java - 图像未在 firebase RecyclerView 中显示

Question

我正在尝试显示 firebase 数据库中的图像。我认为在 firebase 中一切都很好，但没有显示任何内容。它没有在布局中显示图像，我为 recyclerview 和 item_view.xml 创建了 main_layout 这是我的代码。

MainActivity.java

<i>public class MainActivity extends AppCompatActivity {
private RecyclerView mRecyclerView;
private ImageAdapter mAdapter;


private DatabaseReference mDatabaseRef;
private List<Upload> mUploads;

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

    mRecyclerView = findViewById(R.id.recycler_view);
    mRecyclerView.setHasFixedSize(true);
    mRecyclerView.setLayoutManager(new GridLayoutManager(this, 2));

    mUploads = new ArrayList<>();

    mDatabaseRef = FirebaseDatabase.getInstance().getReference("Data");

    mDatabaseRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                Upload upload = postSnapshot.getValue(Upload.class);
                mUploads.add(upload);
            }

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


        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Toast.makeText(MainActivity.this, databaseError.getMessage(), Toast.LENGTH_SHORT).show();
        }
    });
}

}

ImageAdapter.java

<i>public class ImageAdapter extends RecyclerView.Adapter<ImageAdapter.ImageViewHolder> {
private Context mContext;
private List<Upload> mUploads;

public ImageAdapter(Context context, List<Upload> uploads) {
    mContext = context;
    mUploads = uploads;
}

@Override
public ImageViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View v = LayoutInflater.from(mContext).inflate(R.layout.item_view, parent, false);
    return new ImageViewHolder(v);
}

@Override
public void onBindViewHolder(ImageViewHolder holder, int position) {
    Upload uploadCurrent = mUploads.get(position);
    picasso.get()
            .load(uploadCurrent.getImageUrl())
            .placeholder(R.mipmap.ic_launcher)
            .fit()
            .centerCrop()
            .into(holder.imageView);
}

@Override
public int getItemCount() {
    return mUploads.size();
}

public class ImageViewHolder extends RecyclerView.ViewHolder {
    public ImageView imageView;

    public ImageViewHolder(View itemView) {
        super(itemView);

        imageView = itemView.findViewById(R.id.image_view);
    }
}
Picasso.Builder picassoBuilder = new Picasso.Builder(mContext);
Picasso picasso = picassoBuilder.build();

}

上传.java

<i>

公开课上传{ 私有(private)字符串mImageUrl;

public Upload() {

}

public Upload( String imageUrl) {
    mImageUrl = imageUrl;
}

public String getImageUrl() {
    return mImageUrl;
}

public void setImageUrl(String imageUrl) {
    mImageUrl = imageUrl;
}

}

调试输出:

E/RecyclerView:未连接适配器；跳过布局

Answer 1

您没有为 recyclerView 设置适配器，并且在从 Firebase 接收数据后也没有更新它。我将通过以下方式解决这个问题。首先，在适配器中添加一个 setter:

public void setUploads(List<Upload> uploads) {
    this.mUploads = uploads;
    this.notifyDataSetChanged();
}

这将允许您轻松修改适配器的数据集。然后，在您的 Activity 中，更改 onCreate 方法，如下所示:

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

    mRecyclerView = findViewById(R.id.recycler_view);
    mRecyclerView.setHasFixedSize(true);
    mRecyclerView.setLayoutManager(new GridLayoutManager(this, 2));

    mAdapter = new ImageAdapter(MainActivity.this, new ArrayList<>());
    mRecyclerView.setAdapter(mAdapter);

    mDatabaseRef = FirebaseDatabase.getInstance().getReference("Data");

    mDatabaseRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            List<Upload> readUploads = new ArrayList<>();
            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                Upload upload = postSnapshot.getValue(Upload.class);
                readUploads.add(upload);
            }
            mAdapter.setUploads(readUploads);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Toast.makeText(MainActivity.this, databaseError.getMessage(), Toast.LENGTH_SHORT).show();
        }
    });
}

这样，您将从一个空列表开始，因此您的 recyclerView 将不会显示任何内容。然后，当您收到数据时，您只需更新它，在适配器中设置数据集并通知适配器某些内容发生了变化，因此它必须重新绘制列表