OR-5270 Operators:: Sequence Operators:: Querying values:: reduce(_:_:)

[crazymanish]

Context

• Close ticket: [w25(18june-24june)] Operators:: Sequence Operators: Query the publisher #33

Operators are publishers

• In Combine, methods that perform an operation on values coming from a publisher are called operators.
• Each Combine operator actually returns a publisher. Generally speaking, that publisher receives the upstream values, manipulates the data, and then sends that data downstream.

Sequence operators

• Sequence operators are easiest to understand when you realize that publishers are just sequences themselves.
• Sequence operators work with the collection of a publisher’s values, much like an array or set — which, of course, are just finite sequences!

Querying values

• The following operators also deal with the entire set of values emitted by a publisher, but they don't produce any specific value that it emits.
• Instead, these operators emit a different value representing some query on the publisher as a whole. A good example of this is the count operator.

In this PR

• Operators:: Sequence Operators:: Finding values:: reduce(_:_:)
• reduce(_:_:) Applies a closure that collects each element of a stream and publishes a final result upon completion.
• Param: initialResult:: The value that the closure receives the first time it’s called.
• Param: nextPartialResult:: A closure that produces a new value by taking the previously-accumulated value and the next element it receives from the upstream publisher.
• Use reduce(::) to collect a stream of elements and produce an accumulated value based on a closure you provide.
• https://developer.apple.com/documentation/combine/publishers/reduce/reduce(_:_:)