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toKeySet()

Operator function that maps a stream of KeyChanges into a Set that contains the active keys.

function toKeySet<K>(): OperatorFunction<KeyChanges<K>, Set<K>>

Returns​

An OperatorFunction that will aggregate the key changes from the stream into a Set with those keys.

Example​

// Call to service mocked for the example
const priceStream$ = from([
  {
    symbol: "AUDCHF",
    price: 1.32,
  },
  {
    symbol: "GOLD",
    price: 1.213,
  },
  {
    symbol: "CRYPTO",
    price: 2.45,
  },
  {
    symbol: "GOLD",
    price: 1.27,
  },
]).pipe(
  // Add delay to every new price.
  concatMap((value) => of(value).pipe(delay(100))),
)

const [lastPriceOfSymbol, symbolChange$] = partitionByKey(
  priceStream$,
  (x) => x.symbol,
)

const activeSymbols$ = toKeySet(symbolChange$).pipe(
  map((keySet) => Array.from(keySet)),
)

activeSymbols$.subscribe((keys) => console.log(keys))
// Will log:
// ['AUDCHF']
// ['AUDCHF', 'GOLD']
// ['AUDCHF', 'GOLD', 'CRYPTO']

See also​