performSelectorOnMainThread

#import <UIKit/UIKit.h>

@interface RootViewController : UIViewController
{
    UILabel *label;
}

@end

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    self.title=@"performSelectorOnMainThread";
    
}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    // performSelectorOnMainThread的作用就如其名，是在主线程执行某个selector， 所有的UIKit里的调用必须是在主线程的，在非主线程调用会产生非预期的结果也很可能会造成crash,所以当你在其他线程想执行一段UI调用的代码时，就需要用到这个方法了
    
    [self performSelectorOnMainThread:@selector(loadData) withObject:nil waitUntilDone:NO];
    
}
-(void)loadData
{
    if (!label) {
        label=[[UILabel alloc]init];
        label.frame=CGRectMake(50, 80, 200, 200);
        label.backgroundColor=[UIColor redColor];
        [self.view addSubview:label];
    }else
    {
        [label removeFromSuperview];
        label=nil;
    }
    
}