tryMap(_:)
Transforms all elements from the upstream publisher with a provided error-throwing closure.
Declaration
func tryMap<T>(_ transform: @escaping (Self.Output) throws -> T) -> Publishers.TryMap<Self, T>Parameters
- transform:
A closure that takes one element as its parameter and returns a new element. If the closure throws an error, the publisher fails with the thrown error.
Return Value
A publisher that uses the provided closure to map elements from the upstream publisher to new elements that it then publishes.
Discussion
Combine’s tryMap(_:) operator performs a function similar to that of map(_:) in the Swift standard library: it uses a closure to transform each element it receives from the upstream publisher. You use tryMap(_:) to transform from one kind of element to another, and to terminate publishing when the map’s closure throws an error.
The following example uses an array of numbers as the source for a collection based publisher. A tryMap(_:) operator consumes each integer from the publisher and uses a dictionary to transform it from its Arabic numeral to a Roman equivalent, as a String. If the tryMap(_:)’s closure fails to look up a Roman numeral, it throws an error. The tryMap(_:) operator catches this error and terminates publishing, sending a Subscribers.Completion.failure(_:) that wraps the error.
struct ParseError: Error {}
func romanNumeral(from:Int) throws -> String {
let romanNumeralDict: [Int : String] =
[1:"I", 2:"II", 3:"III", 4:"IV", 5:"V"]
guard let numeral = romanNumeralDict[from] else {
throw ParseError()
}
return numeral
}
let numbers = [5, 4, 3, 2, 1, 0]
cancellable = numbers.publisher
.tryMap { try romanNumeral(from: $0) }
.sink(
receiveCompletion: { print ("completion: \($0)") },
receiveValue: { print ("\($0)", terminator: " ") }
)
// Prints: "V IV III II I completion: failure(ParseError())"If your closure doesn’t throw, use map(_:) instead.