RxSwift: Scan Operator

.scan { }

• Applies a supplied predicate to the elements emitted by an Observable, sequentially, emitting each successful result.
• Applies function to first element and then uses the result of this as the parameter for the second emitted elements applied function, and so on.
• Acts like an accumulator.
• Useful for for running totals, statistics, states…

--1--2--3--4--------5-------->

scan((x, y) => x * y)

--1--2--6--24-------120------>

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