So your client or designer has asked you for this crazy non-standard control. What should you do now? On the one hand, a unique UI can help your app stand out from the crowd. On the other hand, would this new idea really improve the user experience? Is it something that's even feasible within your current timeframe / budget?
Know when to say "yes" and when to push back. Know what options are available for customizing the appearance of standard controls in different iOS versions, so that you can make an informed decision about when a customized standard control may be a better choice than a true custom control. And when you do say "yes" to a custom control, know the best way to design and implement the requested control.

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