它是在SQLite中初始化MainActivity的正确方法吗？

Question

我在android上工作，遇到了一个问题，我不能在我的DataHandler类中初始化sqlite实例，但是我可以在我的MainActivity类中初始化它。

示例: MainActivity类：

public class MainActivity extends AppCompatActivity {

private RecyclerView recyclerView;
private RecyclerView.LayoutManager layoutManager;
private RecyclerView.Adapter adapter;

//valid
private SQLiteDatabase sqLiteDatabase = openOrCreateDatabase("Database", Context.MODE_PRIVATE , null); //...

在数据处理程序类中：

public class DataHandler {
public static String[] names,dates;

//invalid: cannot resolve method
SQLiteDatabase sqLiteDatabase = openOrCreateDatabase("HMDB", Context.MODE_PRIVATE , null);

所以，我做了这个

主要活动：

public SQLiteDatabase getSqLiteDatabase() {
    return sqLiteDatabase;
}

在数据处理程序中：

MainActivity mainActivity;
SQLiteDatabase sqLiteDatabase = mainActivity.getSqLiteDatabase();

这样做对吗？有什么更好的方法吗？如果很难理解sry

Answer 1

最好的方法是使用Singleton模式，如本文所述，guides/wiki/Local-Databases-with-SQLiteOpenHelper

public class PostsDatabaseHelper extends SQLiteOpenHelper { 
  private static PostsDatabaseHelper sInstance;

  // ...

  public static synchronized PostsDatabaseHelper getInstance(Context context) {
    // Use the application context, which will ensure that you 
    // don't accidentally leak an Activity's context.
    if (sInstance == null) {
      sInstance = new PostsDatabaseHelper(context.getApplicationContext());
    }
    return sInstance;
  }

  /**
   * Constructor should be private to prevent direct instantiation.
   * Make a call to the static method "getInstance()" instead.
   */
  private PostsDatabaseHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
  }
}

在您的MainActivity中使用它就像

// In any activity just pass the context and use the singleton method
PostsDatabaseHelper helper = PostsDatabaseHelper.getInstance(this);
// or
PostsDatabaseHelper helper = PostsDatabaseHelper.getInstance(context);
// or
PostsDatabaseHelper helper = PostsDatabaseHelper.getInstance(getActivity());
// Do something with data via Cursor and
helper.close();

这种方法非常适合于从您的应用程序的不同位置访问数据库，如服务、片段等。

Answer 2

这样做对吗？

当项目运行时，它会抛出错误吗？

这就是我通常在DatabaseHandler中所做的(只是分享)

public class DatabaseHandler extends SQLiteOpenHelper {
    public static final String DATABASE_NAME = "DATA.db";
    public static final String TABLE_NAME = "TITLE";
    public static final String COL0 = "ID";
    public static final String COL1 = "TODO";
    public static final String COL2 = "DETAIL";

    public DatabaseHandler(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, TODO TEXT, DETAIL TEXT)";
        db.execSQL(createTable);    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("drop if database is exists" + TABLE_NAME);
        onCreate(db);
    }
}

在MainActivity中，声明以下一行

DatabaseHandler db;

在DatabaseHandler方法中，初始化onCreate

db = new DatabaseHandler(this);

当您想使用SQLite数据库时，请确保您有以下行

SQLiteDatabase db = this.getWritableDatabase();