async let does not compile because of the "Reference to captured var 'imageDescriptors' in concurrently-executing code" - swift

Question

I am trying to use withThrowingTaskGroup to get some data from the web. I found that my code runs a bit slow, so I tried to use actors with async/await. I use Descriptors to get the
func getAllStocksList() async throws -> [SingleStockViewModel] {
var stockViewModels = [SingleStockViewModel]()
let urlString = URLBuilder.getAllStocks.makeString()
let (data, response) = try await URLSession.shared.data(from: URL(string: urlString)!)
if let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode != 200 {
throw "Invalid HttpResponseCode"
}
let dataResponse = try JSONDecoder().decode([StockDetails].self, from: data)
let stocksDetailsList = dataResponse[..<25]
let stockSymbolsList = stocksDetailsList.map {
$0.title
}
let dataResponseDict = dataResponse.toDictionary {
$0.title
}
var stockPrices: [String: Double] = [:]
let imageUrlStringsDict = try await imageService.makeStockImageUrlStringsList(for: stockSymbolsList)
let nonEmptyImageUrlStringsList = imageUrlStringsDict.filter {
!$0.value.isEmpty
}
var imageDescriptors = [Descriptor]()
var chartDescriptors = [Descriptor]()
for item in nonEmptyImageUrlStringsList {
imageDescriptors.append(
Descriptor(
stockSymbol: item.key,
stockImageUrlString: nonEmptyImageUrlStringsList[item.key]!,
type: .image)
)
chartDescriptors.append(
Descriptor(stockSymbol: item.key,
stockImageUrlString: "",
type: .marketData)
)
let price = try await fetchStockPrice(for: item.key)
stockPrices[item.key] = price
}
print(chartDescriptors)
async let taskResults = try fetchGroupedStocksInfo(descriptors: imageDescriptors)
async let marketDataTaskResult = try fetchGroupedStocksInfo(descriptors: chartDescriptors)
let (taskResultsDict, marketDataTaskResultDict) = try await (taskResults, marketDataTaskResult)
}
and the stock descriptor is passed to
func fetchGroupedStocksInfo(descriptors: [Descriptor]) async throws -> [String: TaskResult] {
try await withThrowingTaskGroup(of: (String, TaskResult).self, returning: [String: TaskResult].self) { group in
for descriptor in descriptors {
group.addTask { [self] in
switch descriptor.type {
case .image:
let (_, image) = try await self.imageService.makeStockImageTuple(descriptor.stockImageUrlString)
let (symbol, marketResponse) = try await merketInfoSerice.fetchMarketInfo(descriptor.stockSymbol, numberOfDays: 3)
print(image)
return (descriptor.stockSymbol, TaskResult.image(image))
case .marketData:
let (symbol, marketResponse) = try await merketInfoSerice.fetchMarketInfo(descriptor.stockSymbol, numberOfDays: 3)
print(marketResponse.close)
return (symbol, TaskResult.marketData(marketResponse))
}
}
}
return try await group.reduce(into: [:]) {
$0[$1.0] = $1.1
}
}
}
and the errors found in compile time are
(63, 73) Reference to captured var 'imageDescriptors' in concurrently-executing code
(64, 82) Reference to captured var 'chartDescriptors' in concurrently-executing code
How can I make these error disappear, though I sense that my code can cause some problems at runtime.

Answer 1

You received this error:
Reference to captured var 'imageDescriptors' in concurrently-executing code
The issue is that you are supplying a mutable array to async let, an expression that will be await’ed later. As SE-0317 async let bindings says:
The right-hand side of a async let expression can be thought of as an implicit #Sendable closure ... the closure is #Sendable and nonisolated, meaning that it cannot access non-sendable state of the enclosing context.
Bottom line, the problem goes away if you replace it with an immutable array.
You can initialize them to constants directly:
let imageDescriptors = items.map { item in
Descriptor(
stockSymbol: item.key,
stockImageUrlString: nonEmptyImageUrlStringsList[item.key]!,
type: .image
)
}
let chartDescriptors = items.map { item in
Descriptor(
stockSymbol: item.key,
stockImageUrlString: "",
type: .marketData
)
}
You can make your own copy:
let descriptors = imageDescriptors
async let taskResults = try fetchGroupedStocksInfo(descriptors: descriptors)
Instead of async let, you can make your own Task with an explicit capture list:
let subtask1 = Task { [imageDescriptors] in
await self.fetchGroupedStocksInfo(descriptors: imageDescriptors)
}
let subtask2 = Task { [chartDescriptors] in
await try fetchGroupedStocksInfo(descriptors: chartDescriptors)
}
let (taskResultsDict, marketDataTaskResultDict) = try await (subtask1.value, subtask2.value)
NB: This opts out of structured concurrency.