tryFirst(where:)
Publishes the first element of a stream to satisfy a throwing predicate closure, then finishes normally.
Declaration
func tryFirst(where predicate: @escaping (Self.Output) throws -> Bool) -> Publishers.TryFirstWhere<Self>Parameters
- predicate:
A closure that takes an element as a parameter and returns a Boolean value that indicates whether to publish the element.
Return Value
A publisher that only publishes the first element of a stream that satisfies the predicate.
Discussion
Use tryFirst(where:) when you need to republish only the first element of a stream that satisfies an error-throwing closure you specify. The publisher ignores all elements after the first. If this publisher doesn’t receive any elements, it finishes without publishing. If the predicate closure throws an error, the publisher fails.
In the example below, a range publisher emits the first element in the range then finishes normally:
let numberRange: ClosedRange<Int> = (-1...50)
numberRange.publisher
.tryFirst {
guard $0 < 99 else {throw RangeError()}
return true
}
.sink(
receiveCompletion: { print ("completion: \($0)", terminator: " ") },
receiveValue: { print ("\($0)", terminator: " ") }
)
// Prints: "-1 completion: finished"
// If instead the number range were ClosedRange<Int> = (100...200), the tryFirst operator would terminate publishing with a RangeError.