Asynchronous Database Access with Qt 4.x
Listing 6. You must register any custom data types in order to share them.
// first, make the object system aware qRegisterMetaType< QList<QSqlRecord> >("QList<QSqlRecord>"); // now set up the queued connection connect( m_worker, SIGNAL( results( const QList<QSqlRecord>& ) ), this, SIGNAL( queryFinished( const QList<QSqlRecord>& ) ) );
The sample application provided with this article implements the strategy outlined above, in which queries are executing in parallel with the rest of the application [the application is available for download from the Linux Journal FTP site, ftp.linuxjournal.com/pub/lj/listings/issue158/9602.tgz]. The UI is not disturbed while the queries are underway. A queued connection is created between the QueryThread interface and the encapsulated worker thread after the appropriate types are registered with the Qt metaobject system. This allows the separate threads to communicate safely with one another, with minimal overhead and code complexity. The sample application was tested with SQLite and PostgreSQL; however, it will work with any database connection supported by Qt that enforces the same connection-per-thread limitation.
The following points should be kept in mind when designing asynchronous database applications with Qt:
Create a database connection per thread. Use the name parameter of the thread-safe QSqlDatabase::addDatabase() method in order to distinguish various database connections.
Encapsulate the database connection within worker thread objects as much as possible. Never share a database connection with another thread. Never use a database connection from any thread other than the one that created it.
Manage communication between threads using the tools provided by Qt. In addition to QMutex, QSemaphore and QWaitCondition, Qt provides much more direct mechanisms: events and signals/slots. The implementation of signals/slots across thread boundaries relies on events; therefore, ensure that your threads start their own event loop using QThread::exec().
Register unknown types with the Qt metaobject system. Any unknown types cannot be marshaled properly without first invoking qRegisterMetaType(). This enables a queued connection to invoke a slot in a separate thread within that thread's context using new types.
Utilize queued connections to communicate between the application and the database threads. The queued connection provides all the advantages for dealing with asynchronous database connections, but maintains a simple and familiar interface using QObject::connect().
Dave Berton is a professional programmer working for Eventide, Inc. He welcomes your comments at email@example.com.