My custom UI elements are not being updated while UIScrollView is scrolled

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While scrolling, the main thread’s run loop is in UITrackingRunLoopMode. So what you need to do is schedule your timer in that mode (possibly in addition to the default mode). I believe NSRunLoopCommonModes includes both the default and event tracking modes, so you can just do this:

NSTimer *timer = [NSTimer timerWithTimeInterval:0.42 target:foo selector:@selector(doSomething) userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];

By the way, you should do this all on the main thread. No need to spawn a background thread (unless each timer invocation is going to do some lengthy processing).

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