Understanding CADisplayLink, NSTimer, and RunLoop in iOS: Implementation, Issues, and Solutions

The article provides a comprehensive overview of iOS timing mechanisms, focusing on CADisplayLink and NSTimer , their properties such as timestamp , duration , preferredFramesPerSecond , and how they interact with the system's display refresh cycle.

It demonstrates how to create, configure, and use these timers in a view controller, including code snippets for initializing a CADisplayLink , adding it to a run loop, handling pause/resume, and calculating actual frame rates.

The article also discusses common problems like retain cycles when timers strongly reference view controllers, and presents a proxy‑based solution using an NSProxy subclass to break the strong reference chain.

Further, it delves into the RunLoop architecture, explaining the roles of CFRunLoopSourceRef , CFRunLoopTimerRef , and CFRunLoopObserverRef , the concept of RunLoop modes (e.g., kCFRunLoopDefaultMode , UITrackingRunLoopMode ), and how timers and display links are scheduled within these modes.

Differences between iOS 14 and iOS 15 implementations of CADisplayLink are examined, showing the shift from a VSync‑driven source to a UIKit‑driven source and the impact on callback behavior.

Finally, the article offers a practical fix for monitoring dynamic refresh rates on ProMotion devices by reading the duration property and adjusting preferredFrameRateRange accordingly, along with references to official documentation and further reading.