贴几段斯坦福大学关于gcd的代码，这段代码逐步演示了如何修正错误，其中用到的既是串行队列

 

1。这个是原始代码

- (void)viewWillAppear:(BOOL)animated
{
    NSData *imageData = [FlickrFetcher imageDataForPhotoWithURLString:photo.URL];
    UIImage *image = [UIImage imageWithData:imageData];
    self.imageView.image = image;
    self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);
    self.scrollView.contentSize = image.size;
}

2。这个是采用gcdd的代码，里面有错误3处

- (void)viewWillAppear:(BOOL)animated
{
    dispatch_queue_t downloadQueue = dispatch_queue_create(“Flickr downloader”, NULL);
    dispatch_async(downloadQueue, ^{
         NSData *imageData = [FlickrFetcher imageDataForPhotoWithURLString:photo.URL];
         UIImage *image = [UIImage imageWithData:imageData];
         self.imageView.image = image;
         self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);
         self.scrollView.contentSize = image.size;
    });
}

3。第一个错误， UI更新只能在主线程中 Problem! UIKit calls can only happen in the main thread!

改正后如下：

- (void)viewWillAppear:(BOOL)animated
{
    dispatch_queue_t downloadQueue = dispatch_queue_create(“Flickr downloader”, NULL);
    dispatch_async(downloadQueue, ^{
         NSData *imageData = [FlickrFetcher imageDataForPhotoWithURLString:photo.URL];
         <span style="color:#ff0000;">dispatch_async(dispatch_get_main_queue(), ^{        </span>
             UIImage *image = [UIImage imageWithData:imageData];
             self.imageView.image = image;
             self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);
             self.scrollView.contentSize = image.size;
         });
}); }

4。第二个错误，NSManagedObjectContext并不是线程安全的，gcd中访问成员变量有危险

Problem! NSManagedObjectContext is not thread safe,

so we can’t call photo.URL in downloadQueue’s t

改正后如下：

- (void)viewWillAppear:(BOOL)animated
{
  <span style="color:#ff0000;">  NSString *url = photo.URL;</span>
    dispatch_queue_t downloadQueue = dispatch_queue_create(“Flickr downloader”, NULL);
    dispatch_async(downloadQueue, ^{
        NSData *imageData = [FlickrFetcher <span style="color:#ff0000;">imageDataForPhotoWithURLString:url];</span>
        dispatch_async(dispatch_get_main_queue(), ^{
            UIImage *image = [UIImage imageWithData:imageData];
            self.imageView.image = image;
            self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);
            self.scrollView.contentSize = image.size;
}); });
}

5。第三个错误，队列创建后没有释放，内存泄露

- (void)viewWillAppear:(BOOL)animated
{
    NSString *url = photo.URL;
    dispatch_queue_t downloadQueue = dispatch_queue_create(“Flickr downloader”, NULL);
    dispatch_async(downloadQueue, ^{
        NSData *imageData = [FlickrFetcher imageDataForPhotoWithURLString:url];
        dispatch_async(dispatch_get_main_queue(), ^{
            UIImage *image = [UIImage imageWithData:imageData];
            self.imageView.image = image;
            self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);
            self.scrollView.contentSize = image.size;
}); });
<span style="color:#ff0000;">dispatch_release(downloadQueue); //won’tactuallygoawayuntilqueueisemp