4.线程间通信

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    //进入子线程做任务
    [self performSelectorInBackground:@selector(download3) withObject:nil];
}

- (void)download3
{

    NSURL *url = [NSURL URLWithString:@"http://img.pconline.com.cn/images/photoblog/9/9/8/1/9981681/200910/11/1255259355826.jpg"];
    

    NSData *data = [NSData dataWithContentsOfURL:url];
    

    UIImage *image = [UIImage imageWithData:data];
    
    // 回到主线程，操作UI
    [self.imageView performSelector:@selector(setImage:) onThread:[NSThread mainThread] withObject:image waitUntilDone:NO];

}