# Swift Concurrency Part 1
---
# [Swift Concurrency Manifesto](https://gist.github.com/yxztj/7744e97eaf8031d673338027d89eea76)
----
## Callback hell
> 俗稱 金字塔(`波動拳`)
```swift=
func processImageData1(completionBlock: (result: Image) -> Void) {
loadWebResource("dataprofile.txt") { dataResource in
loadWebResource("imagedata.dat") { imageResource in
decodeImage(dataResource, imageResource) { imageTmp in
dewarpAndCleanupImage(imageTmp) { imageResult in
completionBlock(imageResult)
}
}
}
}
}
```
---
# Term
----
## async/await
----
## Continuations
----
## Task
* Unstructured Task
* cancellation
* Atached/Detached Task
* `Task {}`
* `Task.detached {}`
----
## TaskGroup
* Parent/Child Task
* Structured Task
* async let
* TaskGroup
----
## `Actor`
* `actor`
* Sendable
* @unchecked
* @Sendable
* isolated/nonisolated
----
## `GlobalActor`
* `@globalActor`
* `@MainActor`
----
## TaskLocal
----
## AsyncSequence
----
## AsyncStream
----
## Job 相關
* Excutor
* SerialExecutor
* UnownedSerialExecutor
* UnownedJob
---
### [Concurrency vs. Parallelism](http://tutorials.jenkov.com/java-concurrency/concurrency-vs-parallelism.html)
{%youtube Y1pgpn2gOSg %}
----
### Concurrency
Making progress on more than one task seemingly at the same time.
----
### Concurrency
----
### Parallel Execution
Making progress on more than one task at the exact same time.
----
### Parallel Execution
---
# async/await
----
## 宣告
```swift=
func download(_ url: URL) async throws -> Data {}
func decode(_ data: Data) async throws -> UIImage {}
```
----
## 一般使用方式
> 在 async func 內使用
```swift=
func your_function() async throws -> UIImage {
let data = try await download(url)
let image = try await decode(data)
return image
}
```
----
## 在 sync func 內使用
> 透過 `Task`
```swift=
func your_function_task() -> Task<UIImage, Error> {
let task = Task {
let data = try await download(url)
let image = try await decode(data)
// image ...
return image
}
return taks
}
```
----
## [get async throws](https://github.com/apple/swift-evolution/blob/main/proposals/0310-effectful-readonly-properties.md)
```swift=
protocol RemoteData {
var data: Data { get async throws }
}
struct Resouce: RemoteData {
let url: URL
var data: Data {
get async throws {
try await URLSession.shared.data(from: url).0
}
}
}
```
----
## closure
``` swift
() async -> Void
```
----
## 隱式 closure
``` swift
// () async -> Int
let closure = { await getInt() } // implicitly async
```
----
## 對現有 api 的衝擊
```swift=
// old api
func doSomething(completionHandler: ((String) -> Void)? = nil) { ... }
// new api
func doSomething() async -> String { ... }
```
----
### 對現有 api 的衝擊(rule)
> 如果在 `同步` 預設優先呼叫 `同步`
> 如果在 `非同步` 預設優先呼叫 `非同步`
----
### 對現有 api 的衝擊(rule)
```swift=
func xxx() {
// old api
// doSomething(completionHandler: nil)
doSomething()
}
func xxx() async {
// new api
let str: String = await doSomething()
}
```
----
## 自動隱式轉換
```swift=
var syncNonThrowing: () -> Void = {}
var asyncNonThrowing: () async -> Void = {}
asyncNonThrowing = syncNonThrowing
```
----
### 可以用同步函數替代非同步
```swift=
protocol Asynchronous {
func f() async
}
struct S2: Asynchronous {
func f() { } // okay, synchronous function satisfying async requirement
}
```
----
## Test
``` swift
class AsyncTests: XCTestCase {
func testCallback() throws {
let expectation = XCTestExpectation (description: "download completion")
download(url) { result, error in
XCTAssertEqual(result, Data())
expectation.fulfill()
}
wait(for: [expectation], timeout: 5.0)
}
func testAsync() async throws {
let result = try await download(url)
XCTAssertEqual(result, Data())
}
}
```
----
## 限制
> 不能在 defer 內使用 await
```swift=
defer {
await xxx() // x
}
```
----
## 限制
> @autoclosure sync closure 只能放在 async func
```swift=
// x
func computeArgumentLater<T>
(_ fn: @escaping @autoclosure () async -> T) { }
// o
func computeArgumentLater<T>
(_ fn: @escaping @autoclosure () async -> T) async { }
```
----
## 限制
同步函數 不能呼叫 異步函數
---
### 同步函數 vs 異步函數
----
## 同步函數
> sync func
```swift=
func xxx() {}
```
----
## 異步函數
> async func
```swift=
func xxx() async {}
```
----
## 同步函數
* 執行過程中，只能在同一個 thread 執行
* call stack
----
## 同步函數
```swift=
func a() {
// in main thread
b()
// in main thread
}
func b() {
// in main thread
}
```
----
## 異步函數
* 執行過程中，可在任意 thread 切換執行
* 獨立存儲
----
## 異步函數
```swift=
func a() async {
// in thread 1
await b()
// in thread 2
await c()
// in thread 3
}
func b() async {
// in thread 4
}
func c() async {
// in thread 5
}
```
----
## 暫停點(Suspension point)
* 只會出現在 `顯試操作`
* 潛在暫停點
* `await`
* 暫停會中斷 `原子性`
* 讓異步函數離開其 `thread`
----
## 暫停點
```swift=
func a() async {
// in thread 1
await b() // 暫停點
// in thread 2
await c() // 暫停點
// in thread 3
}
func b() async {
// in thread 4
// return 暫停點
}
func c() async {
// in thread 5
// return 暫停點
}
```
---
## CheckedContinuation
----
```swift=
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
public func withCheckedContinuation<T>(
function: String = #function,
_ body: (CheckedContinuation<T, Never>) -> Void
) async -> T
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
public struct CheckedContinuation<T, E> where E : Error {}
```
----
### [Converting closure-based code into async/await in Swift](https://www.andyibanez.com/posts/converting-closure-based-code-into-async-await-in-swift/)
----
## async code
```swift=
func downloadMetadata(for id: Int) async throws -> ImageMetadata {
let metadataUrl = URL(string: "https://www.andyibanez.com/fairesepages.github.io/tutorials/async-await/part1/\(id).json")!
let metadataRequest = URLRequest(url: metadataUrl)
let (data, metadataResponse) = try await URLSession.shared.data(for: metadataRequest)
guard (metadataResponse as? HTTPURLResponse)?.statusCode == 200 else {
throw ImageDownloadError.invalidMetadata
}
return try JSONDecoder().decode(ImageMetadata.self, from: data)
}
```
----
## callback code
```swift=
let metadataUrl = URL(string: "https://www.andyibanez.com/fairesepages.github.io/tutorials/async-await/part1/\(imageNumber).json")!
let metadataTask = URLSession.shared.dataTask(with: metadataUrl) { data, response, error in
guard let data = data, let metadata = try? JSONDecoder().decode(ImageMetadata.self, from: data), (response as? HTTPURLResponse)?.statusCode == 200 else {
completionHandler(nil, ImageDownloadError.invalidMetadata)
return
}
let detailedImage = DetailedImage(image: image, metadata: metadata)
completionHandler(detailedImage, nil)
}
metadataTask.resume()
```
----
## Continuations code
```swift=
guard let data = data, let metadata = try? JSONDecoder().decode(ImageMetadata.self, from: data), (response as? HTTPURLResponse)?.statusCode == 200 else {
completionHandler(nil, ImageDownloadError.invalidMetadata)
return
}
let detailedImage = DetailedImage(image: image, metadata: metadata)
completionHandler(detailedImage, nil)
```
----
## Explicit continuations
* `withCheckedContinuation`
* `CheckedContinuation<T, Never>`
* `withCheckedThrowingContinuation`
* `CheckedContinuation<T, Error>`
----
## 使用 Continuation 封裝 callback code
```swift=
func downloadImageAndMetadata(imageNumber: Int) async throws -> DetailedImage {
return try await withCheckedThrowingContinuation({
(continuation: CheckedContinuation<DetailedImage, Error>) in
downloadImageAndMetadata(imageNumber: imageNumber) { image, error in
if let image = image {
continuation.resume(returning: image)
} else {
continuation.resume(throwing: error!)
}
}
})
}
```
----
## 注意事項
* 請確保 value/error 有被呼叫過一次
* `continuation.resume(returning: value)`
* `continuation.resume(throwing: error)`
* 呼叫兩次以上屬於未定義行為
* 未呼叫的話會讓 task 永遠停住
----
## Rx VS Continuation
----
## Rx
```swift=
public var data: Observable<Data> {
return Observable.create { observer -> Disposable in
self.dataRequest.responseData { (res) in
do {
try observer.onNext(res.result.get())
observer.onCompleted()
} catch {
observer.onError(error)
}
}
return Disposables.create {
self.dataRequest.cancel()
}
}
}
```
```swift=
data.subscribe(onNext: { data in
// ...
})
```
----
## Continuation
```swift=
public var data: Data {
get async throws {
return try await withCheckedThrowingContinuation { continuation in
self.dataRequest.responseData { (res) in
do {
let value = try res.result.get()
continuation.resume(returning: value)
} catch {
continuation.resume(throwing: error)
}
}
}
}
}
```
```swift=
let value = try await data
```
----
## Demo
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/vt5r2gbheff3xh3d63xepdo2gm/embedded/">
</iframe>
---
## UnsafeContinuation
```swift=
@frozen struct UnsafeContinuation<T, E> where E : Error
func withUnsafeContinuation<T>(_ fn: (UnsafeContinuation<T, Never>) -> Void) async -> T
```
----
跟 `CheckedContinuation` 擁有一樣的介面
可直接切換
無 run time check
---
## Task
----
```swift=
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
@frozen public struct Task<Success, Failure>
: Sendable where Failure : Error {
}
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
extension Task where Failure == Never {
public init(
priority: TaskPriority? = nil,
operation: @escaping @Sendable () async -> Success
)
}
```
----
A unit of asynchronous work.
一個非同步工作的單元
----
When you create an instance of `Task`,
you provide a closure that contains the work for that task to perform.
當你建立 `Task` 實體時，你需要提供 closure 以供 task 執行
```swift=
let task = Task {
return 1
}
```
----
Tasks can start running immediately after creation;
任務可以在創建後立即開始運行
----
you don't explicitly start or schedule them.
您不用明確啟動或安排它們。
----
After creating a task, you use the instance to interact with it
for example, to wait for it to complete or to cancel it.
創建任務後，您使用實例與其交互
例如，等待它完成或取消它。
----
等待它完成
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/lvpgzooinvfrtegfg2pagllyuu/embedded/">
</iframe>
```swift=
await task.value
// or
try await task.value
await task.result
```
----
取消它
```swift=
task.cancel()
```
----
It's not a programming error to discard a reference to a task
without waiting for that task to finish or canceling it.
丟棄對任務的引用不是編程錯誤
無需等待該任務完成或取消它。
----
A task runs regardless of whether you keep a reference to it.
無論您是否保留對它的引用，任務都會運行。
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/2u2ayn7s5ffqznql5tm54lobmy/embedded/">
</iframe>
----
However, if you discard the reference to a task,
you give up the ability
to wait for that task's result or cancel the task.
但是，如果您放棄持有 `task`
你會失去等待該任務的結果或取消該任務之能力
----
Only code that's running as part of the task can interact with that task.
To interact with the current task,
you call one of the static methods on `Task`.
只有運行在 `task` 內的 code
可以用 `static method` 跟 `當前 task` 互動
```swift=
Task {
Task.isCancelled
Task.currentPriority
try Task.checkCancellation()
await Task.yield()
try await Task.sleep(nanoseconds: UInt64)
}
```
---
# Task Cancellation
----
Tasks include a shared mechanism for indicating cancellation,
but not a shared implementation for how to handle cancellation.
任務包括指示取消的共享機制，但不包括如何處理取消的共享實現。
----
Depending on the work you're doing in the task,
the correct way to stop that work varies.
根據您在任務中所做的工作，停止該工作的正確方法會有所不同。
----
Likewise,
it's the responsibility of the code running as part of the task
to check for cancellation whenever stopping is appropriate.
同樣，作為任務的一部分運行的代碼有責任在適當的停止時檢查取消。
----
In a long-task that includes multiple pieces,
you might need to check for cancellation at several points,
and handle cancellation differently at each point.
在包含多個部分的長任務中，您可能需要在多個點檢查取消，並在每個點以不同的方式處理取消。
```swift=
Task {
while !Task.isCancelled {
// Long run
}
}
```
----
If you only need to throw an error to stop the work,
call the `Task.checkCancellation()` function to check for cancellation.
如果只需要拋出一個錯誤來停止工作，調用`Task.checkCancellation()`函數來檢查是否取消。
```swift=
Task {
try Task.checkCancellation()
}
```
----
Other responses to cancellation include
returning the work completed so far, returning an empty result, or returning `nil`.
對取消的其他響應包括返回到目前為止已完成的工作、返回空結果或返回“nil”。
```swift=
Task {
if Task.isCancelled {
return nil
}
// return ...
}
```
----
Cancellation is a purely Boolean state;
there's no way to include additional information
like the reason for cancellation.
取消是一個純粹的 `bool` 狀態； 無法包含其他信息，例如取消原因。
----
This reflects the fact that a task can be canceled for many reasons,
and additional reasons can accrue during the cancellation process.
這反映了一個任務可以由於多種原因被取消的事實，並且在取消過程中可能會產生其他原因。
----
## Demo
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/vgn4s5cbf5h5jd6me4fhpgsokq/embedded/">
</iframe>
---
### Job
----
A task's execution can be seen as a series of periods where the task ran.
Each such period ends at a suspension point or the
completion of the task.
----
運行中 task 可以被視為一連串的執行週期(series of periods)
這些週期的結束點位於
* task 暫停點
* 完成 task
----
These periods of execution are represented by instances of `PartialAsyncTask`.
Unless you're implementing a custom executor,
you don't directly interact with partial tasks.
這些執行週期可以 ``PartialAsyncTask`` 的實例呈現
除非你要實作 custom executor
否則你不必直接跟它做互動
----
## UnownedJob
`PartialAsyncTask` 已改名為 `UnownedJob`
---
## [Unstructured Concurrency](https://docs.swift.org/swift-book/LanguageGuide/Concurrency.html#ID643)
Unlike tasks that are part of a task group, an unstructured task doesn’t have a parent task.
----
### Attached Task
To create an unstructured task that runs on the current actor, call the Task.init(priority:operation:) initializer.
```swift=
Task {}
```
----
### Detached Task
To create an unstructured task that’s not part of the current actor, known more specifically as a detached task, call the Task.detached(priority:operation:) class method.
```swift=
Task.detached {}
```
---
## Task 小結
----
Task 建立後會自行執行
----
cancel 只能停止
* ~~尚未開始的 Task~~
* 有支援取消流程的 Task
----
Task 的 cancel 流程必須自行掌控
----
一個 Task 可以拆分成多個 UnownedJob
---
## Actor Model
{%youtube ELwEdb_pD0k %}
----
{%youtube 7erJ1DV_Tlo %}
----
## 信箱
----
### 擁有復數地址
----
### 地址 != ID
----
### 沒有 ID
----
### 允許操作
* 建立 `子 actor`
* 傳送訊息(message)
* 決定如何處理下一條訊息
* FIFO 處理訊息
----
### ISOLATED
* 擁有私有`狀態`
* 不共享記憶體
----
## 各語言實作方式不同
---
## Swift Actor
```swift=
actor MyActor {}
```
----
### protocol Actor
```swift=
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
public protocol Actor : AnyObject, Sendable {
nonisolated
var unownedExecutor: UnownedSerialExecutor { get }
}
```
----
### UnownedSerialExecutor
```swift=
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
@frozen public struct UnownedSerialExecutor {
@inlinable
public init<E>(ordinary executor: E)
where E : SerialExecutor
}
```
----
### SerialExecutor
```swift=
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
public protocol SerialExecutor : Executor {
func enqueue(_ job: UnownedJob)
func asUnownedSerialExecutor() -> UnownedSerialExecutor
}
```
----
### UnownedJob
``` swift=
@available(SwiftStdlib 5.1, *)
@frozen
public struct UnownedJob: Sendable {
private var context: Builtin.Job
@_alwaysEmitIntoClient
@inlinable
public func _runSynchronously(
on executor: UnownedSerialExecutor
) {
_swiftJobRun(self, executor)
}
}
```
----
### UnownedJob
``` swift=
@available(SwiftStdlib 5.1, *)
@_silgen_name("swift_job_run")
@usableFromInline
internal func _swiftJobRun(
_ job: UnownedJob,
_ executor: UnownedSerialExecutor
) -> ()
```
---
## Actor reentrancy
* Actor 的 `isolated functions` 是可重入的(reentrant).
* 當 Actor 的 `isolated functions` 暫停時， 重入性會允許在原 isolated function 恢復之前，並在 actor 先執行其他工作，稱之 `interleaving`.
----
### Actor reentrancy
預期是 before before after after
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/okf3jkkjzvcetmpl4dxig25ulq/embedded/">
</iframe>
----
### Actor reentrancy(預期)
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/vrel7oeb3bg4blm2o55kkcb3qu/embedded/">
</iframe>
----
### 結論(坑)
async func 有機會排進 `actor's queue`
---
## Actor 猜想
----
### Thread
* 擁有唯一一個 Thread?
* 擁有可隨時切換 Thread?
* async/await
----
### Thread
```swift=
func action() async {
print("test")
}
actor Tester1 {
func action() {
print("test")
}
}
actor Tester2 {
func action() {
print("test")
}
}
Task.detached {
await action()
await Tester1().action()
await Tester2().action()
exit(0)
}
RunLoop.main.run()
```
----
### Thread
----
### Thread
----
### Thread
`Thread 2 Queue : com.apple.root.default-qos.cooperative (concurrent)`
---
### Queue
> Message Queue?
----
### Queue
> Task Queue?
----
### Queue
> Job(UnownedJob) Queue
----
### Executor
* `UnownedSerialExecutor`
* 以 FIFO 方式，在 Thread 執行 Job
---
## 問題 No.1
```swift=
import Foundation
func process(_ text: String) {
print("\(Date()) \(text) 1")
sleep(2)
print("\(Date()) \(text) 2")
sleep(2)
print("\(Date()) \(text) 3")
}
actor Test {
var counter = 0
func ver1_1() async {
print("\(Date()) \(#function) start")
let a: Task<Void, Never> = Task {
process("a")
}
let b: Task<Void, Never> = Task {
process("b")
}
sleep(2)
let (_, _) = await (a.value, b.value)
print("\(Date()) end")
}
func ver1_2() async {
print("\(Date()) \(#function) start")
let a: Task<Void, Never> = Task {
_ = counter
process("a")
}
let b: Task<Void, Never> = Task {
_ = counter
process("b")
}
sleep(2)
let (_, _) = await (a.value, b.value)
print("\(Date()) end")
}
}
@MainActor
func main() async {
let test = Test()
await test.ver1_1();print("")
await test.ver1_2();print("")
exit(EXIT_SUCCESS)
}
Task.detached {
await main()
}
RunLoop.main.run()
```
----
## 問題 No.2
```swift=
import Foundation
func process(_ text: String) {
print("\(Date()) \(text) 1")
sleep(2)
print("\(Date()) \(text) 2")
sleep(2)
print("\(Date()) \(text) 3")
}
func processAsync() async {
let text = "a"
process(text)
}
func processAsync1(_ text: String) async {
process(text)
}
func processAsync2(_ text: String) async {
process(text)
}
@MainActor
func processMain(_ text: String) async {
process(text)
}
@MainActor
func ver2_1() async {
print("\(Date()) \(#function) start")
let a: Task<Void, Never> = Task {
process("a")
}
let b: Task<Void, Never> = Task {
process("b")
}
sleep(2)
let (_, _) = await (a.value, b.value)
print("\(Date()) end")
}
@MainActor
func ver2_2() async {
print("\(Date()) \(#function) start")
let a: Task<Void, Never> = Task {
await processAsync()
}
let b: Task<Void, Never> = Task {
await processAsync()
}
sleep(2)
let (_, _) = await (a.value, b.value)
print("\(Date()) end")
}
@MainActor
func ver2_3() async {
print("\(Date()) \(#function) start")
let a: Task<Void, Never> = Task {
await processAsync1("a")
await processAsync1("b")
}
let b: Task<Void, Never> = Task {
await processAsync2("c")
await processAsync2("d")
}
sleep(2)
let (_, _) = await (a.value, b.value)
print("\(Date()) end")
}
actor T {
func processAsync1(_ text: String) {
process(text)
}
}
let t1 = T()
let t2 = T()
@MainActor
func ver2_4() async {
print("\(Date()) \(#function) start")
let a: Task<Void, Never> = Task {
await t1.processAsync1("a")
}
let b: Task<Void, Never> = Task {
await t2.processAsync1("b")
}
sleep(2)
let (_, _) = await (a.value, b.value)
print("\(Date()) end")
}
@MainActor
func main() async {
// await ver2_1();print("")
// await ver2_2();print("")
await ver2_3();print("")
// await ver2_4();print("")
exit(EXIT_SUCCESS)
}
Task.detached {
await main()
}
RunLoop.main.run()
```
---
## 坑 No.1
Task 有/無 mutable var
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/xx5n6mvxv5arbkr3hkl6fwk4o4/embedded/">
</iframe>
----
## 坑 No.2
async func 有/無 參數
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/pqq74s2cdzbm5htskkbeiu7wq4/embedded/">
</iframe>
---
### Q & A
---
## ??? No.1
----
### [0296-async-await.md](https://github.com/apple/swift-evolution/blob/main/proposals/0296-async-await.md)
> However, many asynchronous functions are not just asynchronous: they’re also associated with specific actors
----
#### `async function` 跟 `特定 actor` 相關
```swift=
// actor SpecificActor {
func xxx() async {}
//}
// @SpecificActor
func xxx() async {}
```
----
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/77nint5bwrc7dbzpup3hkuykka/embedded/">
</iframe>
----
### 結論
一般 async func 有可能會延續上一個 actor嗎?
---
## ??? NO.2
----
### [swift.org](https://docs.swift.org/swift-book/LanguageGuide/Concurrency.html#ID643)
> To create an unstructured task that runs on the current actor, call the `Task.init(priority:operation:)` initializer
----
#### 跑在 `目前的 actor`
```swift=
func xxx() {
// is actor?
Task {
}
}
```
----
<iframe width="100%" height="300" frameborder="0"
src="https://swiftfiddle.com/ux5wjo4mpfcxrfynxes4fvvsae/embedded/">
</iframe>
---
### Reference
* [SE 296](https://github.com/apple/swift-evolution/blob/main/proposals/0296-async-await.md)
* [SE 306](https://github.com/apple/swift-evolution/blob/main/proposals/0306-actors.md)
* [Task](https://developer.apple.com/documentation/swift/task)
* [Continuation](https://developer.apple.com/documentation/swift/checkedcontinuation)
* ...
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