Main Thread Checker Violation

iOS

TL;DR

Main Thread Checker Violation indicates a concurrency failure on iOS that interrupts the current user flow.

Symptoms

Main Thread Checker Violation appears in crash logs
The app exits or the UI flow stops

Common Causes

Shared state is mutated without coordination
Object is used after lifecycle termination

How to Verify

Reproduce with deterministic input and capture full logsInspect the first throw site and recent changes around it

Quick Fixes

Add guard checks and a safe fallback path
Disable or rollback the risky code path behind a flag

Proper Fixes

Enforce contracts with types, runtime checks, and tests
Refactor ownership and lifecycle boundaries to remove the failure mode

Example Stack Trace

Main Thread Checker Violation in ProfileViewController.swift:42

Related Errors

IllegalStateException Unhandled Promise Rejection Hydration Failed