ios block一定会范的几个错误

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’tactuallygoawayuntilqueueisempty }span>