[TLDR] Using Task Local Value in Swift

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• Declare a mutable static property on a type. Annotate the property with the @TaskLocal property wrapper

• Inside of a task, set the property by accessing its projectedValue and calling the withValue method. All code and subtasks created within the closure will have the value of the static property set to whatever was passed as the first argument of the withValue method.

extension Logger {
    @TaskLocal static var traceId: String?
}

let trace1 = "Trace1"
let trace2 = "Trace2"

print(trace1)
// Trace1
print(trace2)
// Trace2
print("Outside of tasks: \(Logger.traceId)")
// nil

Task {
    await Logger.$traceId.withValue(trace1) {
        print("\(Logger.traceId ?? ""): Doing some work for user with ID")
        // Trace1: Doing some work for user with ID
        try? await Task.sleep(for: .seconds(1))
        print("\(Logger.traceId ?? ""): Finished some work for user with ID")
        // Trace1: Finished some work for user with ID
    }
}

Task {
    await Logger.$traceId.withValue(trace2) {
        print("\(Logger.traceId ?? ""): Doing some work for user with ID")
        // Trace2: Doing some work for user with ID
        try? await Task.sleep(for: .seconds(2))
        print("\(Logger.traceId ?? ""): Finished some work for user with ID")
        // Trace2: Finished some work for user with ID
    }
}