Timer Safety Patterns

Overview

Timer-related crashes are among the hardest to diagnose because they're often intermittent and the crash log points to GCD internals, not your code. Core principle: DispatchSourceTimer has a state machine — violating it causes deterministic EXC_BAD_INSTRUCTION crashes that look random. Timer (NSTimer) has a RunLoop mode trap that silently stops your timer during scrolling. Both are preventable with the patterns in this skill.

Example Prompts

• "My timer stops when the user scrolls"
• "EXC_BAD_INSTRUCTION crash in my timer code"
• "Should I use Timer or DispatchSourceTimer?"
• "How do I safely cancel a DispatchSourceTimer?"
• "My DispatchSourceTimer crashes on dealloc"
• "Timer keeps running after I dismiss the view controller"

Part 1: Timer vs DispatchSourceTimer Decision Tree