tonihei 6f6c72266e Add addTransferListener method to DataSource and provide BaseDataSource.
The new method allows to add transfer listeners (e.g. the BandwidthMeter) after
the data source has been created. To simplify the implementation for
subclasses, this change also introduces a BaseDataSource which handles
the list of listeners and the listener registration.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=202649563
2018-07-03 11:48:28 +01:00
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