java - 保存 arrayList 而不是覆盖抽屉导航中的每个项目单击？

Question

我有一个抽屉导航，其中包含 Home、News、Photos 等项目列表。当我单击一个项目时，会加载并显示相应的 fragment 。

假设如果我点击 News，它会执行 Async Task 并将数据保存在 ArrayList 中。我的问题是，如果我在抽屉导航中再次点击 News 项目，它会删除旧的 ArrayList 值并显示新项目(即覆盖)。

由于我是 fragment 的新手，所以我不知道如何保存我的旧 ArrayList 值。

主 Activity .java

DemoFragment fragment = new DemoFragment();
getSupportFragmentManager().beginTransaction().replace(R.id.frame_container, fragment).commit();

DemoFragment.java

public class DemoFragment extends Fragment {
    RecyclerView recyclerView;
    ArrayList<Post> postArrayList;
    PostRecyclerAdapter postRecyclerAdapter;
    ProgressWheel progressWheel;   

    public DemoFragment() {

        // Required empty public constructor       
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);   
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Inflate the layout for this fragment    
        View v = inflater.inflate(R.layout.fragment_post, container, false);
        recyclerView = (RecyclerView) v.findViewById(R.id.posts_list);
        // progressBar = (ProgressBar)v.findViewById(R.id.progressBar);
        progressWheel = (ProgressWheel) v.findViewById(R.id.progress_wheel);

        recyclerView.setHasFixedSize(true);

        // use a linear layout manager
        LinearLayoutManager mLayoutManager = new LinearLayoutManager(getActivity());
        recyclerView.setLayoutManager(mLayoutManager);


        new LoadPosts(getActivity(), progressWheel, recyclerView).execute();
        return v;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        setRetainInstance(true);


    }

}

LoadPosts.java

class LoadPosts extends AsyncTask<Void, Void, Void>
{
    ProgressDialog progressDialog;
    List<ParseObject> parseObjects;
    Context context;
    FragmentManager supportFragmentManager;
    ArrayList<Post> postArrayList;
    String postC, postT,postI;
    RecyclerView recyclerView;
    ProgressWheel progressWheel;


    public LoadPosts(FragmentActivity context, ProgressWheel progressWheel, RecyclerView recyclerView) {

        this.context = context;
        this.progressWheel = progressWheel;
        this.recyclerView = recyclerView;

    }


    @Override
    protected void onPreExecute() {
        super.onPreExecute();

        progressDialog = new ProgressDialog(context);
        progressDialog.setMessage("Loading...");
        //progressDialog.show();
        progressWheel.setVisibility(View.VISIBLE);
    }

    @Override
    protected Void doInBackground(Void... avoid) {

            postArrayList = new ArrayList<>();
            // dont care about postC, postT. I get these values from parse db
            Post post = new Post();
            post.setContent(postC);
            post.setCreatedAt(postT);
            postArrayList.add(post);


        return null;
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);

        progressWheel.setVisibility(View.GONE);   
        PostRecyclerAdapter postRecyclerAdapter = new PostRecyclerAdapter(context, postArrayList);
        recyclerView.setAdapter(postRecyclerAdapter);

       supportFragmentManager.beginTransaction().replace(R.id.frame_container, fragment).commit();
    }

}

Answer 1

您需要将来自 onCreateView 的数据 postArrayList 返回给调用者。请记住，当您调用 ASyncTask 时，会创建/使用一个 UI 线程，之后所有数据都会从内存中释放(在某个时间点)。

更改自:

public LoadPosts(FragmentActivity context, ProgressWheel progressWheel, RecyclerView recyclerView)

TO(例如，更改第三个参数):

public LoadPosts(FragmentActivity context, ProgressWheel progressWheel, ArrayList<Post> postArrayList)

在 AsyncTask 中，更改 FROM:

@Override
protected void onPostExecute(String result) {

收件人:

@Override
protected void onPostExecute(ArrayList<Post> postArrayList) {
   super.onPostExecute(postArrayList);

   return postArrayList;
...
}

注意事项:

• 基本上，我们的想法是将修改后的数据返回给您的 Fragment 类。我实际上从来没有这样做过。我认为 ASyncTask 是为在后台做事而构建的，然后忘记结果。
• Google 网页说

onPostExecute(Result), invoked on the UI thread after the background computation finishes.

，链接@ ASyncTask .搜索文本。

尝试一下，让我们知道...