Cursor 在 android 中如何工作

Question

我很难准确地可视化程序中的 'Cursor' 功能。我有点明白了，但是谁能详细解释一下它的功能？

我所说的 Cursor 是指 Cursor 界面。我不能简单地理解它对任何事物的作用。

http://developer.android.com/reference/android/database/Cursor.html

Answer 1

您指的是这个 Cursor 用法吗？

public String databaseToString(){
    String dbString = "";
    SQLiteDatabase db = getWritableDatabase();
    String query = "SELECT * FROM " + TABLE_PRODUCTS + " WHERE 1";

    //Cursor points to a location in your results
    Cursor c = db.rawQuery(query, null);
    //Move to the first row in your results
    c.moveToFirst();

Answer 2

来自Wikipedia

In computer science, a database cursor is a control structure that enables traversal over the records in a database. Cursors facilitate subsequent processing in conjunction with the traversal, such as retrieval, addition and removal of database records. The database cursor characteristic of traversal makes cursors akin to the programming language concept of iterator.

来自 Here

A cursor is a tool that allows you to iterate the records in a set. It has concepts of order and current record.

来自The documentation you pointed yourself

provides random read-write access to the result set returned by a database query.

所以不要将 Cursor 视为一种功能，而是一种以更有效的方式从任何数据库获取记录的手段。

Answer 3

游标对象是从对 SQLite 数据库的查询中 return 得到的。它将 return 查询 returns.

的所有行

假设您的数据库数据库中有一个名为 names 的 table，配置如下：

_id     _name
1       Space Ghost
2       Zorak
3       Moltar
4       Brak

如果你想从这个 table 中获取所有数据并使用它，你会这样做像这样：

public HashMap<Integer, String> getNames(){

 HashMap<Integer, String> data = new HashMap<Integer, String>();

 try{
  SQLiteOpenHelper helper = new MyOpenDbHelper(context);
  SQLiteDatabase db = helper.getReadableDatabase();
  String selectQuery = "SELECT  * FROM names";
  Cursor cursor = db.rawQuery(selectQuery, null);
  if (cursor != null && cursor.moveToFirst()){ //make sure you got results, and move to first row
    do{
        int mID = cursor.getInt(0); //column 0 for the current row
        String mName = cursor.getString(1); //column 1 for the current row
        data.put(mID, mName);

      } while (cursor.moveToNext()); //move to next row in the query result

  }

 } catch (Exception ex) {
    Log.e("MyApp", ex.getMessage());
 } finally
 {
    if (cursor != null) {
       cursor.close();

    }
    if (db != null) {
        db.close();
    }

 }

return data;
}

通常您会创建自己的 class 来扩展 SQLiteOpenHelper，例如：

public class MyOpenDbHelper extends SQLiteOpenHelper {
     //........
}

Cursor 在 android 中如何工作

How does Cursor work in android

java

android

cursor