Main Thread Checker 🍎 iOS

Main Thread Checker: UI API called on a background thread: -[UIView setNeedsLayout]

UIKit was used from a background thread; UI must only be touched on the main thread.

Seen on: iOS

Meaning

Network/database callbacks run off the main queue. Updating labels, images or layout there causes this checker warning and random UI glitches or crashes.

Common causes

  • UI updates inside URLSession completion handlers
  • Updates from background Tasks/OperationQueues
  • Third-party SDK callbacks on background queues

⚡ Quick fix

  1. Wrap UI updates in DispatchQueue.main.async or MainActor
  2. Mark UI-facing classes @MainActor

Detailed fix by platform

Swift

  1. swift
    URLSession.shared.dataTask(with: url) { data, _, _ in
    DispatchQueue.main.async { self.imageView.image = data.flatMap(UIImage.init) }
}.resume()

How to diagnose

  1. Stack — Which call path?
  2. Queue — Callback queue

🔧 Still not fixed?

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