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RxDart vs FxDart › #23 · Dedupe a paged feed by id

Dedupe a paged feed by id

Toss-up

• fx
• flatMap
• uniqBy
• map

Requirement

A product feed arrives in three pages whose boundaries overlap, so some items appear on two pages. Flatten the pages and print each item exactly once — first occurrence wins, arrival order kept — keyed by its numeric id. The pages are in the code; both versions must print the lines shown under Expected output.

#101 Desk lamp
#102 Notebook
#103 Pen set
#104 Stapler
#105 Monitor arm
#106 Desk mat

Side by side

Why they differ

The interesting fact is where the duplicates sit. The page boundaries overlap, so a repeated id arrives on a different page — never next to its first occurrence once the pages are flattened. That is exactly the job plain Stream.distinct silently gets wrong: it is adjacent-only, so on this feed it would wave every repeat straight through. The dedup here needs a whole-feed memory, while the flattening itself is fluent in both models — expand on the stream, flatMap on the pull chain.

As in the unique-visitors pair, the global dedup is distinctUnique against uniqBy — an equals/hashCode pair versus one key function — and the adjacent-vs-global naming split is the one the status-changes page walks through. Both sides keep the same set of seen keys and preserve first-arrival order, so the verdict is a tie — the pull version just stays synchronous because the pages are sitting in a local list.