ControlNotificationStream.compactMap(_:)

Description#

Use the compactMap(_:) method to transform every element received from a base asynchronous sequence, while also discarding any nil results from the closure. Typically, you use this to transform from one type of element to another.

In this example, an asynchronous sequence called Counter produces Int values from 1 to 5. The closure provided to the compactMap(_:) method takes each Int and looks up a corresponding String from a romanNumeralDict dictionary. Since there is no key for 4, the closure returns nil in this case, which compactMap(_:) omits from the transformed asynchronous sequence. When the value is 5, the closure throws MyError, terminating the sequence.

let romanNumeralDict: [Int: String] = [1: "I", 2: "II", 3: "III", 5: "V"]

do { let stream = Counter(howHigh: 5) .compactMap { (value) throws -> String? in if value == 5 { throw MyError() } return romanNumeralDict[value] } for try await numeral in stream { print(numeral, terminator: " ") } } catch { print("Error: \(error)") } // Prints "I II III Error: MyError() "

• Parameter transform: An error-throwing mapping closure. transform accepts an element of this sequence as its parameter and returns a transformed value of the same or of a different type. If transform throws an error, the sequence ends.
• Returns: An asynchronous sequence that contains, in order, the non-nil elements produced by the transform closure. The sequence ends either when the base sequence ends or when transform throws an error.