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The upcoming Apple iPhone Duo, set for release on October 23, has developers contemplating a new design challenge: folding displays. Atharva Deosthale, an Appwrite developer advocate, emphasizes that folding and unfolding is a normal part of device use, meaning apps must adapt to preserve state and respond cleanly when the display changes.

Apple's new smartphone boasts two screens: a 5.4-inch outer display atop the case and a 7.6-inch inner display within. Upon opening or closing the device, the app seamlessly transitions to the visible screen. Apple has published extensive documentation to guide developers in preparing for this new form factor.

The iPhone Duo runs iOS 27.1, but will run unaltered iPhone apps in a manner similar to iPad apps. Developers need to learn new techniques to optimize their apps for the foldable form factor, as the user may interact with the app in various postures, including "tabletop posture" or "book posture."

The foldable display allows for smooth transitions, with both screens featuring a 120 Hz adaptive refresh rate thanks to Apple's ProMotion technology. Apps must run smoothly and maintain state during these transitions. The device also supports running two apps simultaneously through "Split View."

Apple is updating its Xcode IDE to accommodate this new topology, and developers will need to adjust their app layouts and behavior to ensure a consistent experience when the display changes. Some challenges include apps with fixed layouts, hardcoded dimensions, or view controllers that reload on size change, which can appear stretched. Developers should also ensure that user data and session identity are preserved when switching screens.