Task groups for parallel execution in Swift - Step-by-Step Execution

Concept Flow - Task groups for parallel execution

Start Task Group

↓

Add Tasks to Group

↓

Run Tasks in Parallel

↓

Wait for All Tasks to Finish

↓

Collect Results

↓

End Task Group

This flow shows how tasks are added to a group, run at the same time, and then results are collected after all finish.

Execution Sample

Swift

func fetchData() async {
  await withTaskGroup(of: Int.self) { group in
    group.addTask { await fetchNumber(1) }
    group.addTask { await fetchNumber(2) }

    var sum = 0
    for await number in group {
      sum += number
    }
    print("Sum: \(sum)")
  }
}

This code runs two tasks in parallel that fetch numbers, then sums and prints the result.

Execution Table

Step	Action	Task Group State	Task Result	Output
1	Start task group	Empty group	-	-
2	Add task 1 (fetchNumber(1))	Group with 1 task	-	-
3	Add task 2 (fetchNumber(2))	Group with 2 tasks	-	-
4	Run tasks in parallel	Running 2 tasks	-	-
5	Task 1 completes	1 task done, 1 running	1	-
6	Task 2 completes	All tasks done	2	-
7	Sum results (1 + 2)	All tasks done	-	Sum: 3
8	End task group	Group closed	-	-

💡 All tasks finished and results summed, task group ends.

Variable Tracker

Variable	Start	After Step 5	After Step 6	After Step 7	Final
sum	0	1	3	3	3
group state	empty	1 task done	all done	all done	closed

Key Moments - 3 Insights

Why do tasks run at the same time instead of one after another?

Because tasks are added to a task group which runs them concurrently, as shown in steps 4 to 6 in the execution table.

How does the program know when all tasks are finished?

The for-await loop waits for each task result until all tasks complete, indicated by step 6 where all tasks are done.

Why is the sum variable updated inside the for-await loop?

Because each task result arrives asynchronously, the loop adds each number as it comes, shown in steps 5 and 6 updating sum.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'sum' after step 5?

A 0

B 1

C 2

D 3

💡 Hint

Check the variable_tracker row for 'sum' after step 5.

At which step does the task group have all tasks done?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at the 'Task Group State' column in the execution_table.

If we add a third task returning 3, what would be the final sum after step 7?

A 6

B 5

C 3

D 4

💡 Hint

Sum is the total of all task results; adding 1 + 2 + 3 = 6.

Concept Snapshot

Task groups let you run many tasks at once.
Add tasks to the group with addTask.
Tasks run in parallel.
Use for-await to get results as they finish.
Waits until all tasks complete before continuing.

Full Transcript

This visual trace shows how Swift's task groups work for parallel execution. First, a task group starts empty. Then tasks are added to it. The group runs all tasks at the same time. As each task finishes, its result is collected. The sum variable adds each result inside a for-await loop. When all tasks finish, the final sum is printed. The task group then ends. This method helps run multiple jobs simultaneously and wait for all to complete before moving on.