Modern Concurrency: Getting Started

Oct 18 2022 Swift 5.5, iOS 15, Xcode 13.4

Part 2: Asynchronous Sequences

14. Canceling Tasks

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Previous episode: 13. Downloading Chunks Next episode: 15. Using Combine

Xcode 14 beta detects a race condition for stopDownloads when you tap Cancel Now while using the Cloud 9 plan. Move stopDownloads and reset() to @MainActor and await the variable in the private downloadWithProgress method:

while await !stopDownloads, !accumulator.checkCompleted() {
...
if await stopDownloads, !Self.supportsPartialDownloads {
...
@MainActor var stopDownloads = false
...
@MainActor func reset() {

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Canceling Tasks

Why is it important to cancel tasks in a timely manner? Try this:

@State var downloadTask: Task<Void, Error>?

downloadTask = 🟥Task {

downloadTask?.cancel()

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What if you want to cancel a download task without leaving DownloadView? Look in SuperStorageModel.

TaskLocal

Say hello to the TaskLocal property wrapper!

@TaskLocal static var supportsPartialDownloads = false

try await SuperStorageModel {
    fileData = try await model.downloadWithProgress(file: file)
}

try await SuperStorageModel
  .$supportsPartialDownloads
  .withValue(file.name.hasSuffix(".jpeg")) {
    fileData = try await model.downloadWithProgress(file: file)
  }

Button(action: { 🟩model.stopDownloads = true

if stopDownloads, !Self.supportsPartialDownloads {
  throw CancellationError()
}