android - 更改数据库中的数据后刷新 fragment

有一个MainActivity，其片段名为FragmentOne。我在此片段中有一个项目列表，这些数据保存在我的数据库中

我从NavigationView中的MainActivity进行更改。.我尝试从数据库中删除所有行。它可以工作，但是在那之后我需要刷新FragmentOne，该如何刷新FragmentOne UI？

我在updateUI中有一个名为FragmentOne的方法，但是如何访问该方法？当我公开该方法时，一切都会出错！

我测试了不同的方法，但是失败了..有人可以告诉我我该怎么做吗？

MainActivity：

public class MainActivity extends AppCompatActivity {

private DrawerLayout mDrawerLayout;
private TabLayout tabLayout;
private ViewPager viewPager;
private Detail mDetail;

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

    //////// TOOLBAR
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    ActionBar actionBar = getSupportActionBar();
    actionBar.setHomeAsUpIndicator(R.drawable.ic_menu);
    actionBar.setDisplayHomeAsUpEnabled(true);

    ///////// DRAWER
    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);

    NavigationView navigationView =
            (NavigationView) findViewById(R.id.navigation_view);
    navigationView.setNavigationItemSelectedListener
            (new NavigationView.OnNavigationItemSelectedListener() {
                @Override
                public boolean onNavigationItemSelected(MenuItem menuItem) {
                    switch (menuItem.getItemId()) {
                        case R.id.nav_item_one:
                            return false;

                        case R.id.nav_item_two:


                            AlertDialog.Builder alert_delete = new AlertDialog.Builder(
                                    MainActivity.this);
                            alert_delete.setTitle("Alert!");
                            alert_delete.setMessage(R.string.alert_delete_all);
                            alert_delete.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                                @Override
                                public void onClick(DialogInterface dialog, int which) {

                                    DetailLab.get(getApplicationContext()).deleteAllDetail();

                                    FragmentOne fragmentOne = new FragmentOne();
                                    fragmentOne.getFragmentManager().beginTransaction().replace(R.id.card_view, fragmentOne)
                                            .commit();

                                            dialog.dismiss();
                                }
                            });
                            alert_delete.setNegativeButton("No", new DialogInterface.OnClickListener() {
                                @Override
                                public void onClick(DialogInterface dialog, int which) {
                                    dialog.dismiss();
                                }
                            });
                            alert_delete.show();

                            return false;
..
..
                        case R.id.nav_item_nine:
                            finish();

                            default:
                                return false;
                    }
                }
            });

    viewPager = (ViewPager) findViewById(R.id.viewpager);
    setupViewPager(viewPager);

    tabLayout = (TabLayout) findViewById(R.id.tablayout);
    tabLayout.setupWithViewPager(viewPager);
    setupTabIcons();
}


private void setupViewPager(ViewPager viewPager) {
    ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());
    adapter.addFragment(new FragmentOne(), "ONE");
    adapter.addFragment(new FragmentTwo(), "TWO");
    viewPager.setAdapter(adapter);
}

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 null;
    }
}


@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();

    switch (id) {
        case android.R.id.home:
            mDrawerLayout.openDrawer(GravityCompat.START);
            return true;
    }
    return super.onOptionsItemSelected(item);
}

}

这是我从另一个名为DetailLab的类中删除数据库中所有行的方法：

public void deleteAllDetail() {
        mDatabase.delete(DetailTable.NAME, null, null); }

这是我的FragmentOne：

public class FragmentOne extends Fragment {
    private static final String SAVED_NUMBER_VISIBLE = "number";

    private RecyclerView mDetailRecyclerView;
    private DetailAdapter mAdapter;
    private boolean mNumberVisible;
    private View view;

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

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        view = inflater.inflate(R.layout.fragment_one_layout,
                container, false);

        mDetailRecyclerView = (RecyclerView)
                view.findViewById(R.id.detail_recycler_view);

        LinearLayoutManager layoutManager = new LinearLayoutManager(getContext());
        layoutManager.setReverseLayout(true); //This will reverse the data order but not scroll the RecyclerView to the last item
        layoutManager.setStackFromEnd(true); //For keeping data order same and simply scrolling the RecyclerView to the last item
        mDetailRecyclerView.setLayoutManager(layoutManager);

        if (savedInstanceState != null) {
            mNumberVisible =
                    savedInstanceState.getBoolean(SAVED_NUMBER_VISIBLE);
        }

        updateUI();
        return view;
    }

    @Override
    public void onResume() {
        super.onResume();
        updateUI();
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putBoolean(SAVED_NUMBER_VISIBLE, mNumberVisible);
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        super.onCreateOptionsMenu(menu, inflater);
        inflater.inflate(R.menu.fragment_one_layout, menu);
        MenuItem numberItem = menu.findItem(R.id.show_numbers);
        if (mNumberVisible) {
            numberItem.setTitle(R.string.hide_numbers);
        } else {
            numberItem.setTitle(R.string.show_numbers);
        }
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {

            case R.id.show_numbers:
                mNumberVisible = !mNumberVisible;
                getActivity().invalidateOptionsMenu();
                updateNumbers();
                return true;

            default:
                return super.onOptionsItemSelected(item);
        }
    }

    private void updateNumbers() {
        DetailLab detailLab = DetailLab.get(getActivity());
        int detailCount = detailLab.getDetails().size();
        String number = getResources().getQuantityString
                (R.plurals.number_plural, detailCount, detailCount);


        if (!mNumberVisible) {
            number = null;
        }

        AppCompatActivity activity = (AppCompatActivity) getActivity();
        activity.getSupportActionBar().setSubtitle(number);
    }

    private void updateUI() {
        DetailLab detailLab = DetailLab.get(getActivity());
        List<Detail> details = detailLab.getDetails();
        if (details.size() == 0) {
            TextView t = (TextView) view.findViewById(R.id.empty);
            t.setText(R.string.empty_view);
            t.setVisibility(View.VISIBLE);
        } else {
            TextView t = (TextView) view.findViewById(R.id.empty);
            t.setVisibility(View.GONE);
        }

//            Toast.makeText(getContext(), "zero", Toast.LENGTH_SHORT).show();
        if (mAdapter == null) {
            mAdapter = new DetailAdapter(details);
            mDetailRecyclerView.setAdapter(mAdapter);

        } else {
            mAdapter.setDetails(details);
            mAdapter.notifyDataSetChanged();
        }

        updateNumbers();

    }

    private class DetailHolder extends RecyclerView.ViewHolder
            implements View.OnClickListener, View.OnLongClickListener {
        private TextView mTitleTextView;
        //        private TextView mDateTextView;
        private Detail mDetail;
        private RatingBar mRatingBar;

        public DetailHolder(LayoutInflater inflater, ViewGroup parent) {
            super(inflater.inflate(R.layout.list_item_detail,
                    parent, false));

            itemView.setOnClickListener(this);
            itemView.setOnLongClickListener(this);
            mTitleTextView = (TextView) itemView.findViewById(R.id.detail_title);
//            mDateTextView = (TextView) itemView.findViewById(R.id.detail_date);
            mRatingBar = (RatingBar) itemView.findViewById(R.id.ratingBar);

        }

        public void bind(Detail detail) {
            mDetail = detail;
            mTitleTextView.setText(mDetail.getTitle());
            mRatingBar.setRating(mDetail.getRate());
        }

        @Override
        public void onClick(View view) {
            Intent intent = DetailPagerActivity.newIntent(getActivity(),
                    mDetail.getId());
            startActivity(intent);
        }

        @Override
        public boolean onLongClick(View v) {

            AlertDialog.Builder alert = new AlertDialog.Builder(
                    getActivity());
            alert.setMessage(R.string.alert);
            alert.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    DetailLab.get(getActivity()).deleteDetail(mDetail);
                    updateUI();
                    updateNumbers();
                    dialog.dismiss();
                }
            });
            alert.setNegativeButton("No", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
                }
            });
            alert.show();
            return true;
        }
    }

    private class DetailAdapter extends RecyclerView.Adapter<DetailHolder> {
        private List<Detail> mDetails;

        public DetailAdapter(List<Detail> details) {
            mDetails = details;
        }

        @Override
        public DetailHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            LayoutInflater layoutInflater =
                    LayoutInflater.from(getActivity());

            return new DetailHolder(layoutInflater, parent);
        }

        @Override
        public void onBindViewHolder(DetailHolder holder, int position) {
            Detail detail = mDetails.get(position);
            holder.bind(detail);
        }

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

        public void setDetails(List<Detail> details) {
            mDetails = details;
        }
    }
}

这是我FragmentOne的布局：

<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/fragment_one_layout">

    <android.support.v7.widget.CardView
    android:id="@+id/card_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/detail_recycler_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

    <TextView
        android:id="@+id/empty"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:padding="50dp"
        android:textAlignment="center"
        android:textSize="20sp"/>
</android.support.v7.widget.CardView>

<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignParentRight="true"
android:layout_marginRight="24dp"
android:layout_marginBottom="24dp"
android:src="@drawable/ic_menu_add"
android:scaleType="center"
app:elevation="8dp"
app:borderWidth="0dp"
    />

我的logcat：

  FATAL EXCEPTION: main
Process: com.drgnme.listhamrah, PID: 23454
java.lang.NullPointerException: Attempt to invoke virtual method 'android.support.v4.app.FragmentTransaction android.support.v4.app.FragmentManager.beginTransaction()' on a null object reference
at com.drgnme.listhamrah.MainActivity$1$1.onClick(MainActivity.java:76)
at android.support.v7.app.AlertController$ButtonHandler.handleMessage(AlertController.java:161)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5451)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

最佳答案

由于MainActivity中的以下代码，我认为您遇到此问题：

FragmentOne fragmentOne = new FragmentOne();
fragmentOne.getFragmentManager().beginTransaction().replace(R.id.card_view, fragmentOne).commit();

您尝试使用getFragmentManager()获取片段管理器的实例，该实例调用Activity.getFragmentManager，但是您的fragmentOne未附加到活动，因此该方法返回null并得到一个java.lang.NullPointerException

代替fragmentOne.getFragmentManager()直接在活动中使用getSupportFragmentManager()。

希望这可以帮助。