A DummySurface is useful with MediaCodec on API levels 23+.
Rather than having to release a MediaCodec instance when the
app no longer has a real surface to output to, it's possible
to retain the MediaCodec, using MediaCodec.setOutputSurface
to target a DummySurface instance instead. When the app has
a real surface to output to again, it can call swap this
surface back in instantaneously. Without DummySurface a new
MediaCodec has to be instantiated at this point, and decoding
can only start from a key-frame in the media.
A future change may hook this up internally in MediaCodecRenderer
for supported use cases, although this looks a little awkward. If
this approach isn't viable, we can require applications wanting
this to set a DummySurface themselves. This isn't easy to do with
the way SimpleExoPlayerView.setPlayer works at the moment, however,
so some changes will be needed either way.
Issue: #677
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