A modal dialog demands attention; while it is present, the user can do nothing other than work within it or dismiss it. You might need to put up a simple modal dialog in order to give the user some information or to ask the user how to proceed. Two UIView subclasses construct and present rudimentary modal dialogs:
A UIAlertView pops up unexpectedly with an elaborate animation and may be thought of as an attention-getting interruption. An alert is displayed in the center of the screen; it contains a title, a message, and some number of buttons, one of which may be the cancel button, meaning that it does nothing but dismiss the alert. Often there is only a cancel button, the primary purpose of the alert being to show the user the message (“You won the game”); the additional buttons may be used to give the user a choice of how to proceed (“You won the game; would you like to play another?” “Yes,” “No,” “Replay”).
Starting in iOS 5, an alert view may contain a text field, a password field, or both.
A UIActionSheet may be considered the iOS equivalent of a menu. An action sheet is displayed arising from the interface: on the iPhone, it slides up from the bottom of the screen; on the iPad, it is typically shown in a popover. It consists of some number of buttons (there can be a title, optionally, but there usually isn’t); one may be the cancel button, which appears last (though on the iPad, for a popover, this may not be needed), ...