First working conversion: SAF input -> hardware transcode -> staged cache
file -> SAF export, driven from a Compose screen.
Media3 Transformer is the engine for this path rather than FFmpeg. It is
Apache-2.0, needs no native build, consumes content:// URIs directly, and
runs MediaCodec decode -> GL surface -> MediaCodec encode without frames
round-tripping through the CPU. FFmpeg remains necessary for the long tail
(MP3, GIF, MKV, exotic containers) but is not the right tool here.
Two hazards are designed against rather than discovered later:
Transformer must be driven from a single thread that has a Looper, and
start()/cancel() throw IllegalStateException from anywhere else. The Looper
it binds to is whichever the Builder saw, silently falling back to the main
one. A WorkManager Worker runs on a Looper-less executor thread, so the
naive arrangement builds against the main Looper and then throws on start.
Media3Engine owns a dedicated HandlerThread, passes its Looper explicitly,
and marshals every call onto it, so callers get a plain suspending function
and cannot reintroduce the bug. Media3EngineTest covers this directly by
driving a conversion from a Looper-less thread.
Output never goes through a SAF file descriptor. MP4 faststart rewrites the
moov atom at the end and needs to seek backwards, which a SAF fd does not
reliably support. OutputPublisher stages to app-private cache, a real POSIX
path, and copies out afterwards. That costs transient double disk usage, so
it checks free space before starting.
Input uses ACTION_OPEN_DOCUMENT rather than the photo picker: the picker is
images and video only, offers no audio at all, and does not reliably
surface .mkv/.flac/.webm. SAF needs no runtime permission.
Tests run on an API 37 emulator and assert the output codec by reading the
muxed file with MediaExtractor, so a silent fallback to H.264 fails rather
than passing. Progress reporting is deliberately not asserted as non-empty:
a 3 s fixture can finish inside one 250 ms poll tick, which would be an
intermittent failure rather than a real defect.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
JMR-devandClaude Opus 5
e4efa8a74a