feat(utils/combineKeys): emit set with changed keys

[voliva]

Recently I found a use case where I'd like to have an operator similar to combineKeys, but that instead of emitting a map of values it would just emit the latest individual change.

I couldn't find any way of composing the existing operators to do this, so I had to make a custom operator which had mostly combineKeys' code changing a few lines.

After discussing it we found out that this operator would be easy to implement if the existing combineKeys also emitted the list of keys that have changed. This is a posible implementation.

With this operator, the mergeKeys operator I had in mind would become something like:

const mergeKeys = <K, T>(keys$: Observable<K>, getInner$: (key: K) => Observable<T>): Observable<T> =>
  combineKeys(keys$, getInner$).pipe(
    mergeMap(values => {
      return Array.from(values.changes)
        .map(key => values.get(key))
        .filter(value => value !== undefined)
    })
  )

Doubts on this one:

• Is it good to just add a property into the Map?
• I went for the same pattern to make a copy of the Set... I think an alternative would be to just return the existing Set and instantiate a new one for the next emission

[codecov[bot]]

Codecov Report

Merging #199 (791bac2) into main (13fa3af) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #199   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines          445       448    +3     
  Branches        60        60           
=========================================
+ Hits           445       448    +3     

Impacted Files	Coverage Δ
packages/utils/src/combineKeys.ts	100.00% <100.00%> (ø)

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[peicodes]

Can't wait to use this!