Use UIButtonTypeSystem as default button appearance on iOS


The plain UIButtonTypeSystem (first button in the image) is the default button type on iOS. Displaying background is a customization option and rounding corners is deprecated.

> In iOS 7, the rounded rect button type has been deprecated in favor of the system button, UIButtonTypeSystem.
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> Button objects can be specified as one of five standard types—system, detail disclosure, info light, info dark, and add contact. The detail disclosure, info, and add contact button types are supplied with standard image graphics to indicate their purpose to the user. These images cannot be customized.

https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/UIKitUICatalog/UIButton.html

> Embrace borderless buttons. By default, all bar buttons are borderless. In content areas, a borderless button uses context, color, and a call-to-action title to indicate interactivity. And when it makes sense, a content-area button can display a thin border or tinted background that makes it distinctive.

https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/