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RxDart vs FxDart › #7 · A default line for an empty report

A default line for an empty report

Toss-up

• fx
• filter
• ifEmpty
• map

Requirement

A spending report filters this month's transactions down to the travel category — and there aren't any. An empty report should not print nothing: it should print a single no travel spending line instead. The data is in the code; both versions must print the line shown under Expected output.

no travel spending recorded in August

Side by side

Why they differ

Hardly at all — and in the same place. "Emptiness needs a fallback" is a problem both models hit in exactly the same place — a downstream stage cannot tell filtered to nothing from never had anything unless the pipeline says what to emit in that case — and both libraries answer with one operator. RxDart's defaultIfEmpty injects the default when the source completes without an event; FxDart's defaultIfEmpty (openly borrowed from the Rx vocabulary, with ifEmpty(() => fallback) as the lazy whole-iterable form) yields it when the first pull finds nothing.

The residual difference is the usual one in this Part: the stream version has to know it is empty by waiting for completion, so the whole program goes async over a fixed list, while the pull version discovers emptiness synchronously on the first demand. That cost is real but small here, and the operator parity is the story — a genuine tie, and a nice example of the two libraries trading ideas rather than competing.