IOS AlertView 自动消失

UIAlertView *alert;
- (IBAction)Open:(id)sender {
    alert = [[UIAlertView alloc]
                          initWithTitle:@"msg" message:@"sss" delegate:self cancelButtonTitle:@"取消" otherButtonTitles:@"确定", nil];
    [NSTimer scheduledTimerWithTimeInterval:5.0f target:self selector:@selector(performDismiss:) userInfo:nil repeats:NO];
    [alert show];
}
-(void)performDismiss:(NSTimer*)timer
{
    [alert dismissWithClickedButtonIndex:0 animated:NO];
}