Question

我对这种方法感到困惑，并不明白这个目的。我发现Android 4.4的默认台钟应用程序已经注意到一些奇怪的东西。在ClockDatabaseHelper类中，在数据库中创建了几个表。

   // Database and table names
    static final String DATABASE_NAME = "alarms.db";
    static final String OLD_ALARMS_TABLE_NAME = "alarms";
    static final String ALARMS_TABLE_NAME = "alarm_templates";
    static final String INSTANCES_TABLE_NAME = "alarm_instances";
    static final String CITIES_TABLE_NAME = "selected_cities";

所以第一个字符串是清楚的。第二个字符串为什么命名为OLD_ALARMS并且表的实名是警报。它为什么有这样的名字？什么是真正的puprose。接下来，第三个字符串的值为“alarm_templates”...为什么？ “模板”代表什么词？它为什么有这样的名字？实例表是针对当前的报警状态...但是我仍然不清楚为什么我们不能直接写入报警表。请解释一下做这些事情的目标是什么？

 @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int currentVersion) {
        LogUtils.v("Upgrading alarms database from version "
                + oldVersion + " to " + currentVersion);

        if (oldVersion <= VERSION_6) {
            // These were not used in DB_VERSION_6, so we can just drop them.
            db.execSQL("DROP TABLE IF EXISTS " + INSTANCES_TABLE_NAME + ";");
            db.execSQL("DROP TABLE IF EXISTS " + CITIES_TABLE_NAME + ";");

            // Create new alarms table and copy over the data
            createAlarmsTable(db);
            createInstanceTable(db);
            createCitiesTable(db);

            LogUtils.i("Copying old alarms to new table");
            String[] OLD_TABLE_COLUMNS = {
                    "_id",
                    "hour",
                    "minutes",
                    "daysofweek",
                    "enabled",
                    "vibrate",
                    "message",
                    "alert",
            };
            Cursor cursor = db.query(OLD_ALARMS_TABLE_NAME, OLD_TABLE_COLUMNS,
                    null, null, null, null, null);
            Calendar currentTime = Calendar.getInstance();
            while (cursor.moveToNext()) {
                Alarm alarm = new Alarm();
                alarm.id = cursor.getLong(0);
                alarm.hour = cursor.getInt(1);
                alarm.minutes = cursor.getInt(2);
                alarm.daysOfWeek = new DaysOfWeek(cursor.getInt(3));
                alarm.enabled = cursor.getInt(4) == 1;
                alarm.vibrate = cursor.getInt(5) == 1;
                alarm.label = cursor.getString(6);

                String alertString = cursor.getString(7);
                if ("silent".equals(alertString)) {
                    alarm.alert = Alarm.NO_RINGTONE_URI;
                } else {
                    alarm.alert = TextUtils.isEmpty(alertString) ? null : Uri.parse(alertString);
                }

                // Save new version of alarm and create alarminstance for it
                db.insert(ALARMS_TABLE_NAME, null, Alarm.createContentValues(alarm));
                if (alarm.enabled) {
                    AlarmInstance newInstance = alarm.createInstanceAfter(currentTime);
                    db.insert(INSTANCES_TABLE_NAME, null,
                            AlarmInstance.createContentValues(newInstance));
                }
            }
            cursor.close();

            LogUtils.i("Dropping old alarm table");
            db.execSQL("DROP TABLE IF EXISTS " + OLD_ALARMS_TABLE_NAME + ";");
        }
    }

以下是使用OLD_ALARMS_TABLE_NAME的方法 - onUpgrade我们只是将旧表复制到new，但我们从未写信给她。请解释这里发生的事情。谢谢

修改

感谢 CL。，在写作期间我得到了它。

这是用于更新旧数据库表，这里是来自旧应用程序的代码

private static class DatabaseHelper extends SQLiteOpenHelper {
    private static final String DATABASE_NAME = "alarms.db";
    private static final int DATABASE_VERSION = 5;

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE alarms (" +
                   "_id INTEGER PRIMARY KEY," +
                   "hour INTEGER, " +
                   "minutes INTEGER, " +
                   "daysofweek INTEGER, " +
                   "alarmtime INTEGER, " +
                   "enabled INTEGER, " +
                   "vibrate INTEGER, " +
                   "message TEXT, " +
                   "alert TEXT);");

        // insert default alarms
        String insertMe = "INSERT INTO alarms " +
                "(hour, minutes, daysofweek, alarmtime, enabled, vibrate, message, alert) " +
                "VALUES ";
        db.execSQL(insertMe + "(7, 0, 127, 0, 0, 1, '', '');");
        db.execSQL(insertMe + "(8, 30, 31, 0, 0, 1, '', '');");
        db.execSQL(insertMe + "(9, 00, 0, 0, 0, 1, '', '');");
    }

所以这里的名称是在insert语句中硬编码的。我认为这是不好的方法，首先，常量值的所有变量都不是常量变量。这里是sql openhelper是嵌套的静态类。我不知道，也许我错了。

时钟应用程序中的报警和报警模板表

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