Why Dart Isolates Can Slow Your Flutter App If Used Incorrectly

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A Flutter developer discovered that parsing an 18 MB JSON file on the main thread caused a 900ms UI freeze on a mid-range Android device, prompting a deeper look at Dart isolates. The key distinction is between I/O-bound tasks, which are best handled with async/await on the main isolate, and CPU-bound tasks like JSON parsing or image manipulation, which genuinely require isolates to avoid dropped frames. Dart's event loop efficiently manages async waiting, but it cannot yield during a tight synchronous loop, meaning no amount of async/await will unblock the UI thread for pure computation. Common tools like compute and Isolate.run exist for offloading CPU-heavy work, but misapplying them to I/O-bound tasks adds latency without benefit. The author warns that deferring work with Future() does not offload it to another thread and that many perceived concurrency problems are simply cases of using the wrong async API.

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ProgrammingDEV Community ·

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ProgrammingDEV Community ·

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