tryReduce(_:_:)
Applies an error-throwing closure that collects each element of a stream and publishes a final result upon completion.
Declaration
func tryReduce<T>(_ initialResult: T, _ nextPartialResult: @escaping (T, Self.Output) throws -> T) -> Publishers.TryReduce<Self, T>Parameters
- initialResult:
The value that the closure receives the first time it’s called.
- nextPartialResult:
An error-throwing closure that takes the previously-accumulated value and the next element from the upstream publisher to produce a new value.
Return Value
A publisher that applies the closure to all received elements and produces an accumulated value when the upstream publisher finishes.
Discussion
Use tryReduce(_:_:) to collect a stream of elements and produce an accumulated value based on an error-throwing closure you provide. If the closure throws an error, the publisher fails and passes the error to its subscriber.
In the example below, the publisher’s 0 element causes the myDivide(_:_:) function to throw an error and publish the nan result:
struct DivisionByZeroError: Error {}
func myDivide(_ dividend: Double, _ divisor: Double) throws -> Double {
guard divisor != 0 else { throw DivisionByZeroError() }
return dividend / divisor
}
var numbers: [Double] = [5, 4, 3, 2, 1, 0]
numbers.publisher
.tryReduce(numbers.first!, { accum, next in try myDivide(accum, next) })
.catch({ _ in Just(Double.nan) })
.sink { print("\($0)") }