Understanding Hangs in Your App (Summary)

Context: Apple guidance on identifying hangs caused by long-running main-thread work and understanding the main run loop.

Key concepts

• A hang is a noticeable delay in a discrete interaction (typically >100 ms).
• Hangs almost always come from long-running work on the main thread.
• The main run loop processes UI events, timers, and main-queue work sequentially.

Main-thread work stages

• Event delivery to the correct view/handler.
• Your code: state updates, data fetch, UI changes.
• Core Animation commit to the render server.

Why the main run loop matters

• Only the main thread can update UI safely.
• The run loop is the foundation that executes main-queue work.
• If the run loop is busy, it can’t handle new events; this causes hangs.

Diagnosing hangs

• Observe the main run loop’s busy periods: healthy loops sleep most of the time.
• Hang detection typically flags busy periods >250 ms.
• The Hangs instrument can be configured to lower thresholds.

Practical takeaways

• Keep main-thread work short; offload heavy work from event handlers.
• Avoid long-running tasks on the main dispatch queue or main actor.
• Use run loop behavior as a proxy for user-perceived responsiveness.