问题

I've got two Fragments

`AddActivityFragment` - 此片段负责将数据添加到我的 SQLite 数据库中。

和

`ActivityListFragment` - 此片段负责在 `ListView` 中显示数据。

问题是，每当将数据添加到 SQLite 表中时，表格在应用程序重新启动之前不会更新。因此，如果我尝试将一些数据添加到表中，直到重新启动应用程序之前，数据才会在 `ListView` 中更新。

我在多个线程中遇到过关于类似问题的 `adapter.notifyDataSetChanged()`，但我无论如何都无法弄清楚如何将其实现到我的代码中。

`AddActivityFragment.java`：

```java
public class AddActivityFragment extends Fragment implements View.OnClickListener {
    // ...（此处省略其他代码）
    @Override
    public void onClick(View v) {
        switch(v.getId()) {
            case R.id.addActivityButton:
                // ...（此处省略其他代码）

                // 将活动数据添加到 SQL 数据库
                mDatabaseHelper.addData(activityName, activityDate, activityLocationLat, activityLocationLong);
                mActivityListFragment.updateListView();  // 添加此行以通知 ActivityListFragment 更新 ListView

                break;
            default:
                break;
        }
    }
}

ActivityListFragment.java：

public class ActivityListFragment extends Fragment {
    // ...（此处省略其他代码）
    public void updateListView() {
        listData.clear();
        for (int i = 0; i < mDatabaseHelper.getRowCount(); i++) {
            listData.add(mDatabaseHelper.getDataFromIndex(i));
        }
        adapter.notifyDataSetChanged();  // 更新适配器数据
    }
}

ViewPagerAdapter.java 和 DatabaseHelper.java 中的代码保持不变。

<details>
<summary>英文:</summary>

I&#39;ve got two Fragments

`AddActivityFragment` - This Fragment is responsible for adding data into my SQLite database.

and

`ActivityListFragment` - This Fragment is responsible for displaying the data in a `ListView`

The problem is that whenever data is added to the SQLite table, the table is not updated until the application is restarted. Because of this, if I try to add some data into the table, it is not updated in the `ListView` until I restart the application.

I&#39;ve come across `adapter.notifyDataSetChanged()` on multiple threads regarding similar problems but I can not for the life of me figure out how I&#39;m supposed to implement this into my own code.

`AddActivityFragment.java`:

public class AddActivityFragment extends Fragment implements View.OnClickListener {

private static final String TAG = "AddActivityFragment";

private TextInputLayout activityNameTextField, activityDateTextField;
private TextInputEditText activityNameInput, activityDateInput;
public DatabaseHelper mDatabaseHelper;
private LocationRequest mLocationRequest;
private FusedLocationProviderClient mFusedLocationProviderClient;
private Location mLastLocation;
private ActivityListFragment mActivityListFragment;
private DatePicker mDatePicker;
Context mContext;

private double activityLocationLat;
private double activityLocationLong;

public AddActivityFragment() {
    // Required empty public constructor
}
@Override
public android.view.View onCreateView(LayoutInflater inflater, ViewGroup container,
                                      Bundle savedInstanceState) {
    View activityViewFragment = inflater.inflate(R.layout.fragment_add_activity, container, false);
    // Get context from fragment.
    this.mContext = activityViewFragment.getContext();

    mDatabaseHelper = new DatabaseHelper(mContext);

    // Find the layout components.
    activityNameTextField = activityViewFragment.findViewById(R.id.activityNameTextField);
    activityNameInput = activityViewFragment.findViewById(R.id.activityNameInput);
    mDatePicker = activityViewFragment.findViewById(R.id.datePicker);


    Button addActivityButton = activityViewFragment.findViewById(R.id.addActivityButton);

    // Set listener to addActivityButton in this view.
    addActivityButton.setOnClickListener(this);
    // Set hints for text fields.
    activityNameTextField.setHint("Activity Name");


    return activityViewFragment;
}

@Override
public void onClick(View v) {
    switch(v.getId()) {
        case R.id.addActivityButton:
            StringBuilder sb = new StringBuilder();
            int day = mDatePicker.getDayOfMonth();
            int month = mDatePicker.getMonth() + 1;
            int year = mDatePicker.getYear();
            sb.append(day);
            sb.append("-");
            sb.append(month);
            sb.append("-");
            sb.append(year);

            String activityName = activityNameInput.getText().toString();
            String activityDate = sb.toString();


            activityNameInput.setText("");

            //Add the activity data to the SQL database

            mDatabaseHelper.addData(activityName,activityDate,activityLocationLat,activityLocationLong);


            break;
        default:
            break;
    }

@Override
public void onResume(){
    super.onResume();
    Log.d(TAG,"Log when activity is swiped to.");
}

@Override
public void onPause() {
    super.onPause();
    Log.d(TAG, "List paused.");
}

}

`ActivityListFragment.java`:

public class ActivityListFragment extends Fragment {

//Class tag
private static final String TAG = "ActivityListFragment";
private static final String ACTIVITY_LIST_FRAGMENT_TAG = "ACTIVITY_LIST_FRAGMENT";

DatabaseHelper mDatabaseHelper;
Context mContext;
private ListView mListView;
public ArrayAdapter adapter;
private ArrayList<ArrayList> listData;

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

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View listViewFragment = inflater.inflate(R.layout.fragment_activity_list, container, false);
    this.mContext = listViewFragment.getContext();
    mListView = listViewFragment.findViewById(R.id.activity_list_view);

    mDatabaseHelper = new DatabaseHelper(mContext);
    listData = new ArrayList<>();

    for(int i = 0; i < mDatabaseHelper.getRowCount(); i++) {
        listData.add(mDatabaseHelper.getDataFromIndex(i));
    }

    adapter = new ArrayAdapter<>(mContext, android.R.layout.simple_list_item_1, listData);
    mListView.setAdapter(adapter);
    handleOnItemClick();
    return listViewFragment;
}
@Override
public void onResume(){
    super.onResume();
    Log.d(TAG,"Log when activity is swiped to.");
}

@Override
public void onPause() {
    super.onPause();
    Log.d(TAG, "List paused.");
}

`ViewPagerAdapter.java`:

public class ViewPagerAdapter extends FragmentStateAdapter {

public ViewPagerAdapter(@NonNull FragmentManager fragmentManager, @NonNull Lifecycle lifecycle) {
    super(fragmentManager, lifecycle);
}

@NonNull
@Override
public Fragment createFragment(int position) {

    switch (position) {
        case 0:
            return new MapViewFragment();
        case 1:
            return new AddActivityFragment();
        case 2:
            return new ActivityListFragment();
    }
    return null;
}

@Override
public int getItemCount() {
    return 3;
}

}

`DataBaseHelper.java`:

public class DatabaseHelper extends SQLiteOpenHelper {

private static final String TAG = "DatabaseHelper";

//Table name, column names.
private static final String TABLE_NAME = "activity_table";
private static final String COL_ID = "ID";
private static final String COL_NAME = "name";
private static final String COL_DATE = "date";
private static final String COL_LOCATION_LAT = "location_lat";
private static final String COL_LOCATION = "location_long";

private Cursor mCursor;
ArrayList latLongList;
public DatabaseHelper(Context context) {
    super(context, TABLE_NAME, null, 1);
}

/**
 * Method: onCreate
 * @param db - The database that we are creating the table in.
 */
@Override
public void onCreate(SQLiteDatabase db) {
    String CREATE_ACTIVITES_TABLE = "CREATE TABLE " + TABLE_NAME + "("
            + COL_ID + " INTEGER PRIMARY KEY," + COL_NAME + " VARCHAR,"
            + COL_DATE + " VARCHAR," + COL_LOCATION_LAT + " DOUBLE," + COL_LOCATION + " DOUBLE" + ");";
    db.execSQL(CREATE_ACTIVITES_TABLE);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
    onCreate(db);
}

/**
 * Method: addData
 * @param name - The "name"-value which is added to the table column COL_NAME.
 * @param date - The "date"-value which is added to the table column COL_DATE.
 * @param locationLat - The "locationLat"-value which is added to the table column COL_LOCATION_LAT.
 * @param locationLong - The "locationLong"-value which is added to the table column COL_LOCATION_LONG.
 * @return true or false, depending if the addition to the table was successful or not.
 */
public boolean addData(String name, String date, Double locationLat, Double locationLong) {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(COL_NAME, name);
    contentValues.put(COL_DATE, date);
    contentValues.put(COL_LOCATION_LAT, locationLat);
    contentValues.put(COL_LOCATION, locationLong);

    Log.d(TAG, "addData: Adding " + name + " to " + TABLE_NAME);
    Log.d(TAG, "addData: Adding " + date + " to " + TABLE_NAME);
    Log.d(TAG, "addData: Adding " + locationLat + " to " + TABLE_NAME);
    Log.d(TAG, "addData: Adding " + locationLong + " to " + TABLE_NAME);

    long result = db.insert(TABLE_NAME, null, contentValues);

    if(result == -1) {
        Log.d(TAG, "Something went wrong when adding data to database.");
        return false;
    } else {
        Log.d(TAG, "Data correctly added to database");
        return true;
    }
}

/**
 * Method: getData
 * @return mCursor - Returns entire table from database.
 */
public Cursor getData() {
    SQLiteDatabase db = this.getWritableDatabase();
    String query = "SELECT * FROM " + TABLE_NAME;
    mCursor = db.rawQuery(query, null);
    return mCursor;
}

/**
 * Method: getDataFromIndex
 * @param index - Row index in activity_table table.
 * @return list - ArrayList of data at row index.
 */
public ArrayList getDataFromIndex (int index) {
    ArrayList list = new ArrayList();

    if(mCursor == null) {
        getData();
    }
    mCursor.moveToPosition(index);
    list.add(mCursor.getString(1));
    list.add(mCursor.getString(2));
    list.add(mCursor.getDouble(3));
    list.add(mCursor.getDouble(4));
    return list;
}

/**
 * Method: getRowCount
 * @return mCursor.getCount - Total amount of rows in table.
 */
public int getRowCount() {
    if(mCursor == null) {
        getData();
    }
    return mCursor.getCount();
}

/** TODO: This might be useless as we will need to fetch the names associated to the activity anyway, to be decided.
 * Method: latLongArrayListFromIndex
 * @param index - Row index in activity_table table.
 * @return latLongList - ArrayList of data at row index.
 */
public ArrayList latLongArrayListFromIndex(int index) {
    latLongList = new ArrayList();
    if(mCursor == null) {
        getData();
    }
    mCursor.moveToPosition(index);
    latLongList.add(mCursor.getDouble(mCursor.getColumnIndex("location_lat")));
    latLongList.add(mCursor.getDouble(mCursor.getColumnIndex("location_long")));

    return latLongList;
}

/**
 * Method getItemId
 */
public Cursor getItemId(String name) {
    SQLiteDatabase db = this.getWritableDatabase();
    String query = "SELECT " + COL_ID + " FROM " + TABLE_NAME + " WHERE " + COL_NAME + " = '" + name + "'";
    Cursor data = db.rawQuery(query, null);
    return data;
}

/**
 * Method: updateName
 * @param newName
 * @param id
 * @param oldName
 */
public void updateName(String newName, int id, String oldName) {
    SQLiteDatabase db = this.getWritableDatabase();
    String query = "UPDATE " + TABLE_NAME + " SET " + COL_NAME + " = '" + newName + "' WHERE " + COL_ID + " = '" + id + "'" + " AND " + COL_NAME + " = '" + oldName + "'";
    Log.d(TAG, "updateName: query: " + query);
    Log.d(TAG, "updateName: Setting new name of activity to: " + newName);
    db.execSQL(query);
}

public void deleteActivity(int id, String name) {
    SQLiteDatabase db = this.getWritableDatabase();
    String query = "DELETE FROM " + TABLE_NAME + " WHERE " + COL_ID + " = '" + id + "'" + " AND " + COL_NAME + " = '" + name + "'";
    Log.d(TAG, "deleteActivity: query: " + query);
    Log.d(TAG, "deleteActivity: Deleting " + name + " from table");
    db.execSQL(query);
}

}

    


</details>


# 答案1
**得分**: 0

你可以通过使用内容观察器（content observer）来实现所需的行为。我在[这里有一个Github项目][1]，可能会帮助你理解这是如何工作的。

第一步是定义一个唯一的内容URI（Uniform Resource Identifier）。URI可能看起来如下所示。

```java
public static final Uri DB_TABLE_USER_URI = Uri
    .parse("sqlite://" + com.package.your.app + "/" + "user_table");

然后在你的ActivityListFragment中使用LoaderManager.LoaderCallbacks<Cursor>来加载该片段中的用户，使用CursorLoader的回调函数（例如onCreateLoader、onLoadFinished等）。还要在onCreateLoader中为内容观察器进行注册。

this.registerContentObserver(cursor, DBConstants.DB_TABLE_USER_URI);

在你的DataBaseHelper类中添加/更新用户之后，你可能还希望通知观察器数据库中的数据已更改，以便它自动在RecyclerView中刷新数据。

context.getContentResolver().notifyChange(DBConstants.DB_TABLE_USER_URI, null);

我建议你分叉（fork）这个Github项目并运行代码，以查看它是如何工作的。

public static final Uri DB_TABLE_USER_URI = Uri
    .parse("sqlite://" + com.package.your.app + "/" + "user_table");

this.registerContentObserver(cursor, DBConstants.DB_TABLE_USER_URI);

context.getContentResolver().notifyChange(DBConstants.DB_TABLE_USER_URI, null);