Choreographer.getInstance() is per-thread and throws on threads
without a Looper. The previous impl called it from the SocketClient
reader thread, which surfaced as snapshot failures with empty
state during the sample-app round trip. Now the reader thread
posts a Runnable to the main Handler that, on the main thread,
gets the Choreographer and posts the frame callback.
Single-activity Android app that calls Uatu.start() in
SampleApplication.onCreate and registers three extractors
(app_state, click_count, uptime_millis). MainActivity has a
button so click_count exercises non-trivial state.
Uatu singleton owns an internal UatuRuntime that wires the socket
client, pauser, and extractor registry. start() sends HELLO on
connect, responds to PAUSE by running extractors inside the pause
window and replying with STATE, and releases the gate on RESUME.
LinkedHashMap preserves extractor registration order so trace
output is deterministic. Failed extractors log a warning and
record null rather than aborting the whole snapshot.
Worker thread calls pauseAndSnapshot with an extractor closure;
the Pauser posts a frame callback that runs extractors on the
main thread and then blocks that thread on a semaphore until
release() fires. FrameCallbackPoster abstracts Choreographer so
JVM unit tests drive it with a single-thread executor, while the
Android runtime uses ChoreographerPoster.
SocketClient owns a background reader thread, dispatches incoming
messages to a handler, and auto-reconnects with exponential backoff
when the transport drops. AgentTransport abstracts the socket so
JVM unit tests drive the client through piped streams while the
Android runtime uses LocalSocket over the abstract namespace.
Send is exposed to consumers via a MessageSender passed to
onConnected. Writes are serialized under the output stream lock.
Uses org.json (already on Android; org.json:json test dep for JVM
unit tests). Field names and framing mirror internal/agent/protocol.go
so Go-encoded frames decode on the SDK without a shared schema file.
pj