Snapchat·Software Engineer·Online Assessment (OA)·Intermediate

Intermediate

May 2026Remote

Summary

Snapchat iOS interview on HackerRank, pure Swift and GCD concurrency stuff, no UI questions at all. The interviewer literally writes code in front of you and asks what it prints and where it runs. Niche format but kind of refreshing compared to the usual leetcode grind.

Questions Asked (1)

Given this Swift snippet using a serial DispatchQueue with one async block and one sync block followed by a print on the calling thread, predict the output and identify which thread each print statement runs on.

Technical Trade-offsSystem Design

Author's notes

The async block goes first so print(1) gets dispatched and starts running on the queue's worker thread while the main thread moves on.

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AI HintsAI Generated

Suggested Approach

First, clarify the queue's serial nature and the difference between async and sync dispatch. Then trace the execution order: the async block is enqueued and runs later on the queue's thread, while the sync block blocks the calling thread until the queue executes it, so the sync block runs before the final print. Finally, map each print to its thread: async block on the queue's thread, sync block on the queue's thread (but called from the calling thread), and the final print on the calling thread.

Pro tip: Emphasize that sync dispatch does not cause a deadlock here because the calling thread is not the queue's thread; if it were, it would deadlock. This shows you understand the underlying threading model.

1. Identify queue type and dispatch methods

Recognize that the queue is serial, meaning tasks execute one at a time in FIFO order. Note that async returns immediately, while sync blocks the calling thread until the task completes.

2. Trace execution order

The async block is enqueued and will run when the queue is free. The sync block is submitted next; since the queue is serial and currently idle, it executes the sync block immediately, blocking the calling thread until done. The final print runs after sync returns.

3. Determine thread for each print

The async block runs on the queue's associated thread (often a background thread). The sync block also runs on the queue's thread, but it is invoked from the calling thread. The final print runs on the calling thread (likely the main thread).

4. Predict output order

The sync block's print occurs before the final print because sync blocks. The async block's print may occur before or after the final print depending on timing, but typically after the sync block and possibly after the final print.

5. Address potential pitfalls

Mention that if the sync block were dispatched to the same queue from within that queue, it would deadlock. Also note that the async block's execution time is non-deterministic relative to the calling thread's final print.

Key Points to Mention

Serial queue guarantees FIFO order and one task at a time.
async returns immediately; sync blocks the calling thread until the task finishes.
The sync block runs on the queue's thread, not the calling thread, but the calling thread waits.
The final print runs on the calling thread after sync returns.
The async block runs on the queue's thread, but its timing relative to the final print is non-deterministic.
No deadlock occurs because the calling thread is not the queue's thread; deadlock would happen if sync were called from the queue itself.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.