当我返回片段时，防止重复条目

Question

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

Home FragmentcontainsRecyclerView当我从Home Fragment导航到Profile Fragment或More Fragment时，它像预期的那样工作。

但是当我导航回Home Fragment时，RecyclerView中的项目重复…

当我再次导航并返回时，它再次复制

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
}
});
}

}

这里是ViewPagerAdapterCode:

//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FragmentHome.javaCode:

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);
}
});

}
}

，这是一个显示项目重复

的图像任何人都可以帮助我如何防止这种重复…

Answer 1

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

当你需要使用FragmentPagerAdapter?

FragmentStatePagerAdapter在有大量时更有用当页面对不可见时用户，他们的整个片段可能被销毁，只保留该片段的保存状态。这允许分页保持到与。相比，与每个访问页面关联的内存要少得多FragmentPagerAdapter以潜在的更多开销为代价切换页面

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;
}
}

另外，在你想显示ViewPager的地方设置适配器为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));