Question

代码1是来自网页的示例项目，我不知道代码1中override fun onCreate(db: SQLiteDatabase)中的var db是如何传递值的。

我检查了源代码ManagedSQLiteOpenHelper和SQLiteOpenHelper，我还不明白！

代码1

class ForecastDbHelper(ctx: Context = App.instance) : ManagedSQLiteOpenHelper(ctx,
        ForecastDbHelper.DB_NAME, null, ForecastDbHelper.DB_VERSION) {

    companion object {
        val DB_NAME = "forecast.db"
        val DB_VERSION = 1
        val instance by lazy { ForecastDbHelper() }
    }

    override fun onCreate(db: SQLiteDatabase) {
        db.createTable(CityForecastTable.NAME, true,
                CityForecastTable.ID to INTEGER + PRIMARY_KEY,
                CityForecastTable.CITY to TEXT,
                CityForecastTable.COUNTRY to TEXT)

        db.createTable(DayForecastTable.NAME, true,
                DayForecastTable.ID to INTEGER + PRIMARY_KEY + AUTOINCREMENT,
                DayForecastTable.DATE to INTEGER,
                DayForecastTable.DESCRIPTION to TEXT,
                DayForecastTable.HIGH to INTEGER,
                DayForecastTable.LOW to INTEGER,
                DayForecastTable.ICON_URL to TEXT,
                DayForecastTable.CITY_ID to INTEGER)
    }

    override fun onUpgrade(db: SQLiteDatabase, oldVersion: Int, newVersion: Int) {
        db.dropTable(CityForecastTable.NAME, true)
        db.dropTable(DayForecastTable.NAME, true)
        onCreate(db)
    }
}

ManagedSQLiteOpenHelper

abstract class ManagedSQLiteOpenHelper(
    ctx: Context,
    name: String,
    factory: SQLiteDatabase.CursorFactory? = null,
    version: Int = 1
): SQLiteOpenHelper(ctx, name, factory, version) {

    private val counter = AtomicInteger()
    private var db: SQLiteDatabase? = null

    fun <T> use(f: SQLiteDatabase.() -> T): T {
        try {
            return openDatabase().f()
        } finally {
            closeDatabase()
        }
    }

    @Synchronized
    private fun openDatabase(): SQLiteDatabase {
        if (counter.incrementAndGet() == 1) {
            db = writableDatabase
        }
        return db!!
    }

    @Synchronized
    private fun closeDatabase() {
        if (counter.decrementAndGet() == 0) {
            db?.close()
        }
    }
}

SQLiteOpenHelper

public abstract class SQLiteOpenHelper {
    public SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version) {
        throw new RuntimeException("Stub!");
    }

    public SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version, DatabaseErrorHandler errorHandler) {
        throw new RuntimeException("Stub!");
    }

    public String getDatabaseName() {
        throw new RuntimeException("Stub!");
    }

    public void setWriteAheadLoggingEnabled(boolean enabled) {
        throw new RuntimeException("Stub!");
    }

    public SQLiteDatabase getWritableDatabase() {
        throw new RuntimeException("Stub!");
    }

    public SQLiteDatabase getReadableDatabase() {
        throw new RuntimeException("Stub!");
    }

    public synchronized void close() {
        throw new RuntimeException("Stub!");
    }

    public void onConfigure(SQLiteDatabase db) {
        throw new RuntimeException("Stub!");
    }

    public abstract void onCreate(SQLiteDatabase var1);

    public abstract void onUpgrade(SQLiteDatabase var1, int var2, int var3);

    public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        throw new RuntimeException("Stub!");
    }

    public void onOpen(SQLiteDatabase db) {
        throw new RuntimeException("Stub!");
    }
}

Answer 1

SQLHelper类旨在简化对SQLite数据库的访问。将值传递给super允许它存储用于创建或升级数据库的值。

第一次调用getWriteable数据库时，它将根据构造函数中提供的数据库验证数据库的版本，如果它不存在，则会调用onCreate，如果它更新，则会调用onUpgrade，否则将发生正常操作

你也在SQLiteOpenHelper上面发布了虚假来源，实际上就是这样：

 public SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version,
        DatabaseErrorHandler errorHandler) {
    if (version < 1) throw new IllegalArgumentException("Version must be >= 1, was " + version);

    mContext = context;
    mName = name;
    mFactory = factory;
    mNewVersion = version;
    mErrorHandler = errorHandler;
}

所以我不确定你在哪里获得了SQLiteOpenHelper的源代码，但它是编码的，比你上面粘贴的抽象类要大得多。

db如何在Kotlin中传递价值？

1 个答案: