db如何在Kotlin中傳遞值？

Question

代碼1是一個來自網頁的示例項目，我不知道代碼1中的override fun onCreate(db: SQLiteDatabase)中的var db如何傳遞值。

我檢查了源代碼都ManagedSQLiteOpenHelper和SQLiteOpenHelper，我還不明白！

代碼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

的提供SQLHelper類是爲了讓訪問你的SQLite數據庫更簡單。將值傳遞給super允許它存儲用於創建或升級數據庫的值。

您第一次調用getWriteable數據庫時，它會根據構造函數中提供的數據庫驗證數據庫的版本，如果它不存在，將調用onCreate，如果更新，onUpgrade將被調用，否則正常的操作將會發生。

而且你貼假源上方的SQLiteOpenHelper它實際上是這樣的：

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; 
} 

所以我不知道你在哪裏得到了SQLiteOpenHelper的源代碼，但它是編碼，遠遠大於你的粘貼上面的抽象類。