一，效果图。

二，代码。

RootViewController.h
#import <UIKit/UIKit.h>
@interface RootViewController : UIViewController
<UIAlertViewDelegate>
@end
RootViewController.m
- (void)viewDidLoad
{
 [super viewDidLoad];
 // Do any additional setup after loading the view.
 //alert
 UIAlertView *alert=[[UIAlertView alloc]initWithTitle:nil message:@"一个可以自动关闭的Alert窗口" delegate:self cancelButtonTitle:nil otherButtonTitles:nil, nil];
 [alert show];
 //显示框
 UIActivityIndicatorView *indicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
 indicator.backgroundColor=[UIColor redColor];
 indicator.center = CGPointMake(alert.bounds.size.width/2, alert.bounds.size.height-40.0);
 [indicator startAnimating];
 [alert insertSubview:indicator atIndex:0];
 //定时器
 [NSTimer scheduledTimerWithTimeInterval:3.0f target:self selector:@selector(dismissAlert:) userInfo:[NSDictionary dictionaryWithObjectsAndKeys:alert, @"alert", @"testing ", @"key" ,nil] repeats:NO];
}
//alert 自动消失
-(void) dismissAlert:(NSTimer *)timer{
 UIAlertView *alert = [[timer userInfo] objectForKey:@"alert"];
 [alert dismissWithClickedButtonIndex:0 animated:YES];
}