- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    //GCD的用法
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        
        NSURL * url = [NSURL URLWithString:@"http://www.baidu.com"];
        NSError * error;
        NSString * data = http://www.mamicode.com/[NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:&error];
        if (data != nil) {
            dispatch_async(dispatch_get_main_queue(), ^{
                NSLog(@"call back, the data is: %@", data);
            });
        } else {
            NSLog(@"error when download:%@", error);
        }
    });
    
    
}