Programming the iPhone User Experience

Sleight of Hand

The layout of Cocoa Touch user interfaces can have a big impact on usability. This is especially true in regards to user interface controls. A control is any interface element that causes the application to do something when a user touches the screen. Standard controls include buttons, sliders, and switches. Table cells can act as controls, as can entire views.

The Multi-Touch interface on the iPhone and iPod Touch requires that applications implement virtual controls. Unlike physical knobs or buttons, virtual controls provide no tactile (or haptic) feedback. A great example of tactile feedback on a hardware control is the raised bumps on the F and J keys of a standard English QWERTY keyboard. These bumps indicate to users that their fingers are in the proper place on the keyboard. On a touchscreen, muscle memory is built with repeated use, but orientation is based purely on the portion of the screen a user is touching.

The sleight of hand anti-pattern occurs when developers change the perceived meaning of a touch-sized area of the screen by placing controls that perform different actions in the same place across views.

Apple provides a good example of the sleight of hand anti-pattern. The iPhone displays one of two screens in response to an incoming call. In both scenarios, the screen shows the name of the caller and, if available, a photograph assigned to the incoming phone number. If the iPhone is locked, users see a large slider labeled “slide to answer” and a slider ...