UIAlertViewDelegate

@protocol UIAlertViewDelegate <NSObject>
@optional

// Called when a button is clicked. The view will be automatically dismissed after this call returns
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex;

// Called when we cancel a view (eg. the user clicks the Home button). This is not called when the user clicks the cancel button.
// If not defined in the delegate, we simulate a click in the cancel button
- (void)alertViewCancel:(UIAlertView *)alertView;

- (void)willPresentAlertView:(UIAlertView *)alertView;  // before animation and showing view
- (void)didPresentAlertView:(UIAlertView *)alertView;  // after animation

- (void)alertView:(UIAlertView *)alertView willDismissWithButtonIndex:(NSInteger)buttonIndex; // before animation and hiding view
- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex;  // after animation

// Called after edits in any of the default fields added by the style
- (BOOL)alertViewShouldEnableFirstOtherButton:(UIAlertView *)alertView;

@end

self.usageAlertView = [[UIAlertView alloc] initWithTitle:@"Usage" 
                                                 message:message 
                                                delegate:self 
                                       cancelButtonTitle:@"Launch Safari" 
                                       otherButtonTitles:nil];

如果需要处理UIAlertView对象所触法的所有事件，设置成 delegate：self，否则设置呈delegate：nil