withTaskGroup(of:returning:isolation:body:)

A group waits for all of its child tasks to complete or be canceled before it returns. After this function returns, the task group is always empty.

To collect the results of the group’s child tasks, you can use a for-await-in loop:

var sum = 0
for await result in group {
    sum += result
}

If you need more control or only a few results, you can call next() directly:

guard let first = await group.next() else {
    group.cancelAll()
    return 0
}
let second = await group.next() ?? 0
group.cancelAll()
return first + second

Task Group Cancellation

You can cancel a task group and all of its child tasks by calling the cancelAll() method on the task group, or by canceling the task in which the group is running.

If you call addTask(priority:operation:) to create a new task in a canceled group, that task is immediately canceled after creation. Alternatively, you can call addTaskUnlessCancelled(priority:operation:), which doesn’t create the task if the group has already been canceled Choosing between these two functions lets you control how to react to cancellation within a group: some child tasks need to run regardless of cancellation, but other tasks are better not even being created when you know they can’t produce useful results.

Because the tasks you add to a group with this method are nonthrowing, those tasks can’t respond to cancellation by throwing CancellationError. The tasks must handle cancellation in some other way, such as returning the work completed so far, returning an empty result, or returning nil. For tasks that need to handle cancellation by throwing an error, use the withThrowingTaskGroup(of:returning:body:) method instead.