My sound preview worked with the Xine Phonon backend and failed as soon as I switched the machine to GStreamer. Same file, same button, no sound. So much for my five-line multimedia triumph.

The application called play() and immediately checked state(). Xine reached PlayingState quickly on my machine. GStreamer was still in LoadingState, which my code treated as failure and answered with stop(). I had accidentally made backend speed part of the program’s contract.

Phonon state changes are asynchronous. The fix was to let the MediaObject report stateChanged() and handle ErrorState there instead of demanding synchronous playback:

connect(media, SIGNAL(stateChanged(Phonon::State, Phonon::State)),
        this, SLOT(mediaStateChanged(Phonon::State, Phonon::State)));
media->setCurrentSource(fileName);
media->play();

The slot updates the button when playback really starts and displays errorString() if the object enters ErrorState. LoadingState is just progress, not proof that the backend rejected the file.

Switching backends was useful precisely because it broke this. The common API kept Xine and GStreamer details out of the application, but it did not turn asynchronous operations into synchronous ones. One backend had merely been fast enough to hide my bad assumption.

The preview button is boring again. I left both backends in the test pass; otherwise the next timing assumption will wait for a user to find it.