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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.