数据库如何在Kotlin中传递值？

代码1是从网页的示例项目，我不知道代码1中的override fun onCreate(db: SQLiteDatabase)中的var db是如何传递的值。

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类是为了使访问您的SQLite数据库更简单。 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，它会根据构造函数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，如果更新，则会调用UpdateUp，否则将会发生正常的操作。

另外，你在SQLiteOpenHelper上面发布了一个虚假源代码，实际上是这样的：

  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得到了你的SQLiteOpenHelper的源代码，但是它被编码并且比你上面粘贴的抽象类要大得多。