Async sequences in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an async sequence using AsyncStream.

iOS Swift

let numbers = AsyncStream<Int> { continuation in
    continuation.yield(1)
    continuation.yield(2)
    continuation.finish()
}

Task: Use a for await loop to iterate over [1].

Drag options to blanks, or click blank then click option'

Anumbers

BArray

CDispatchQueue

DTask

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to iterate over Array with for await (Array is not async sequence).

Using DispatchQueue or Task instead of the async sequence variable.

The variable numbers is the async sequence created with AsyncStream. You use for await to iterate over it.

2fill in blank

medium

Complete the async function to return an AsyncStream of integers from 1 to 3.

iOS Swift

func makeNumbers() -> AsyncStream<Int> {
    AsyncStream { continuation in
        for i in 1...3 {
            continuation.[1](i)
        }
        continuation.finish()
    }
}

Drag options to blanks, or click blank then click option'

Asend

Byield

Cemit

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using send, emit, or push which are not valid methods for AsyncStream.

The method to send values to an AsyncStream is yield.

3fill in blank

hard

Fix the error in the async sequence iteration by completing the missing keyword.

iOS Swift

func printNumbers() async {
    let stream = AsyncStream<Int> { continuation in
        continuation.yield(10)
        continuation.finish()
    }

    for [1] number in stream {
        print(number)
    }
}

Drag options to blanks, or click blank then click option'

Alet

Basync

Cawait

Dyield

Attempts:

3 left

💡 Hint

Common Mistakes

Using for async or just for which causes errors.

When iterating over an async sequence, you must use for await to wait for each value asynchronously.

4fill in blank

hard

Fill both blanks to create an AsyncStream that yields strings and finishes properly.

iOS Swift

let greetings = AsyncStream<String> { [1] in
    [2].yield("Hello")
    [2].finish()
}

Drag options to blanks, or click blank then click option'

Acontinuation

Bstream

Ctask

Dsequence

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and calls causing errors.

The closure parameter is usually named continuation. You call yield and finish on it.

5fill in blank

hard

Fill all three blanks to create a filtered async sequence of integers greater than 5.

iOS Swift

let filtered = AsyncStream<Int> { continuation in
    for i in 1...10 {
        if i [1] 5 {
            continuation.[2](i)
        }
    }
    continuation.[3]()
}

Drag options to blanks, or click blank then click option'

A>

Byield

Cfinish

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < or wrong method names causing logic or syntax errors.

The condition checks if i > 5. Then yield sends the value, and finish ends the stream.

Practice

1. What is the main purpose of using async sequences in Swift?

easy

A. To perform synchronous loops faster

B. To create static arrays of data

C. To handle data that arrives over time asynchronously

D. To store data permanently on disk

Solution

1. Step 1: Understand async sequences concept

   Async sequences allow handling data that comes in over time, not all at once.

2. Step 2: Compare options with async sequence use

   Only To handle data that arrives over time asynchronously describes handling data arriving asynchronously, which matches async sequences.

3. Final Answer:

   To handle data that arrives over time asynchronously -> Option C

4. Quick Check:

   Async sequences = handle data over time [OK]

Hint: Async sequences handle data arriving over time [OK]

Common Mistakes:

• Thinking async sequences create static data
• Confusing async sequences with synchronous loops
• Assuming async sequences store data permanently

2. Which of the following is the correct syntax to iterate over an async sequence named stream?

easy

A. for value in stream.await { print(value) }

B. for value in await stream { print(value) }

C. await for value in stream { print(value) }

D. for await value in stream { print(value) }

Solution

1. Step 1: Recall async sequence iteration syntax

   Swift uses for await to loop over async sequences.

2. Step 2: Match syntax with options

   Only for await value in stream { print(value) } uses for await value in stream, which is correct syntax.

3. Final Answer:

   for await value in stream { print(value) } -> Option D

4. Quick Check:

   Use 'for await' to loop async sequences [OK]

Hint: Use 'for await' before the loop variable [OK]

Common Mistakes:

• Placing 'await' after 'for' incorrectly
• Using 'await' inside the loop variable
• Trying to access '.await' property on sequence

3. Given the async sequence numbers that yields 1, 2, 3, what will this code print?

for await num in numbers {
  print(num * 2)
}

medium

A. 2 4 6

B. 1 2 3

C. 0 2 4

D. Error: Cannot multiply async sequence

Solution

1. Step 1: Understand the async sequence values

   The sequence yields 1, 2, and 3 asynchronously.

2. Step 2: Calculate output inside loop

   Each number is multiplied by 2 and printed: 2, 4, 6.

3. Final Answer:

   2 4 6 -> Option A

4. Quick Check:

   Multiply each yielded value by 2 [OK]

Hint: Multiply each async value inside the loop [OK]

Common Mistakes:

• Printing original values without multiplication
• Expecting synchronous output order
• Thinking async sequences cause errors here

4. What is wrong with this code snippet?

for num in numbers {
  print(num)
}

Assuming numbers is an async sequence.

medium

A. Should use 'while' loop instead of 'for'

B. Missing 'await' keyword before the loop variable

C. Async sequences cannot be iterated

D. Missing 'async' keyword in function declaration

Solution

1. Step 1: Identify iteration syntax for async sequences

   Async sequences require 'for await' to iterate properly.

2. Step 2: Check code snippet

   The code uses 'for num in numbers' without 'await', which is incorrect.

3. Final Answer:

   Missing 'await' keyword before the loop variable -> Option B

4. Quick Check:

   Async sequence loops need 'for await' [OK]

Hint: Always use 'for await' with async sequences [OK]

Common Mistakes:

• Omitting 'await' in the loop
• Thinking async sequences can't be iterated
• Confusing async function declaration with loop syntax

5. You want to collect the first 3 values from an async sequence updates into an array. Which code correctly does this?

hard

A. var results = []
for await update in updates {
results.append(update)
if results.count == 3 { break }
}

B. var results = []
for update in updates {
results.append(update)
if results.count == 3 { break }
}

C. var results = []
while let update = await updates.next() {
results.append(update)
if results.count == 3 { break }
}

D. var results = []
updates.forEach { update in
results.append(update)
if results.count == 3 { break }
}

Solution

1. Step 1: Understand async sequence iteration and breaking

   Use 'for await' to iterate async sequences and break when count reaches 3.

2. Step 2: Evaluate each option

   var results = [] for await update in updates { results.append(update) if results.count == 3 { break } } uses 'for await' with break after 3 items, correct. var results = [] for update in updates { results.append(update) if results.count == 3 { break } } lacks 'await'. var results = [] while let update = await updates.next() { results.append(update) if results.count == 3 { break } } uses invalid syntax for async sequences. var results = [] updates.forEach { update in results.append(update) if results.count == 3 { break } } uses 'forEach' which doesn't support async iteration or breaking.

3. Final Answer:

   var results = [] for await update in updates { results.append(update) if results.count == 3 { break } } -> Option A

4. Quick Check:

   Use 'for await' and break after 3 items [OK]

Hint: Use 'for await' and break after collecting 3 items [OK]

Common Mistakes:

• Omitting 'await' in the loop
• Using synchronous loops on async sequences
• Trying to use 'forEach' with async sequences