OR-5259 FilteringOperators:: Limiting values

[crazymanish]

Context

• Close ticket: [w21(21may-27may)] Operators:: Filtering Operators: Limiting values #22

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.

In this PR

• Limiting values means receiving values until some condition is met, and then forcing the publisher to complete.
• For example, consider a request that may emit an unknown amount of values, but you only want a single emission and don’t care about the rest of them.
• Combine solves this set of problems with the prefix family of operators.
• This is opposite of dropping values operators: drop(), drop(while:), drop(:untilOutputFrom)

---

• prefix(_:) Republishes elements up to the specified maximum count.
• https://developer.apple.com/documentation/combine/publishers/collect/prefix(_:)
• prefix(while:) Republishes elements while a predicate closure indicates publishing should continue.
• https://developer.apple.com/documentation/combine/publishers/collect/prefix(while:)
• tryPrefix(while:) Republishes elements while an error-throwing predicate closure indicates publishing should continue.
• https://developer.apple.com/documentation/combine/publishers/collect/tryprefix(while:)
• prefix(untilOutputFrom:) Republishes elements until another publisher emits an element.
• https://developer.apple.com/documentation/combine/publishers/collect/prefix(untiloutputfrom:)