java - 为什么在切换 fragment 时，recyclerview 丢失了？

Question

您好，我是 java 编程 (android) 的新手。目前我正在学习 recyclerview 在 fragment 中使用 volley 并需要帮助。为什么在将 fragment 一切换到 fragment 二以及返回 fragment 一时，recycleview 丢失了？即使在开始 recyleview 可用？谢谢帮助

这是backgroundtask.java

Context mContext;
ArrayList<List> mArrayList = new ArrayList<>();
String json_url = "http://192.168.43.215/belajar/contact_mad.php";
private static final String TAG = "BackgroundTask";

public BackgroundTask(Context context) {
    this.mContext = context;

}

public ArrayList<List> getList()
{
    JsonArrayRequest jsonArrayRequest = new JsonArrayRequest(Request.Method.POST, json_url, (String) null,
            new Response.Listener<JSONArray>() {
                @Override
                public void onResponse(JSONArray response) {

                    int count = 0;
                    while (count<response.length())
                    {
                        try {
                            JSONObject jsonObject = response.getJSONObject(count);
                            List list = new List(jsonObject.getString("Id"),jsonObject.getString("ImageUrl"),jsonObject.getString("Name"));
                            mArrayList.add(list);
                            count++;

                        } catch (JSONException e) {
                            e.printStackTrace();
                        }
                    }

                }
            }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            Toast.makeText(mContext,"Errorr.............",Toast.LENGTH_SHORT).show();
            error.printStackTrace();

        }
    }
    );

    MySingleton.getInstance(mContext).addToRequestQue(jsonArrayRequest);
    Log.e(TAG, "Log Dari background");


    return mArrayList;
}

这是recycleradapter.java

public class RecyclerAdapter extends RecyclerView.Adapter<RecyclerAdapter.MyViewHolder> {
ArrayList<List> mArrayList = new ArrayList<>();

private static final String TAG = "RecyclerAdapter";

public RecyclerAdapter(ArrayList<List> arrayList){

    this.mArrayList = arrayList;

}




@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.row_item,parent,false);
    MyViewHolder myViewHolder = new MyViewHolder(view);
    return myViewHolder;
}

@Override
public void onBindViewHolder(MyViewHolder holder, int position) {

    final String name = mTxtName.getText().toString();
    final String email = mTxtEmail.getText().toString();
    final String nameFood = this.mArrayList.get(position).getName();
    final String imageUrl = this.mArrayList.get(position).getImageUrl();

    holder.Name.setText(mArrayList.get(position).getName());
    final Context context = holder.mImageView.getContext();
    Picasso.with(context).load(mArrayList.get(position).getImageUrl()).into(holder.mImageView);
    Log.e(TAG, "Log Dari On BindView");


    holder.cvMain.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            /*Intent intent = new Intent(v.getContext(),ActivityClick.class);
            Bundle bundle = new Bundle();
            bundle.putString("imageUrl",imageUrl);
            bundle.putString("name",name);
            intent.putExtras(bundle);
            v.getContext().startActivity(intent);*/
            Toast.makeText(context, name, Toast.LENGTH_LONG).show();
        }
    });

}

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

public static class MyViewHolder extends RecyclerView.ViewHolder{

    TextView Name;
    RelativeLayout cvMain;
    ImageView mImageView;

    public MyViewHolder(View itemView) {
        super(itemView);
        mImageView = (ImageView) itemView.findViewById(R.id.imageView);
        Name = (TextView)itemView.findViewById(R.id.txtName);
        cvMain = (RelativeLayout) itemView.findViewById(R.id.cvMain);

    }
}

这是MainMenu.java

public class MainMenu extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

public static TextView mTxtName, mTxtEmail;
private Fragment fragment;

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


    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    mTxtName   = (TextView)navigationView.getHeaderView(0).findViewById(R.id.txtNavName);
    mTxtEmail   = (TextView)navigationView.getHeaderView(0).findViewById(R.id.txtNavEmail);
    Bundle bundle = getIntent().getExtras();
    mTxtName.setText(bundle.getString("name"));
    mTxtEmail.setText(bundle.getString("email"));


    android.support.v4.app.FragmentManager fm = getSupportFragmentManager();
    android.support.v4.app.FragmentTransaction transaction = fm.beginTransaction();
    transaction.replace(R.id.tempatfragment, new ListMakanan()).addToBackStack(null).commit();
}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main_menu, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.cara_pesan) {

        return true;
    }else if(id == R.id.keluar){
        finish();
        return true;
    }

    return super.onOptionsItemSelected(item);
}

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {


    // Handle navigation view item clicks here.
    int id = item.getItemId();

    android.support.v4.app.FragmentManager fm = getSupportFragmentManager();
    android.support.v4.app.FragmentTransaction transaction = fm.beginTransaction();

    if (id == R.id.list_makanan) {
        transaction.replace(R.id.tempatfragment, new ListMakanan()).addToBackStack(null).commit();

    } else if (id == R.id.pesan_makanan) {
        transaction.replace(R.id.tempatfragment, new PesanMakanan()).addToBackStack(null).commit();

    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

这是 fragment ListMakanan.java

public class ListMakanan extends Fragment {

RecyclerView mRecyclerView;
RecyclerView.Adapter mAdapter;
RecyclerView.LayoutManager mLayoutManager;
ArrayList<List> mArrayList = new ArrayList<>();
private static final String TAG = "ListMakanan";


public ListMakanan() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_list__makanan, container, false);
}

@Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {

    super.onViewCreated(view, savedInstanceState);

    mRecyclerView = (RecyclerView)view.findViewById(R.id.recyclervView);
    mLayoutManager = new LinearLayoutManager(getActivity());
    mRecyclerView.setLayoutManager(mLayoutManager);
    mRecyclerView.setHasFixedSize(true);

    BackgroundTask backgroundTask = new BackgroundTask(getActivity());
    mArrayList = backgroundTask.getList();
    mAdapter = new RecyclerAdapter(mArrayList);
    mRecyclerView.setAdapter(mAdapter);
    Log.e(TAG, "Log Dari View Created");


}

Answer 1

每当您在 FragmentManager 上使用 replace 来切换 fragment 时，您都在执行删除(从容器中删除所有 fragment )，然后添加(新 fragment )。这会清除前一个 fragment 的状态并在您返回时强制刷新，为避免这种情况，您可以只调用添加 fragment 而不是替换。