1# Understanding Hangs in Your App (Summary)
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3Context: Apple guidance on identifying hangs caused by long-running main-thread work and understanding the main run loop.
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5## Key concepts
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7- A hang is a noticeable delay in a discrete interaction (typically >100 ms).
8- Hangs almost always come from long-running work on the main thread.
9- The main run loop processes UI events, timers, and main-queue work sequentially.
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11## Main-thread work stages
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13- Event delivery to the correct view/handler.
14- Your code: state updates, data fetch, UI changes.
15- Core Animation commit to the render server.
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17## Why the main run loop matters
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19- Only the main thread can update UI safely.
20- The run loop is the foundation that executes main-queue work.
21- If the run loop is busy, it can’t handle new events; this causes hangs.
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23## Diagnosing hangs
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25- Observe the main run loop’s busy periods: healthy loops sleep most of the time.
26- Hang detection typically flags busy periods >250 ms.
27- The Hangs instrument can be configured to lower thresholds.
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29## Practical takeaways
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31- Keep main-thread work short; offload heavy work from event handlers.
32- Avoid long-running tasks on the main dispatch queue or main actor.
33- Use run loop behavior as a proxy for user-perceived responsiveness.
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