java - 当我回到片段时防止重复项目

Question

在我的应用程序中，我有 ViewPager 包含 3 个片段（主页、个人资料等）

Home Fragment当我从或 导航时包含RecyclerViewHome Fragment ，它按预期工作。Profile FragmentMore Fragment

但是当我导航回时Home Fragment，RecyclerView中的项目重复...

当我再次导航时，它再次重复

这是我的MainActivity.java代码：

public class MainActivity extends AppCompatActivity {

 TabLayout mainTabLayout;
 ViewPager mainViewPager;

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



        mainTabLayout = findViewById(R.id.main_tablayout);
        mainViewPager = findViewById(R.id.main_viewpager);
        mainTabLayout.setupWithViewPager(mainViewPager);

        //to put mainTabLayout Icons...
        MainTabLayoutIcons();

    }

 //to Change MainLayout Icons...
    private void MainTabLayoutIcons() {

        ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());

        // add fragments to the adapter
        adapter.addFragment(new Home());
        adapter.addFragment(new MyActivity());
        adapter.addFragment(new More());

        mainViewPager.setOffscreenPageLimit(0);
        mainViewPager.setAdapter(adapter);


        // Select Home tab as default tab on startup
        mainViewPager.setCurrentItem(0);
        mainTabLayout.getTabAt(0).select();

        // Set tabs Initial icons
        mainTabLayout.getTabAt(0).setIcon(R.drawable.home_black);
        mainTabLayout.getTabAt(1).setIcon(R.drawable.user_outline);
        mainTabLayout.getTabAt(2).setIcon(R.drawable.more_outline);

        /* tabSelectedListener to change the icon color once it is selected
        // and change it back once another tab is selected */
        mainTabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {

                if (tab.getPosition() == 0) {
                    mainTabLayout.getTabAt(0).setIcon(R.drawable.home_black);
                    mainTabLayout.getTabAt(1).setIcon(R.drawable.user_outline);
                    mainTabLayout.getTabAt(2).setIcon(R.drawable.more_outline);

                }
                if (tab.getPosition() == 1) {
                    mainTabLayout.getTabAt(0).setIcon(R.drawable.home_outline);
                    mainTabLayout.getTabAt(1).setIcon(R.drawable.user_);
                    mainTabLayout.getTabAt(2).setIcon(R.drawable.more_outline);
                }

                if (tab.getPosition() == 2) {
                    mainTabLayout.getTabAt(0).setIcon(R.drawable.home_outline);
                    mainTabLayout.getTabAt(1).setIcon(R.drawable.user_outline);
                    mainTabLayout.getTabAt(2).setIcon(R.drawable.more_black);
                }
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {
                //no need for it
            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {
               //no need for it

           }

        });
    }


}

这是ViewPagerAdapter代码：

  //Adapter for mainActivity ...
    public class ViewPagerAdapter extends FragmentStatePagerAdapter {

        private final List<Fragment> fragmentList = new ArrayList<>();

        public ViewPagerAdapter(@NonNull FragmentManager fm) {
            super(fm);
        }
        
        @Override
        public Fragment getItem(int position) {
            
            return fragmentList.get(position);
        }

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


        public void addFragment(Fragment fragment) {
            fragmentList.add(fragment);
        }

    }

这是第一个片段Home.java代码：

public class Home extends Fragment {

    View v;

    public SwipeRefreshLayout mSwipeRefreshLayout;

    public RecyclerView mainRecyclerView;

    private LinearLayoutManager linearLayoutManager;

    public final List<MyActivityModel> mainPosts = new ArrayList<>();
    private PostAdapter mAdapter;


    public Home() {
        /*Required Empty Constructor... */
    }


    @Override
    public View onCreateView(LayoutInflater layoutInflater, ViewGroup container, Bundle bundle) {

        v = layoutInflater.inflate(R.layout.fragment_home, container, false);

        mSwipeRefreshLayout = (SwipeRefreshLayout) v.findViewById(R.id.mainSwipeRefresh);

     
        mainRecyclerView = (RecyclerView) v.findViewById(R.id.MainRecyclerView);

        linearLayoutManager = new LinearLayoutManager(getActivity());

        mainRecyclerView.setLayoutFrozen(true);

mainRecyclerView.setNestedScrollingEnabled(false);

        mainRecyclerView.setLayoutManager(linearLayoutManager);

        mAdapter = new PostAdapter(mainPosts);

mainRecyclerView.setAdapter(mAdapter);

        Swipe();
        LoadPosts();
          
        return v;
    }


    /**
     * -----------------TODO: to load posts in Home RecyclerView...----------------
     */
    public void LoadPosts() {

        Query database = FirebaseDatabase.getInstance().getReference().child("Posts");
        database.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

                if (snapshot.exists()) {

                    MyActivityModel post = snapshot.getValue(MyActivityModel.class);

                    mainPosts.add(0, post); 
                    mAdapter.notifyDataSetChanged();
                }


            }

            @Override
            public void onChildChanged(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
            }

            @Override
            public void onChildRemoved(@NonNull DataSnapshot snapshot) {
            }

            @Override
            public void onChildMoved(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
            }

        });
 

   

    }

     
  private void Swipe() {

        //to refresh page after swipe
        mSwipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
            @Override
            public void onRefresh() {

                Handler handler = new Handler();
                handler.postDelayed(new Runnable() {
                    @Override
                    public void run() {

                        try {

                            mainPosts.clear();
                          
                            LoadPosts();

                          mSwipeRefreshLayout.setRefreshing(false);

                            Toast.makeText(v.getContext(),"Refreshed", Toast.LENGTH_SHORT).show();

                        } catch (Exception e) {

                            mSwipeRefreshLayout.setRefreshing(false);

                        }

                    }
                }, 500);
            }

        });


    }
}

这是一张显示物品重复的图像

任何人都可以帮助我如何防止这种重复......

标签： javaandroidandroid-fragmentsandroid-recyclerviewandroid-viewpager

Answer 1

您可能需要使用FragmentPagerAdapter而不是FragmentStatePagerAdapter。此外，设置mainRecyclerView.setLayoutFrozen(true);为 false，因为它将停止 recyclerView 中的所有内容。

什么时候需要使用 FragmentPagerAdapter？

当有大量页面时，FragmentStatePagerAdapter 更有用，更像列表视图。当页面对用户不可见时，它们的整个片段可能会被销毁，只保留该片段的已保存状态。与 FragmentPagerAdapter 相比，这允许分页器保留与每个访问页面相关联的内存更少，但代价是在页面之间切换时可能会产生更多开销。

FragmentPagerAdapter 示例：

    class viewpager_class extends FragmentPagerAdapter {
    int tabcount;

    public viewpager_class(@NonNull FragmentManager fm, int tab_count) {
        super(fm, tab_count);
        this.tabcount = tab_count;
    }

    @NonNull
    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                Fragment1 fragment1 = new Fragment1 ();
                return todayBAFragment;
            case 1:
                Fragment2 fragment2 = new Fragment2 ();
                return todayReminderDue;
            case 2:
                Fragment3 fragment3 = new Fragment3 ();
                return currentTransaction;
            default:
                return null;
        }
    }

    @Override
    public int getCount() {
        return tabcount;
    }
}

此外，将适配器设置为 ViewPager，无论您想要显示 viewPager：

    //set adapter into viewpager
    viewpager_class viewpagerClass = new viewpager_class(getChildFragmentManager(), tabLayout.getTabCount());
    viewPager.setAdapter(viewpagerClass);
    viewPager.setOffscreenPageLimit(2);
    viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));