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https://github.com/dfed/swift-async-queue
A queue that enables ordered sending of events from synchronous to asynchronous code
https://github.com/dfed/swift-async-queue
Last synced: 4 months ago
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A queue that enables ordered sending of events from synchronous to asynchronous code
- Host: GitHub
- URL: https://github.com/dfed/swift-async-queue
- Owner: dfed
- License: mit
- Created: 2022-11-23T02:28:24.000Z (over 1 year ago)
- Default Branch: main
- Last Pushed: 2024-01-11T14:55:45.000Z (6 months ago)
- Last Synced: 2024-01-12T07:36:51.521Z (6 months ago)
- Language: Swift
- Size: 62.5 KB
- Stars: 85
- Watchers: 4
- Forks: 4
- Open Issues: 1
-
Metadata Files:
- Readme: README.md
- Contributing: Contributing.md
- License: LICENSE
Lists
- awesome-ios - swift-async-queue
README
# swift-async-queue
[](https://github.com/dfed/swift-async-queue/actions?query=workflow%3ACI+branch%3Amain)
[](https://codecov.io/gh/dfed/swift-async-queue)
[](https://spdx.org/licenses/MIT.html)
[](https://swiftpackageindex.com/dfed/swift-async-queue)
[](https://swiftpackageindex.com/dfed/swift-async-queue)
A library of queues that enable sending ordered tasks from synchronous to asynchronous contexts.
## Task Ordering and Swift Concurrency
Tasks sent from a synchronous context to an asynchronous context in Swift Concurrency are inherently unordered. Consider the following test:
```swift
@MainActor
func testMainActorTaskOrdering() async {
actor Counter {
func incrementAndAssertCountEquals(_ expectedCount: Int) {
count += 1
let incrementedCount = count
XCTAssertEqual(incrementedCount, expectedCount) // often fails
}
private var count = 0
}
let counter = Counter()
var tasks = [Task]()
for iteration in 1...100 {
tasks.append(Task {
await counter.incrementAndAssertCountEquals(iteration)
})
}
// Wait for all enqueued tasks to finish.
for task in tasks {
_ = await task.value
}
}
```
Despite the spawned `Task` inheriting the serial `@MainActor` execution context, the ordering of the scheduled asynchronous work is not guaranteed.
While [actors](https://docs.swift.org/swift-book/LanguageGuide/Concurrency.html#ID645) are great at serializing tasks, there is no simple way in the standard Swift library to send ordered tasks to them from a synchronous context.
### Executing asynchronous tasks in FIFO order
Use a `FIFOQueue` to execute asynchronous tasks enqueued from a nonisolated context in FIFO order. Tasks sent to one of these queues are guaranteed to begin _and end_ executing in the order in which they are enqueued. A `FIFOQueue` executes tasks in a similar manner to a `DispatchQueue`: enqueued tasks executes atomically, and the program will deadlock if a task executing on a `FIFOQueue` awaits results from the queue on which it is executing.
A `FIFOQueue` can easily execute asynchronous tasks from a nonisolated context in FIFO order:
```swift
func testFIFOQueueOrdering() async {
actor Counter {
nonisolated
func incrementAndAssertCountEquals(_ expectedCount: Int) {
queue.enqueue {
await self.increment()
let incrementedCount = await self.count
XCTAssertEqual(incrementedCount, expectedCount) // always succeeds
}
}
func flushQueue() async {
await queue.enqueueAndWait { }
}
func increment() {
count += 1
}
var count = 0
private let queue = FIFOQueue()
}
let counter = Counter()
for iteration in 1...100 {
counter.incrementAndAssertCountEquals(iteration)
}
// Wait for all enqueued tasks to finish.
await counter.flushQueue()
}
```
FIFO execution has a key downside: the queue must wait for all previously enqueued work – including suspended work – to complete before new work can begin. If you desire new work to start when a prior task suspends, utilize an `ActorQueue`.
### Sending ordered asynchronous tasks to Actors from a nonisolated context
Use an `ActorQueue` to send ordered asynchronous tasks to an `actor`’s isolated context from nonisolated or synchronous contexts. Tasks sent to an actor queue are guaranteed to begin executing in the order in which they are enqueued. However, unlike a `FIFOQueue`, execution order is guaranteed only until the first [suspension point](https://docs.swift.org/swift-book/LanguageGuide/Concurrency.html#ID639) within the enqueued task. An `ActorQueue` executes tasks within the its adopted actor’s isolated context, resulting in `ActorQueue` task execution having the same properties as `actor` code execution: code between suspension points is executed atomically, and tasks sent to a single `ActorQueue` can await results from the queue without deadlocking.
An instance of an `ActorQueue` is designed to be utilized by a single `actor` instance: tasks sent to an `ActorQueue` utilize the isolated context of the queue‘s adopted `actor` to serialize tasks. As such, there are a couple requirements that must be met when dealing with an `ActorQueue`:
1. The lifecycle of any `ActorQueue` should not exceed the lifecycle of its `actor`. It is strongly recommended that an `ActorQueue` be a `private let` constant on the adopted `actor`. Enqueuing a task to an `ActorQueue` instance after its adopted `actor` has been deallocated will result in a crash.
2. An `actor` utilizing an `ActorQueue` should set the adopted execution context of the queue to `self` within the `actor`’s `init`. Failing to set an adopted execution context prior to enqueuing work on an `ActorQueue` will result in a crash.
An `ActorQueue` can easily enqueue tasks that execute on an actor’s isolated context from a nonisolated context in order:
```swift
func testActorQueueOrdering() async {
actor Counter {
init() {
// Adopting the execution context in `init` satisfies requirement #2 above.
queue.adoptExecutionContext(of: self)
}
nonisolated
func incrementAndAssertCountEquals(_ expectedCount: Int) {
queue.enqueue { myself in
myself.count += 1
XCTAssertEqual(expectedCount, myself.count) // always succeeds
}
}
func flushQueue() async {
await queue.enqueueAndWait { _ in }
}
private var count = 0
// Making the queue a private let constant satisfies requirement #1 above.
private let queue = ActorQueue()
}
let counter = Counter()
for iteration in 1...100 {
counter.incrementAndAssertCountEquals(iteration)
}
// Wait for all enqueued tasks to finish.
await counter.flushQueue()
}
```
### Sending ordered asynchronous tasks to the `@MainActor` from a nonisolated context
Use a `MainActorQueue` to send ordered asynchronous tasks to the `@MainActor`’s isolated context from nonisolated or synchronous contexts. Tasks sent to this queue type are guaranteed to begin executing in the order in which they are enqueued. Like an `ActorQueue`, execution order is guaranteed only until the first [suspension point](https://docs.swift.org/swift-book/LanguageGuide/Concurrency.html#ID639) within the enqueued task. A `MainActorQueue` executes tasks within its adopted actor’s isolated context, resulting in `MainActorQueue` task execution having the same properties as a `@MainActor`'s' code execution: code between suspension points is executed atomically, and tasks sent to a single `MainActorQueue` can await results from the queue without deadlocking.
A `MainActorQueue` can easily execute asynchronous tasks from a nonisolated context in FIFO order:
```swift
@MainActor
func testMainActorQueueOrdering() async {
@MainActor
final class Counter {
nonisolated
func incrementAndAssertCountEquals(_ expectedCount: Int) {
MainActorQueue.shared.enqueue {
self.increment()
let incrementedCount = self.count
XCTAssertEqual(incrementedCount, expectedCount) // always succeeds
}
}
func flushQueue() async {
await MainActorQueue.shared.enqueueAndWait { }
}
func increment() {
count += 1
}
var count = 0
}
let counter = Counter()
for iteration in 1...100 {
counter.incrementAndAssertCountEquals(iteration)
}
// Wait for all enqueued tasks to finish.
await counter.flushQueue()
}
```
## Requirements
* Xcode 15.0 or later.
* iOS 13 or later.
* tvOS 13 or later.
* watchOS 6 or later.
* macOS 10.15 or later.
* Swift 5.9 or later.
## Installation
### Swift Package Manager
To install swift-async-queue in your project with [Swift Package Manager](https://github.com/apple/swift-package-manager), the following lines can be added to your `Package.swift` file:
```swift
dependencies: [
.package(url: "https://github.com/dfed/swift-async-queue", from: "0.5.0"),
]
```
### CocoaPods
To install swift-async-queue in your project with [CocoaPods](http://cocoapods.org), add the following to your `Podfile`:
```
platform :ios, '13.0'
pod 'AsyncQueue', '~> 0.5.0'
```
## Contributing
I’m glad you’re interested in swift-async-queue, and I’d love to see where you take it. Please read the [contributing guidelines](Contributing.md) prior to submitting a Pull Request.
Thanks, and happy queueing!
## Developing
Double-click on `Package.swift` in the root of the repository to open the project in Xcode.