnapshot.getValue(Upload.class);
                upload.setKey(postSnapshot.getKey());

                mUploads.add(upload);
            }

            mAdapter = new ImageAdapter(ImagesActivity.this, mUploads);

            mRecyclerView.setAdapter(mAdapter);

            mAdapter.setOnItemClickListener(ImagesActivity.this);

            mProgressCircle.setVisibility(View.INVISIBLE);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Toast.makeText(ImagesActivity.this, databaseError.getMessage(), Toast.LENGTH_LONG).show();
            mProgressCircle.setVisibility(View.INVISIBLE);
        }
    });
}

@Override
public void onDeleteClick(final int position) {
    Upload selectedItem = mUploads.get(position);
    final String selectedKey = selectedItem.getKey();

    StorageReference imageRef = mStorage.getReferenceFromUrl(selectedItem.getImageUrl());
    imageRef.delete().addOnSuccessListener(new OnSuccessListener<Void>() {
        @Override
        public void onSuccess(Void aVoid) {
            mDatabaseRef.child(selectedKey).removeValue();
            mUploads.remove(position);
            mAdapter.notifyItemRemoved(position);
        }
    });

}
}

Answer 1

mUploads在添加数据之前先清除，addValueEventListener如下所示

 mDatabaseRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            mUploads.clear(); //change here

            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                Upload upload = postSnapshot.getValue(Upload.class);
                upload.setKey(postSnapshot.getKey());

                mUploads.add(upload);
            }

            mAdapter = new ImageAdapter(ImagesActivity.this, mUploads);

            mRecyclerView.setAdapter(mAdapter);

            mAdapter.setOnItemClickListener(ImagesActivity.this);

            mProgressCircle.setVisibility(View.INVISIBLE);
        }