swift-async-operations

A library extending the capability of async operations.

Motivation

Swift concurrency is powerful language feature, but there are not APIs operating an array for swift concurrency. A developer is required to write redundant code.

var results: [Int] = [] // ☹️ var is required.
for await element in [0, 1, 2, 3, 4] [
    let newElement = try await twice(element)
    result.append(newElement)
}
print(results) // [0, 2, 4, 6, 8]

In a case where the loop needs to run concurrently, a developer is required to write more redundant code.

 // ☹️ Long redundant code
let array = [0, 1, 2, 3, 4]
let results = try await withThrowingTaskGroup(of: (Int, Int).self) { group in
    for (index, number) in array.enumerated() {
        group.addTask {
            (index, try await twice(number))
        }
    }
    var results: [Int: Int] = [:]
    for try await (index, result) in group {
        results[index] = result
    }
    // ☹️ Need to take the order into account.
    return results.sorted(by: { $0.key < $1.key }).map(\.value)
}
print(results) // [0, 2, 4, 6, 8]

Solution

This library provides async functions as extensions of Sequence like asyncMap.

let converted = try await [0, 1, 2, 3, 4].asyncMap { number in
    try await twice(number)
}
print(converted) // [0, 2, 4, 6, 8]

The closure runs sequentially by default. And by specifying a max number of tasks, the closure can also run concurrently.

let converted = try await [0, 1, 2, 3, 4].asyncMap(numberOfConcurrentTasks: 8) { number in
    try await twice(number)
}
print(converted) // [0, 2, 4, 6, 8]

Feature Details

The library provides two features.

1. Async functions of Sequence.
2. Ordered Task Group

Async functions of Sequence

This library provides async operations like asyncForEach and asyncMap.

try await [1, 2, 3].asyncForEach { number in
    print("Start: \(number)")
    try await doSomething(number)
    print("End: \(number)")
}

The closure runs sequential by default.

Start: 1
End: 1
Start: 2
End: 2
Start: 3
End: 3

As an advanced usage, numberOfConcurrentTasks can be specified and the closure can run in parallel if the value is 2 or more.

try await [1, 2, 3].asyncForEach(numberOfConcurrentTasks: 3) { number in
    print("Start: \(number)")
    try await doSomething(number)
    print("End: \(number)")
}

Start: 2
End: 2
Start: 1
Start: 3
End: 3
End: 1

The extended functions perform parallel execution even for order-sensitive functions like map function, transforming the array while preserving the original order.

let result = try await [1, 2, 3].asyncMap(numberOfConcurrentTasks: 3) { number in
    print("Start: \(number)")
    let result = try await twice(number)
    print("End: \(number)")
    return result
}
print(result)

Start: 1
Start: 3
End: 3
End: 1
Start: 2
End: 2
[2, 4, 6]

This library provides

• asyncForEach
• asyncMap
• asyncFlatMap
• asyncCompactMap
• asyncFilter
• asyncFirst
• asyncAllSatisfy
• asyncContains
• asyncReduce

Ordered Task Group

The original utility function withTaskGroup and withThrowingTaskGroup don't ensure the order of for await.

let results = await withTaskGroup(of: Int.self) { group in
    (0..<5).forEach { number in
        group.addTask {
            await Task.yield()
            return number * 2
        }
    }
    var results: [Int] = []
    for await number in group {
        results.append(number)
    }
    return results
}
print(results) // ☹️ [0, 4, 2, 6, 10, 8]

However, ordered for await is required in some of situations like converting an array to a new array.

withOrderedTaskGroup and withThrowingOrderedTaskGroup satisfy such requirements.

let results = await withOrderedTaskGroup(of: Int.self) { group in
    (0..<5).forEach { number in
        group.addTask {
            await Task.yield()
            return number * 2
        }
    }
    var results: [Int] = []
    for await number in group {
        results.append(number)
    }
    return results
}
print(results) // 😁 [0, 2, 4, 6, 8, 10]

They are also used for async functions of Sequence.

Requirements

Swift 5.10 or later.

Installation

You can install the library via Swift Package Manager.

dependencies: [
  .package(url: "https://github.com/mtj0928/swift-async-operations", from: "0.1.0")
]

Documentations

Please see the DocC pages