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RxDart vs FxDart › #18 · Two feeds, strictly in order

Two feeds, strictly in order

Toss-up

• fx
• concat
• map

Requirement

An incident review needs the tail of yesterday's log followed by today's log as one numbered list — today's first entry must never appear before yesterday's last. The data is in the code; both versions must print the lines shown under Expected output.

1. 23:41  cache flushed
2. 23:52  nightly backup started
3. 23:58  nightly backup finished
4. 00:04  cache warmed
5. 00:12  first request served
6. 06:30  daily report generated
7. 07:02  queue drained

Side by side

Why they differ

Both libraries write "this feed, then that one" as a single operator, but each model guarantees the order with its own mechanism. RxDart's concatWith is a subscription sequencer: it doesn't subscribe to today's stream until yesterday's fires done, so ordering holds even if both sources are live and today's would have been ready to emit earlier. FxDart's concat is a demand sequencer: the chain pulls from the second iterable only after the first is exhausted, and with two in-memory lists that is all the ordering there is to arrange.

That mechanism gap matters exactly when the sources are genuinely push-driven — a stream that starts emitting the moment it is subscribed needs concatWith's deferred subscription, where a pull pipeline would first have to buffer the not-yet-wanted feed. Over fixed data the two collapse into the same seven lines and the same map. A tie: the operator is shared vocabulary, and each model implements it with the tool it already had.