多线程中UIAlertView无法消失的问题

 1 /** 定义按钮 */
 2 -(void)setupAllBtns{  
 3     UIButton *btn2 = [UIButton buttonWithType:UIButtonTypeRoundedRect];
 4     btn2.frame = CGRectMake(100, 140, 100, 50);
 5     [btn2 setTitle:@"读取信息" forState:UIControlStateNormal];
 6     [btn2 setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
 7     btn2.backgroundColor = [UIColor brownColor];
 8     [btn2 addTarget:self action:@selector(readSimWithNewThread) forControlEvents:UIControlEventTouchUpInside];
 9     [self.view addSubview:btn2];
10 }
11 
12 -(void)readSimWithNewThread{
13     NSThread *button3Thread = [[NSThread alloc] initWithTarget:self selector:@selector(readInfo) object:nil];
14     [button3Thread setName:@"thread-3"];
15     [button3Thread start];
16 }
17 
18 /** 新线程读取信息 */
19 -(void)readInfo{
20     [[NSOperationQueue mainQueue] addOperationWithBlock:^
21      {
22          self.mAlertView = [[UIAlertView alloc] initWithTitle:nil message:@"开始读取，请稍候..." delegate:self cancelButtonTitle:nil otherButtonTitles:nil, nil];
23          [self.mAlertView show];
24      }];
25     
26     char sn[64] = {0};
27     int ret = [self.sim GetInfo:sn];
28     
29     //alert消失的方法
30     if (self.mAlertView.visible) {
31         [self.mAlertView dismissWithClickedButtonIndex:0 animated:YES];
32     }
33     
34     NSString *msg = @"";
35     msg=[NSString stringWithFormat:@"MobileNum=%@",[NSString stringWithUTF8String:sn]];
36 }

[NSThreadsleepForTimeInterval:0.5]; 

[self performSelector:@selector(dimissAlert) withObject:alert afterDelay:2.0];
//将其添加在[self.mAlertView show]之后

-(void)dimissAlert{
    if (alert.visible) {
        [alert dismissWithClickedButtonIndex:0 animated:YES ];
    }
}