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
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  1. - (void)viewWillAppear:(BOOL)animated  
  2. {  
  3.     NSData *imageData = [FlickrFetcher imageDataForPhotoWithURLString:photo.URL];  
  4.     UIImage *image = [UIImage imageWithData:imageData];  
  5.     self.imageView.image = image;  
  6.     self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);  
  7.     self.scrollView.contentSize = image.size;  
  8. }  

2。这个是采用gcdd的代码,里面有错误3处
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  1. - (void)viewWillAppear:(BOOL)animated  
  2. {  
  3.     dispatch_queue_t downloadQueue = dispatch_queue_create(“Flickr downloader”, NULL);  
  4.     dispatch_async(downloadQueue, ^{  
  5.          NSData *imageData = [FlickrFetcher imageDataForPhotoWithURLString:photo.URL];  
  6.          UIImage *image = [UIImage imageWithData:imageData];  
  7.          self.imageView.image = image;  
  8.          self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);  
  9.          self.scrollView.contentSize = image.size;  
  10.     });  
  11. }  

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:
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  1. - (void)viewWillAppear:(BOOL)animated  
  2. {  
  3.     dispatch_queue_t downloadQueue = dispatch_queue_create(“Flickr downloader”, NULL);  
  4.     dispatch_async(downloadQueue, ^{  
  5.          NSData *imageData = [FlickrFetcher imageDataForPhotoWithURLString:photo.URL];  
  6.          <span style="color:#ff0000;">dispatch_async(dispatch_get_main_queue(), ^{span>  
  7.              UIImage *image = [UIImage imageWithData:imageData];  
  8.              self.imageView.image = image;  
  9.              self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);  
  10.              self.scrollView.contentSize = image.size;  
  11.          });  
  12. }); }  

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:
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  1. - (void)viewWillAppear:(BOOL)animated  
  2. {  
  3.   <span style="color:#ff0000;">  NSString *url = photo.URL;span>  
  4.     dispatch_queue_t downloadQueue = dispatch_queue_create(“Flickr downloader”, NULL);  
  5.     dispatch_async(downloadQueue, ^{  
  6.         NSData *imageData = [FlickrFetcher <span style="color:#ff0000;">imageDataForPhotoWithURLString:url];span>  
  7.         dispatch_async(dispatch_get_main_queue(), ^{  
  8.             UIImage *image = [UIImage imageWithData:imageData];  
  9.             self.imageView.image = image;  
  10.             self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);  
  11.             self.scrollView.contentSize = image.size;  
  12. }); });  
  13. }  
5。第三个错误,队列创建后没有释放,内存泄露
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  1. - (void)viewWillAppear:(BOOL)animated  
  2. {  
  3.     NSString *url = photo.URL;  
  4.     dispatch_queue_t downloadQueue = dispatch_queue_create(“Flickr downloader”, NULL);  
  5.     dispatch_async(downloadQueue, ^{  
  6.         NSData *imageData = [FlickrFetcher imageDataForPhotoWithURLString:url];  
  7.         dispatch_async(dispatch_get_main_queue(), ^{  
  8.             UIImage *image = [UIImage imageWithData:imageData];  
  9.             self.imageView.image = image;  
  10.             self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);  
  11.             self.scrollView.contentSize = image.size;  
  12. }); });  
  13. <span style="color:#ff0000;">dispatch_release(downloadQueue); //won’tactuallygoawayuntilqueueisempty }span>  

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