多线程中UIAlertView无法消失的问题

场景是这样的，点击按钮，开辟新线程，弹出alertView。然后调用dismissWithClickedButtonIndex让其消失。

/** 定义按钮 */
-(void)setupAllBtns{  
    UIButton *btn2 = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    btn2.frame = CGRectMake(100, 140, 100, 50);
    [btn2 setTitle:@"读取信息" forState:UIControlStateNormal];
    [btn2 setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
    btn2.backgroundColor = [UIColor brownColor];
    [btn2 addTarget:self action:@selector(readSimWithNewThread) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:btn2];
}

-(void)readSimWithNewThread{
    NSThread *button3Thread = [[NSThread alloc] initWithTarget:self selector:@selector(readInfo) object:nil];
    [button3Thread setName:@"thread-3"];
    [button3Thread start];
}

/** 新线程读取信息 */
-(void)readInfo{
    [[NSOperationQueue mainQueue] addOperationWithBlock:^
     {
         self.mAlertView = [[UIAlertView alloc] initWithTitle:nil message:@"开始读取，请稍候..." delegate:self cancelButtonTitle:nil otherButtonTitles:nil, nil];
         [self.mAlertView show];
     }];
    
    char sn[64] = {0};
    int ret = [self.sim GetInfo:sn];
    
    //alert消失的方法
    if (self.mAlertView.visible) {
        [self.mAlertView dismissWithClickedButtonIndex:0 animated:YES];
    }
    
    NSString *msg = @"";
    msg=[NSString stringWithFormat:@"MobileNum=%@",[NSString stringWithUTF8String:sn]];
}

但有时会出现alert界面一直出现，造成界面卡死。
原因在于由于用多线程，线程阻塞时，调用alert消失时，alertView还没有绘制出来。当alertView绘制出来时，alert消失的方法已经过时。
解决方法是在执行alert消失的方法时，让线程暂停一段时间。如0.5s

[NSThreadsleepForTimeInterval:0.5]; 

或是用

[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 ];
    }
}

这样在固定时间后，alert总会消失。