- (void)viewDidLoad {
 [super viewDidLoad];
 // Do any additional setup after loading the view, typically from a nib.
 UIAlertView* alert = [[UIAlertView alloc]initWithTitle:nil message:@"此信息３秒后消失" delegate:nil cancelButtonTitle:nil otherButtonTitles:nil, nil];
 [alert show];
 [self performSelector:@selector(dismissAlert:) withObject:alert afterDelay:3.0];
}
- (void)dismissAlert:(UIAlertView*)alert {
 if ( alert.visible ) {
  [alert dismissWithClickedButtonIndex:alert.cancelButtonIndex animated:YES];
 }
}

UIAlertView baseAlert = nil;
- (void) performDismiss: (NSTimer *)timer
{
 [baseAlert dismissWithClickedButtonIndex:0 animated:NO];//important
 [baseAlert release];
 baseAlert = NULL;
}  
- (void) presentSheet
{
 baseAlert = [[UIAlertView alloc] 
        initWithTitle:@"Alert" message:@"\nMessage Message Message " 
        delegate:self cancelButtonTitle:nil
        otherButtonTitles: nil];
 [NSTimer scheduledTimerWithTimeInterval:2.0f target:self selector: @selector(performDismiss:)
         userInfo:nil repeats:NO];
 [baseAlert show];
}

- (void) dimissAlert:(UIAlertView *)alert
{
 if(alert)
 {
  [alert dismissWithClickedButtonIndex:[alert cancelButtonIndex] animated:YES];
  [alert release];
 }
}
- (void)showAlert{   
 UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"title" message:@"message" delegate:nil 
cancelButtonTitle:nil otherButtonTitles:nil];
 [alert show];
 [self performSelector:@selector(dimissAlert:) withObject:alert afterDelay:2.0];
}