使用哈希映射从URL获取图像不会显示它们

Question

初学者在Android应用程序开发这里，我正在制作一个游戏新闻应用程序，在回收站视图网格中显示很多游戏（项目）。 除了来自网址的图片外，所有内容都显示正确。 以下是我的一些代码：

SecondActivity.java

public class SecondActivity extends AppCompatActivity {

public ArrayList<HashMap<String, String>> mDataSource;

protected RecyclerView mRecyclerView;
protected RecyclerView.LayoutManager mLayoutManager;
protected HomeItemAdapter mAdapter;

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


    initDataset();


    mRecyclerView = (RecyclerView) findViewById(R.id.recyclerView);


    mLayoutManager = new GridLayoutManager(this, 3);
    mRecyclerView.setLayoutManager(mLayoutManager);

    mAdapter = new HomeItemAdapter(this, mDataSource);

    mRecyclerView.setAdapter(mAdapter);

}


private void initDataset() {
    mDataSource = new ArrayList();
    HashMap<String, String> h1 = new HashMap();
    h1.put("name", "Super Meat Boy");
    h1.put("city", "Indie");
    h1.put("image_url","https://66.media.tumblr.com/tumblr_ldi4d79gyD1qd9jlto1_500.jpg");
    mDataSource.add(h1);

HomeItemAdapter.java

public class HomeItemAdapter extends RecyclerView.Adapter<HomeItemAdapter.ViewHolder> {
private static final String TAG = "NTR-HomeItemAdapter";


public ArrayList<HashMap<String, String>> mDataSource;
private Context mContext;

public HomeItemAdapter(Context context, ArrayList<HashMap<String, String>> mDataSource) {
    this.mDataSource = mDataSource;
    this.mContext = context;
}

@Override
public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
    View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.row_home_item, viewGroup, false);

    return new ViewHolder(v);
}

@Override
public void onBindViewHolder(final ViewHolder viewHolder, int position) {
    Target target;
    Log.d(TAG, "Element " + position + " set.");

    viewHolder.getName().setText(mDataSource.get(position).get("name"));
    viewHolder.getCity().setText(mDataSource.get(position).get("city"));

    //Get Marquee
    viewHolder.getName().setSingleLine();
    viewHolder.getName().setEllipsize(TextUtils.TruncateAt.MARQUEE);
    viewHolder.getName().setHorizontallyScrolling(true);
    viewHolder.getName().setFocusableInTouchMode(true);
    viewHolder.getName().setSelected(true);
    viewHolder.getName().setMarqueeRepeatLimit(-1);

    viewHolder.getCity().setSingleLine();
    viewHolder.getCity().setEllipsize(TextUtils.TruncateAt.MARQUEE);
    viewHolder.getCity().setHorizontallyScrolling(true);
    viewHolder.getCity().setFocusableInTouchMode(true);
    viewHolder.getCity().setSelected(true);
    viewHolder.getCity().setMarqueeRepeatLimit(-1);

    // Get Dynamic Color
    target = new com.squareup.picasso.Target() {
        @Override
        public void onBitmapLoaded(final Bitmap bitmap, Picasso.LoadedFrom from) {
            Palette.generateAsync(bitmap, new Palette.PaletteAsyncListener() {
                public void onGenerated(Palette palette) {
                    if (palette !=null) {
                        viewHolder.getImageCover().setImageBitmap(bitmap);
                        Palette.Swatch vibrant = palette.getVibrantSwatch();
                        if(vibrant!=null) {
                            viewHolder.getCardItemHome().setBackgroundColor(vibrant.getRgb());
                            viewHolder.getName().setTextColor(vibrant.getTitleTextColor());
                            viewHolder.getCity().setTextColor(vibrant.getTitleTextColor());
                        }
                    }
                }
            });
        }

        @Override
        public void onBitmapFailed(Drawable errorDrawable) {
            viewHolder.getImageCover().setImageResource(R.drawable.cover);
        }

        @Override
        public void onPrepareLoad(Drawable placeHolderDrawable) {

        }
    };
    Log.d(TAG,mDataSource.get(position).get("image_url"));

    Picasso.with(mContext)
            .load(mDataSource.get(position).get("image_url"))
            .into(target);

}

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

public static class ViewHolder extends RecyclerView.ViewHolder {
    private final CardView cardItemHome;
    private final ImageView imgCover;
    private final TextView txtName;
    private final TextView txtCity;

    public ViewHolder(View v) {
        super(v);

        v.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.d(TAG, "Element " + getPosition() + " clicked.");
            }
        });
        cardItemHome = (CardView) v.findViewById(R.id.card_item_home);
        txtName = (TextView) v.findViewById(R.id.txt_name);
        txtCity = (TextView) v.findViewById(R.id.txt_city);
        imgCover = (ImageView) v.findViewById(R.id.img_cover);

    }

    public CardView getCardItemHome() {
        return cardItemHome;
    }

    public TextView getName() {
        return txtName;
    }

    public TextView getCity() {
        return txtCity;
    }

    public ImageView getImageCover() {
        return imgCover;
    }

}

}