在片段之间切换后 RecyclerView 项目消失

Question

我的应用程序中有 4 个片段，它们在 NavActivity 中管理：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_nav);
    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    viewPager = (ViewPager) findViewById(R.id.viewpager);
    setupViewPager(viewPager);


    fab = (FloatingActionButton) findViewById(R.id.fab);


    // Fab for fragments
    viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
        }
        @Override
        public void onPageSelected(int position) {
        }

        @Override
        public void onPageScrollStateChanged(int state) {
            switch (state) {
                case ViewPager.SCROLL_STATE_DRAGGING:
                case ViewPager.SCROLL_STATE_SETTLING:
                    fab.hide(); // Hide animation
                    break;
                case ViewPager.SCROLL_STATE_IDLE:
                    switch (viewPager.getCurrentItem()) {
                        case 0:
                            fragment1.shareFab(fab); 
                        case 1:
                            fragment2.shareFab(fab); 
                            fab.setOnClickListener(new View.OnClickListener() {
                                @Override
                                public void onClick(View v) {
                                    startActivity(new Intent(NavActivity.this, MapsActivity.class));
                                }
                            });
                            break;
                        default:
                            fragment1.shareFab(null);
                            break;
                    }
                    if(viewPager.getCurrentItem() == 0 || viewPager.getCurrentItem() == 1)
                        fab.show(); // Show animation
                    break;
            }

        }
    });

    tabLayout = (TabLayout) findViewById(R.id.tabs);
    tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            viewPager.setCurrentItem(tab.getPosition());

        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) {

        }

        @Override
        public void onTabReselected(TabLayout.Tab tab) {

        }
    });
    tabLayout.setupWithViewPager(viewPager);
}

private void setupViewPager(ViewPager viewPager) {
    ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());
    adapter.addFragment(new MainFragment(), "Main");
    adapter.addFragment(new AboutFragment(), "About");
    adapter.addFragment(new CartFragment(), "Cart");
    adapter.addFragment(new CatalogFragment(), "Catalog");
    viewPager.setAdapter(adapter);
}


///////////new class
class ViewPagerAdapter extends FragmentPagerAdapter {
    private final List<Fragment> mFragmentList = new ArrayList<>();
    private final List<String> mFragmentTitleList = new ArrayList<>();

    public ViewPagerAdapter(FragmentManager manager) {
        super(manager);
    }

    @Override
    public Fragment getItem(int position) {
        return mFragmentList.get(position);
    }

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

    public void addFragment(Fragment fragment, String title) {
        mFragmentList.add(fragment);
        mFragmentTitleList.add(title);
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return mFragmentTitleList.get(position);
    }
}

@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    List<Fragment> fragments = getSupportFragmentManager().getFragments();
    if (fragments != null) {
        for (Fragment fragment : fragments) {
            fragment.onRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }
}

}

我遇到一个问题，在片段之间切换后项目消失我认为 RecyclerView 和 viewHolder 的实现与它有关。

我正在尝试使用 volley 从我的服务器获取数据到片段 C（片段编号 3）中，数据流很好，一切都显示正确。

如果我切换到碎片 B（碎片编号 2）或 D（碎片编号 4），仍然一切正常。但是如果切换到片段 A（片段编号 1）并返回片段 3（我的列表项在哪里），一切都消失了，我得到黑屏，即使我上下交换项目仍然不会再次下载。

这是我的 RecyclerView 的 Frag num 3：

private FloatingActionButton mSharedFab;
private List<MyProducts> productsList;

//Creating Views
private RecyclerView recyclerView;
private RecyclerView.LayoutManager layoutManager;
private RecyclerView.Adapter adapter;

//Volley Request Queue
private RequestQueue requestQueue;


private int requestCount = 1;
private static final String URL_INDEX = "http://myserverip/android/product.php?page=";
//Tag values to read from json
public static final String TAG_IMAGE_URL = "product_img";
public static final String TAG_PRODUCT_SN = "product_serial_num";
public static final String TAG_PRODUCT_TITLE = "product_title";
public static final String TAG_PRODUCT_PRICE = "product_price";
public static final String TAG_PRODUCT_DESCRIPTION = "product_description";


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

 public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.fragment_cart, container, false);
    recyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
    layoutManager = new LinearLayoutManager(getActivity().getApplicationContext(), LinearLayoutManager.VERTICAL, false);
    recyclerView.setLayoutManager(layoutManager);
    recyclerView.setItemAnimator(new DefaultItemAnimator());
    recyclerView.setHasFixedSize(true);

    //Initializing our superheroes list
    productsList = new ArrayList<>();
    requestQueue = Volley.newRequestQueue(getActivity().getApplicationContext());

    //Calling method to get data to fetch data
    getData();

    //Adding an scroll change listener to recyclerview
    if (Build.VERSION.SDK_INT <= Build.VERSION_CODES.LOLLIPOP)
    {
        recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener()
        {
            @Override
            public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
                super.onScrolled(recyclerView, dx, dy);
                if (isLastItemDisplaying(recyclerView)) {
                    getData();
                }
            }
        });
    }
    else
    {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M)
        {
            recyclerView.setOnScrollChangeListener( new RecyclerView.OnScrollChangeListener()
            {
                @Override
                public void onScrollChange(View v, int scrollX, int scrollY, int oldScrollX, int oldScrollY) {
                    if (isLastItemDisplaying(recyclerView)) {
                        getData();
                    }
                }
            });
        }
    }

    //initializing our adapter
    adapter = new CustomAdapter(productsList, getActivity());
    //Adding adapter to recyclerview
    recyclerView.setAdapter(adapter);
    return view ;
}

和自定义适配器 class:

public class CustomAdapter extends RecyclerView.Adapter<CustomAdapter.ViewHolder> {

//Imageloader to load image
private ImageLoader imageLoader;
private Context context;


List<MyProducts> myProducts;

public CustomAdapter(List<MyProducts> myProducts, Context context)
{
    super();
    this.myProducts = myProducts;
    this.context = context;
}

@Override
public CustomAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View v = LayoutInflater.from(parent.getContext())
            .inflate(R.layout.product_list, parent, false);
    ViewHolder viewHolder = new ViewHolder(v);
    return viewHolder;
}

@Override
public void onBindViewHolder(CustomAdapter.ViewHolder holder, int position) {

    if( myProducts != null) {
        MyProducts myProducts1 = myProducts.get(position);
        imageLoader = ImageVolley.getInstance(context).getImageLoader();
        imageLoader.get(myProducts1.getProductImage(), ImageLoader.getImageListener(holder.imageView, R.drawable.android_store_log, android.R.drawable.ic_dialog_alert));

        //Showing data to the views
        holder.imageView.setImageUrl(myProducts1.getProductImage(), imageLoader);
        holder.textViewProductTitle.setText(myProducts1.getProductTitle());
        holder.textViewProductDescription.setText(myProducts1.getProductDescription());
        holder.textViewProductSerialNumber.setText(myProducts1.getProductSn());
        holder.textViewProductPrice.setText(myProducts1.getProductPrice());
    }
}

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

public class ViewHolder extends RecyclerView.ViewHolder {

    //Views
    public NetworkImageView imageView;
    public TextView textViewProductTitle;
    public TextView textViewProductDescription;
    public TextView textViewProductSerialNumber;
    public TextView textViewProductPrice;

    //Initializing Views
    public ViewHolder(View itemView) {
        super(itemView);
        imageView = (NetworkImageView) itemView.findViewById(R.id.imageViewHero);
        textViewProductTitle = (TextView) itemView.findViewById(R.id.textViewProductTitle);
        textViewProductDescription = (TextView) itemView.findViewById(R.id.textViewProductDescription);
        textViewProductSerialNumber = (TextView) itemView.findViewById(R.id.textViewProductSerialNumber);
        textViewProductPrice = (TextView) itemView.findViewById(R.id.textViewProductPrice);
    }
}

Answer 1

试试这个：

viewPager.setOffscreenPageLimit(3);

Set the number of pages that should be retained to either side of the current page in the view hierarchy in an idle state. Pages beyond this limit will be recreated from the adapter when needed.

setOffscreenPageLimit

在片段之间切换后 RecyclerView 项目消失

RecyclerView items disappear after switching between fragments

android

android-fragments

android-viewpager

android-viewholder

android-recyclerview