Async sequences (AsyncSequence) in Swift - Step-by-Step Execution

Concept Flow - Async sequences (AsyncSequence)

Start AsyncSequence

↓

Call async iterator

↓

Wait for next element asynchronously

↓

Receive element or nil

↓

Process element

↓

Repeat wait for next element

This flow shows how an async sequence produces elements one by one, waiting asynchronously for each before continuing or ending.

Execution Sample

Swift

struct Counter: AsyncSequence {
  typealias Element = Int
  struct AsyncIterator: AsyncIteratorProtocol {
    var count = 0
    mutating func next() async -> Int? {
      guard count < 3 else { return nil }
      defer { count += 1 }
      return count
    }
  }
  func makeAsyncIterator() -> AsyncIterator { AsyncIterator() }
}

let counter = Counter()
for await num in counter {
  print(num)
}

This code defines an async sequence that counts from 0 to 2 asynchronously, printing each number.

Execution Table

Step	Action	count value	next() returns	Output
1	Call next(), count=0 < 3	0	0	Print 0
2	Call next(), count=1 < 3	1	1	Print 1
3	Call next(), count=2 < 3	2	2	Print 2
4	Call next(), count=3 !< 3	3	nil	End loop

💡 next() returns nil when count reaches 3, ending the async sequence.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
count	0	1	2	3	3

Key Moments - 3 Insights

Why does next() return nil at step 4?

Because the count reached 3, which fails the guard condition count < 3, so next() returns nil to signal the end of the sequence (see execution_table step 4).

Why is count incremented after returning the value?

The defer statement increments count after returning the current value, so the returned number is the old count before increment (see execution_table steps 1-3).

How does the for-await loop know when to stop?

It stops when next() returns nil, which means no more elements are available (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of count when next() returns 1?

A 0

B 1

C 2

D 3

💡 Hint

Check execution_table row 2 under 'count value' and 'next() returns'.

At which step does the async sequence end?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look for when next() returns nil in execution_table.

If we change the guard to count < 5, how many times will next() return a number?

A 5 times

B 4 times

C 3 times

D 6 times

💡 Hint

The sequence returns numbers while count < 5, so count values 0 to 4 (see variable_tracker).

Concept Snapshot

AsyncSequence lets you produce values asynchronously one by one.
Implement next() async -> Element? to yield values or nil to end.
Use for await to loop over async sequences.
next() suspends until the next value is ready.
Defer can update state after returning a value.

Full Transcript

This visual execution trace shows how an AsyncSequence in Swift works step-by-step. The Counter struct produces numbers 0, 1, and 2 asynchronously. Each call to next() checks if count is less than 3. If yes, it returns the current count and then increments it. When count reaches 3, next() returns nil, signaling the end of the sequence. The for-await loop prints each number as it arrives and stops when nil is returned. The variable tracker shows how count changes after each next() call. Key moments clarify why next() returns nil and how defer affects count. The quiz tests understanding of count values, sequence end, and changing the guard condition.