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FxDart 101 › Seção 3 · Transformando › attach

attach

Pair each value with what you derive from it — the input stays beside its result.

Lecture

map replaces each value with its result — and the moment you need the input again downstream (to fall back, to label, to log), you find yourself hand-building records: .map((x) async => (x, await f(x))). attach(f) is that idiom as an operator: it yields (value, f(value)) pairs, lazily.

It earns its keep in async chains. Look up a price per item and the pair keeps the item next to the (maybe missing) price, so the fallback r.$2 ?? r.$1.listPrice and the "which SKU was that?" label are both still in reach. The async form is built on mapAsync, so it is parallel-safe — put concurrent(n) after it and n lookups run at once.

Dart-native addition (no FxTS counterpart). When you only need the derived value, keep using map; when you need it keyed by the input as a lookup table, that is indexBy.

Demo 1 · The input survives the map

Demo 2 · Async lookups with fallback

Try it yourself

Exercise: keep each query beside its search results.