iPhone Open Application Development

Alert Sheets

The iPhone is a relatively small device with limited screen space and no stylus. This means users are going to fumble with their fingers and tap buttons on accident. When this happens, a well-written application prompts the user for confirmation before just blowing away important data. On a desktop computer, applications pop up windows when they need attention. On the iPhone, an alert sheet is slid up from the bottom, graying out the rest of the screen until the user chooses an option. The term "sheet" continues the page metaphor that Apple uses for the iPhone.

Creating an Alert Sheet

The alert sheet is an object that can be instantiated on top of any view, and a single view can host a number of different alert sheets. A basic alert sheet consists of a title, body text, and whatever choices the user should be presented with. It is instantiated in the same way as other UIView objects, using an initWithFrame method. Because alert sheets appear at the bottom, the window's origin can begin halfway down the screen (Y = 240).

UIAlertSheet *alertSheet = [ [ UIAlertSheet alloc ]
    initWithFrame: CGRectMake(0, 240, 320, 240) ];
[ alertSheet setTitle:@"Computer doesn't like people" ];
[ alertSheet setBodyText: [ NSString stringWithFormat:
    @"I did not complete your request because I don't like humans." ]
];
[ alertSheet setDelegate: self ];

Alert sheets can stretch their frame to accommodate whatever elements it's required to hold, so a static 320×240 rectangle should be sufficient ...