无法在范围外返回arraylist，它变为null

Question

当我将获得的数据（包含在 arraylist 中）复制到新的 arraylist 并返回时，新的 arraylist 不会返回任何值。 onDataReceived(ArrayList arrayList)是arraylist中volley异步获取数据后的监听回调方法。

package com.example.bike.ViewModels;

import android.app.Application;
import android.arch.lifecycle.AndroidViewModel;
import android.arch.lifecycle.LiveData;
import android.support.annotation.NonNull;
import com.example.bike.Models.BikesInfo;
import com.example.bike.Repository.BikesRepository;
import com.example.bike.Repository.Listener;
import com.example.bike.Views.ApplicationContext;
import java.util.ArrayList;
import java.util.Collections;

public class BikesViewModel extends AndroidViewModel {
 private BikesRepository bikesRepository;
 public static ArrayList<BikesInfo> vmarrayList = new ArrayList<>();
 public BikesViewModel(@NonNull Application application) {
    super(application);
     bikesRepository = new BikesRepository(application, new Listener() {
            @Override
            public void onDataReceived(ArrayList<BikesInfo> arrayList) {

                vmarrayList.addAll(arrayList);
            }

            @Override
            public void onError(int error) {

            }
        });
        bikesRepository.getArrayList();

    }

public ArrayList<BikesInfo> getbikesinfolistobservable(){
       return vmarrayList;
  }

}

编辑： MainActivity.java

package com.example.bike.Views.ui;

import android.arch.lifecycle.AndroidViewModel;
import android.arch.lifecycle.Observer;
import android.arch.lifecycle.ViewModelProviders;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import com.example.bike.Models.BikesInfo;
import com.example.bike.R;
import com.example.bike.ViewModels.BikesViewModel;
import com.example.bike.Views.Adapters.BikesListAdapter;
import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {
    BikesViewModel bikesViewModel;

    RecyclerView recyclerView;
    LinearLayoutManager linearLayoutManager;
    BikesListAdapter bikesListAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        recyclerView = findViewById(R.id.bikes_recycler);
        linearLayoutManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(linearLayoutManager);
        recyclerView.setHasFixedSize(true);

        bikesViewModel = ViewModelProviders.of(MainActivity.this).get(BikesViewModel.class);

        bikesListAdapter = new BikesListAdapter(bikesViewModel.getbikesinfolistobservable());
        recyclerView.setAdapter(bikesListAdapter);

    }

}

Answer 1

如果您使用 volley 发出请求，则无法确保 getbikesinfolistobservable() 的调用将在请求完成后执行，因为它是异步请求。

尝试在 vmarrayList.addAll(arrayList) 之后在 vmarrayList 上执行控制台日志。如果仍然是空的，那么ArrayList arrayList也是空的，所以请求的响应会有问题。

vmarrayList 也是一个静态变量，而 BikesViewModel 类是公共的，因此您不需要 getter。