java - 刷新 ListView 数据库

Question

我是 Android studio 的新手，我想制作一个 2 选项卡应用程序，将一个选项卡中的数据保存到数据库中，然后在 ListView 的第二个选项卡上显示数据库。

问题是它不会自动刷新，我必须退出应用程序并重新启动它才能在第二个选项卡上查看新数据。解决办法是什么？

DatabaseHelper 类:

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

import androidx.annotation.Nullable;

public class DatabaseHelper extends SQLiteOpenHelper {
    public static final String DATABASE_NAME = "HISTORY.db";
    public static final String TABLE_NAME = "history_table";
    public static final String COL1= "SUM";



    public DatabaseHelper(@Nullable Context context) {
        super(context, DATABASE_NAME, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String createTable = "CREATE TABLE " + TABLE_NAME + " (ID INTEGER PRIMARY KEY AUTOINCREMENT, " + COL1 + " TEXT)";
        db.execSQL(createTable);
    }

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

    public boolean addData(String item){
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put(COL1, item);

        Log.d("DatabaseHelper", "addData: Adding " + item + " to " + TABLE_NAME);

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


        if (result == -1) {
            return false;
        }
        else{
            return true;
        }

    }

    public Cursor getData(){
        SQLiteDatabase db = this.getWritableDatabase();
        String query = "SELECT * FROM " + TABLE_NAME;
        Cursor data = db.rawQuery(query,null);
        return data;
    }
}

第二个选项卡 fragment :

import android.database.Cursor;
import android.os.Build;
import android.os.Bundle;

import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentTransaction;

import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.List;

import static com.example.scratch.DatabaseHelper.TABLE_NAME;


/**
 * A simple {@link Fragment} subclass.
 */
public class History extends Fragment {



    private ListView mListView;
    private DatabaseHelper db=null;
    private Cursor constantsCursor=null;
    DatabaseHelper mDatabaseHelper;


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


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment


        View histv = inflater.inflate(R.layout.fragment_history, container, false);

        mListView = (ListView) histv.findViewById(R.id.listview);
        mDatabaseHelper = new DatabaseHelper(getActivity());

        Log.d("ListDataActivity", "populateListView: Displaying data in listview.");

        Cursor data = mDatabaseHelper.getData();
        ArrayList<String> listData = new ArrayList<>();
        if (data.getCount() == 0) {
            Toast.makeText(this.getActivity(), "EMPTY DB", Toast.LENGTH_LONG).show();
        } else {
            while (data.moveToNext()) {
                listData.add(data.getString(1));
                ArrayAdapter<String> adapter = new ArrayAdapter<>(getActivity(), android.R.layout.simple_list_item_1, listData);

                mListView.setAdapter(adapter);
            }
        }

        return histv;
    }


}

Answer 1

使用Room存储数据和 LiveData来观察变化。它将在所有情况下为您提供帮助，而无需重新启动应用程序。

根据您的需要，您可以将数据保存在第一个选项卡中，以便在另一个选项卡中获取并观察变化。