Home > Posts > Gotcha: MarkerReached event of MediaElement returns new Markers

Gotcha: MarkerReached event of MediaElement returns new Markers

I just checked in the implementation code for a new feature for ClipFlair Studio’s Captions/Revoicing component:

When playing back recorded (or loaded from a WAV or MP3 file) audio for a caption/revoicing entry, the playback is now limited to the duration of the respective caption, (End-Start) time that is (btw that component has a duration column too that is hidden by default and can be shown by flipping it with the gear button on its titlebar and selecting the respective option to show the column).

The original audio is not affected and is stored in whole inside the saved state of the component/activity, so that you can adjust the caption entries timerange at any time to fit all or part of that recorded audio entry if you wish.

Will see into adding an “Limit playback” option to the backpanel of that component (it will default to true/checked) for any ClipFlair activities that don’t use the Start/End/Duration columns (e.g. if some activity just wants a grid of Captions and Audio entries for practicing and maybe for comparing to audio samples provided by the activity author or teacher at the optional “Comments (Audio)” column).

While implementing this feature, there were some “gotchas” that caused me some headache to spot:

1) When MediaOpened event is called by the MediaElement control, the Markers collection has just been reset and you need at that point to add your TimelineMediaMarker that will notify you when the playback limit point has been reached to stop the playback. In the case above this event is called after recording some audio or loading some WAV or MP3 audio file at an AudioRecorderControl (one is used at each row of the captions grid)

2) One shouldn’t remove an added marker at MediaEnded or MediaFailed events. This is since those will fire at each revoicing entry playback, whereas the MediaOpened while only occur once when the Audio property is populated at the AudioRecorderControl. As I mention above, MediaElement clears the Markers collection every time new content is loaded to it, so we need not worry about removing the marker we had added before.

3) Maybe the least obvious issue and the one that caused me most of headache to spot was that the MediaElement’s MarkerReached event gets back a MediaMarker that isn’t the same Marker instance as the one you had added to the Markers collection. So you have to use the Text property of the marker when you create it and then compare with the text from the one you got in the event to see if they are equal strings (btw, when C# compares strings it does it by content even if you use == instead of Equals method, unlike Java, where you shouldn’t use == to compare strings)

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