Up to now we use a boolean "preventListenerNotification" to suppress updates
while other operations are still in progress. This ensures, for example, that
only one initial timeline is issued even for multiple child sources.
As soon as we allow to reuse the same instance, it becomes increasingly difficult
to manage this manual listener notification suppression. Therefore, this change
schedules an update as a new message on the playback thread immediately after the
current message. This way, we also ensure that all simultaneous updates within one
looper message iteration are reported together.
Issue:#3498
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Created by MOE: https://github.com/google/moe
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