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有谁知道如何SQLiteDatabase.CursorFactory在Android上使用？我希望获得一个持久的 SQLiteCursor 对象，该对象使用 SQL SELECT 语句，其中几个 WHERE 子句表达式使用可以在重新查询之前以编程方式更改的参数。

例如：

SELECT LocationID FROM Locations WHERE Latitude < northlimit AND Latitude > southlimit AND Longitude < eastlimit AND Longitude > westlimit;

限制参数会动态变化，每 200ms 一次

我正在尝试解决一个问题，即如果我只是使用SQLiteDatabase.rawQuery("SELECT ...");系统为每个新查询创建一个新的 CursorWindow，最终内存不足。所以我试图找到一种方法来强制系统不要创建新的 CursorWindows。

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2 回答 2

0

您可以查看本教程..它完美地总结了它：

http://www.youtube.com/watch?v=j-IV87qQ00M

并且您应该在任何使用后关闭数据库.. 另一种方法是创建类变量 Cursor cr 并且每次使用它时都检查一下： if(cr==null) cr = new .... else //do something / /

cr.close();

于 2013-07-06T13:09:44.140 回答
0

我不相信使用CursorFactory会有所帮助或需要。

在以下示例中，创建了一个基于您的表并加载了 100000 行。

单个游标用于每 50 毫秒提取可变数量的行 10000 次。

当 Cursor 返回时，将获得计数（导致扫描 Cursor 并因此访问数据库）并更新 TextView 以显示迭代和获得的行数。

该应用程序运行时不会崩溃，并且分析器显示内存正在按以下方式进行管理：-

在此处输入图像描述

使用的代码是：-

数据库助手：-

public class DBHelper extends SQLiteOpenHelper {

    public static final String DBNAME = "mydb";
    public static final int DBVERSION = 1;

    public static final String TBL_LOCATIONS = "Locations";
    public static final String COl_LOCATIONS_ID = "LocationId";
    public static final String COl_LATITUDE = "Lattitude";
    public static final String COl_LONGITUDE = "Longitude";
    public static final String COL_OTHER = "other";


    public DBHelper(@Nullable Context context) {
        super(context, DBNAME, null, DBVERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE IF NOT EXISTS " + TBL_LOCATIONS +
                "(" +
                COl_LOCATIONS_ID + " INTEGER PRIMARY KEY," +
                COl_LATITUDE + " REAL," +
                COl_LONGITUDE + " REAL," +
                COL_OTHER + " TEXT," +
                " UNIQUE(" + COl_LONGITUDE + "," +  COl_LATITUDE + ")" +
                ")"
        );
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }

    public long insertLocation(double longitude, double latitude, String other) {
        ContentValues cv = new ContentValues();
        cv.put(COl_LONGITUDE,longitude);
        cv.put(COl_LATITUDE,latitude);
        cv.put(COL_OTHER,other);
        return this.getWritableDatabase().insert(TBL_LOCATIONS,null,cv);
    }

    public Cursor getLocations(double northlimit, double southlimt, double eastlimit, double westlimit) {
        return this.getWritableDatabase().query(
                TBL_LOCATIONS,
                new String[]{COl_LOCATIONS_ID},
                COl_LATITUDE + " <? AND " + COl_LATITUDE + ">? AND " + COl_LONGITUDE + "<? AND " + COl_LONGITUDE + "<?",
                new String[]{
                        String.valueOf(northlimit),
                        String.valueOf(southlimt),
                        String.valueOf(eastlimit),
                        String.valueOf(westlimit)},
                null,null,null );
    }
}

活动是：-

public class MainActivity extends AppCompatActivity {

    private int mInterval = 50;
    private int mCounter = 0;
    private final int mLimit = 10000;
    private Handler mHandler;
    private DBHelper mDBHlpr;
    private Cursor mCsr; //<<<<<<<<<< Just this 1 Cursor is used.
    TextView mTV;
    Random mRnd;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTV = this.findViewById(R.id.tv);
        mRnd = new Random();
        mDBHlpr = new DBHelper(this);
        addSomeDataIfNone();
        mHandler = new Handler();
        mRunIt.run();

    }

    private void addSomeDataIfNone() {

        SQLiteDatabase db = mDBHlpr.getWritableDatabase();
        if (DatabaseUtils.queryNumEntries(db,DBHelper.TBL_LOCATIONS) > 0) return;
        Random rnd = new Random();
        db.beginTransaction();
        for (int i=0; i < 100000; i++ ) {
            mDBHlpr.insertLocation(abs(mRnd.nextDouble()),abs(mRnd.nextDouble()),"LOCATION" + String.valueOf(i));
        }
        db.setTransactionSuccessful();
        db.endTransaction();
    }

    Runnable mRunIt = new Runnable() {
        @Override
        public void run() {
            if (mCounter > mLimit) return;
            getData(mCounter);
            mCounter++;
            mHandler.postDelayed(mRunIt, mInterval);
        }
    };

    private void getData(int iteration) {
        Log.d("GETTINGDATA","Getting data for iteration " + String.valueOf(iteration));
        double latlim1 = abs(mRnd.nextDouble());
        double latlim2 = abs(mRnd.nextDouble());
        double  lnglim1 = abs(mRnd.nextDouble());
        double lnglim2 = abs(mRnd.nextDouble());
        double nlim = 0.0, slim = 0.0, elim = 0.0, wlim = 0.0;

        if (latlim1 > latlim2) {
            nlim = latlim1;
            slim = latlim2;
        } else {
            nlim = latlim2;
            slim = latlim1;
        }
        if (lnglim1 > lnglim2) {
            elim = lnglim1;
            wlim = lnglim2;
        } else {
            elim = lnglim2;
            wlim = lnglim1;
        }

        mCsr = mDBHlpr.getLocations(nlim,slim,elim,wlim);
        mTV.setText("Iteration " + String.valueOf(iteration) + " Rows returned = " + String.valueOf(mCsr.getCount()));
    }
}
于 2019-07-08T03:03:59.457 回答