Task cancellation in SwiftUI - ios

Question

I have come across a strange behavior (or at least one I don't understand) while trying to cancel a Task. Here is a minimal example: I have a Task that sleeps 30 seconds and then increment a counter.
However, if I call .cancel() on that Task before 30 seconds have passed then the counter is incremented immediately.
I would have expected that cancelling the Task would not increment the counter value; does anyone have an idea of what is going on here?
Thank you!
import SwiftUI
struct ContentView: View {
#State var task: Task<Void, Never>? = nil // reference to the task
#State var counter = 0
var body: some View {
VStack(spacing: 50) {
// display counter value and spawn the Task
Text("counter is \(self.counter)")
.onAppear {
self.task = Task {
try? await Task.sleep(nanoseconds: 30_000_000_000)
self.counter += 1
}
}
// cancel button
Button("cancel") {
self.task?.cancel() // <-- when tapped before 30s, counter value increases. Why?
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}

Answer 1

When a task is canceled an error is thrown but you are ignoring the thrown error by using try?
Here is a variant of your code that will react properly to the cancellation
self.task = Task {
do {
try await Task.sleep(nanoseconds: 30_000_000_000)
self.counter += 1
} catch is CancellationError {
print("Task was cancelled")
} catch {
print("ooops! \(error)")
}

Answer 2

How about adding a check for isCancelled like this:
Text("counter is \(self.counter)")
.onAppear {
print("onappear..")
self.task = Task {
try? await Task.sleep(nanoseconds: 30_000_000_000)
if !self.task!.isCancelled {
self.counter += 1
}
}
}

Answer 3

Placing the code inside a do catch block is enough.
self.task = Task {
do {
try await Task.sleep(nanoseconds: 3_000_000_000)
self.counter += 1
} catch {
print(error)
}
}

Answer 4

In SwiftUI it's best to use the .task modifier to use async/await, e.g.
#State var isStarted = false
...
Button(isStarted ? "Stop" : "Start") {
isStarted.toggle()
}
.task(id: isStarted) {
if !isStarted {
return
}
do {
try await Task.sleep(for: .seconds(3))
counter += 1
} catch {
print("Cancelled")
}
}