Android ListView通过Volley从数据库动态加载数据但无条目显示问题排查求助

我尝试用Volley从数据库拉取数据动态填充ListView，已经确认Volley返回了正确的数据，但ListView始终没有内容。我的实现步骤如下：

1. OrderRetriever类（获取订单数据） #

public class OrderRetriever {
    private final Context d;
    String user, token;
    private OrderListViewAdapter orderAdapter;
    final ArrayList<Order> myOrders = new ArrayList<>();

    public OrderRetriever(Context d, String user, String token) {
        this.d = d;
        this.user = user;
        this.token = token;
    }

    public void retrieveOrder(final ListView listView, final KProgressHUD myProgressBar){
        myProgressBar.show();
        StringRequest serviceStringRequest = new StringRequest(Request.Method.POST, NetworkConstants.URL_MY_ORDERS, response -> {
            JSONObject jsonObject,photoObject;
            Order order;
            try{
                JSONObject productObject = new JSONObject(response);
                JSONArray productArray = productObject.getJSONArray("orders");
                JSONArray photoArray = productObject.getJSONArray("photos");
                if(productArray.length()>0){
                    String image = "", name, code,status;
                    int id;
                    double cost;
                    for(int i=0; i<productArray.length();i++){
                        jsonObject = productArray.getJSONObject(i);
                        id= jsonObject.getInt("id");
                        name = jsonObject.getString("name");
                        status = jsonObject.getString("status");
                        code = jsonObject.getString("code");
                        cost = jsonObject.getDouble("cost");
                        if(photoArray.length()>0){
                            for(int j=0; j<photoArray.length(); j++) {
                                photoObject = photoArray.getJSONObject(j);
                                if(photoObject.getInt("p_id")==jsonObject.getInt("id")) {
                                    image = photoObject.getString("path");
                                }
                            }
                        }
                        order= new Order(name, status, code,NetworkConstants.URL_IMAGE +image,id,cost);
                        Log.d("Products", "retrieveFinalOrder: "+order);
                        myOrders.add(order);
                    }
                }else{
                    myProgressBar.dismiss();
                    Toasty.warning(d,"No Orders found", 5000).show();
                }
                Log.d("TAG", "retrieveOrder: ");
                orderAdapter = new OrderListViewAdapter(d,myOrders);
                Log.d("TAG", "retrieveOrder: ading adapter");
                listView.setAdapter(orderAdapter);
                myProgressBar.dismiss();
            }catch (JSONException e){
                myProgressBar.dismiss();
                e.printStackTrace();
            }
        }, error -> {
            myProgressBar.dismiss();
            Log.d("Error", "Failed with error msg:\t" + error.getMessage());
            Log.d("Error", "Error StackTrace: \t" + Arrays.toString(error.getStackTrace()));
            // edited here
            try {
                byte[] htmlBodyBytes = error.networkResponse.data;
                Log.e("Error", new String(htmlBodyBytes), error);
            } catch (NullPointerException e) {
                e.printStackTrace();
            }
            Toast.makeText(d,"Error " + error, Toast.LENGTH_SHORT).show();
        }){
            protected Map<String, String> getParams() {
                Map<String, String> params = new HashMap<>();
                params.put("user",user);
                params.put("api_token", token);
                return params;
            }
        };
        serviceStringRequest.setRetryPolicy(new DefaultRetryPolicy(
                1000*5, /*DefaultRetryPolicy.DEFAULT_MAX_RETRIES*/
                3,
                DefaultRetryPolicy.DEFAULT_BACKOFF_MULT));
        RequestQueue productRequestQue = Volley.newRequestQueue(d);
        productRequestQue.add(serviceStringRequest);
    }
}

2. OrderListViewAdapter适配器 #

public class OrderListViewAdapter extends BaseAdapter {
    Context c;
    ArrayList<Order> orders;

    public OrderListViewAdapter(Context c, ArrayList<Order> orders) {
        this.c = c;
        this.orders = orders;
    }

    @Override
    public int getCount() {
        return orders.size();
    }

    @Override
    public Object getItem(int position) {
        return orders.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View view, ViewGroup viewGroup) {
        if(view==null){
            view = LayoutInflater.from(c).inflate(R.layout.order_list_layout,null,false);
        }
        TextView name = view.findViewById(R.id.name);
        TextView cost = view.findViewById(R.id.cost);
        TextView status = view.findViewById(R.id.status);
        ImageView image = view.findViewById(R.id.image);

        final Order order = (Order) this.getItem(position);
        name.setText(order.getName());
        cost.setText("KSH " + order.getCost());
        status.setText(order.getStatus());

        //check if there is an image returned
        if(order.getImage()!= null && order.getImage().length()>0){
            Picasso.with(c).load(order.getImage()).placeholder(R.drawable.drawerback).into(image);
        }else{
            Picasso.with(c).load(R.drawable.drawerback).into(image);
        }

        view.setOnClickListener(v -> {
            Intent intent = new Intent(c, ProductActivity.class);
            intent.putExtra("string_name","string_to_pass");
            c.startActivity(intent);
        });
        return view;
    }
}

3. Order数据模型 #

public class Order {
    private String name, status,code,image;
    private int id;
    double cost;

    public Order(String name, String status, String code, String image, int id, double cost) {
        this.name = name;
        this.status = status;
        this.code = code;
        this.image = image;
        this.id = id;
        this.cost = cost;
    }
}

4. 在OrdersActivity中调用 #

container = findViewById(R.id.orderContainer);
new OrderRetriever(this, clientid, new Db(this).getToken()).retrieveOrder(container,myProgressBar);

明明Volley返回了正确数据，可ListView就是没内容，我哪里漏了或者错了？

我帮你梳理几个大概率的问题点，都是Android开发中填充ListView时容易踩的坑：

• 核心问题：Order类缺少getter方法
  你看适配器里调用了order.getName()、order.getCost()这些方法，但你的Order模型只写了构造方法，完全没定义这些getter！适配器根本拿不到数据，自然显示空内容。赶紧给Order类补上所有属性的getter方法：

  public String getName() {
      return name;
  }
  
  public String getStatus() {
      return status;
  }
  
  public String getCode() {
      return code;
  }
  
  public String getImage() {
      return image;
  }
  
  public int getId() {
      return id;
  }
  
  public double getCost() {
      return cost;
  }
  

  这个应该是最关键的问题，甚至奇怪你编译的时候没报错？可能是IDE的提示被忽略了，补上之后应该就能正常显示了。

• 次要排查点：ListView布局设置
  检查你的order_list_layout布局文件，是不是根布局高度设成wrap_content但内部控件没有正确填充？或者ListView本身在父布局中的高度/权重设置错误，导致它的显示区域为0？另外注意不要把ListView嵌套在ScrollView里，这会导致ListView无法正确计算高度，从而显示为空。

• 优化建议：适配器数据刷新方式
  你现在是每次拿到数据后新建一个适配器，虽然理论上可行，但更稳妥的方式是提前初始化适配器并绑定数据源，拿到数据后只更新集合再通知适配器刷新。比如在OrderRetriever里提前初始化orderAdapter = new OrderListViewAdapter(d, myOrders);，然后在循环添加完订单后调用orderAdapter.notifyDataSetChanged()，这样能避免一些线程或上下文导致的刷新问题。

• 调试辅助：确认Order对象数据
  你打了Log.d("Products", "retrieveFinalOrder: "+order);，可以看看Log里的输出是不是对象的内存地址（比如com.xxx.Order@xxxx），如果是，说明Order类没重写toString()方法，虽然这不影响ListView显示，但重写后能更直观地看到订单数据是否正确：

  @Override
  public String toString() {
      return "Order{" +
              "name='" + name + '\'' +
              ", status='" + status + '\'' +
              ", cost=" + cost +
              '}';
  }
  

先把getter方法补上，这应该能解决你的问题，如果还有问题再排查布局和适配器刷新的问题~

内容的提问来源于stack exchange，提问作者The Only Smart Boy