iphone-GCD应用

//网络资源
// from: http://blog.csdn.net/favormm/article/details/6453260

//***************************************************************************//
//功能
1. 预定义了大量的要下载的图片URL
2. 创建一个GCD queue并开始下载图片
3. 在动画出现消失的时候，同时显示图片

//代码

@interface UIImageView (DispatchLoad)  
- (void) setImageFromUrl:(NSString*)urlString;  
- (void) setImageFromUrl:(NSString*)urlString   
              completion:(void (^)(void))completion;  
@end

#import "UIImageView+DispatchLoad.h"  
@implementation UIImageView (DispatchLoad)  
- (void) setImageFromUrl:(NSString*)urlString {  
    [self setImageFromUrl:urlString completion:NULL];  
}  
- (void) setImageFromUrl:(NSString*)urlString   
              completion:(void (^)(void))completion {  
      
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
          
        NSLog(@"Starting: %@", urlString);  
        UIImage *avatarImage = nil;   
        NSURL *url = [NSURL URLWithString:urlString];  
        NSData *responseData = [NSData dataWithContentsOfURL:url];  
        avatarImage = [UIImage imageWithData:responseData];  
        NSLog(@"Finishing: %@", urlString);  
          
        if (avatarImage) {  
            dispatch_async(dispatch_get_main_queue(), ^{  
                self.image = avatarImage;  
            });  
            dispatch_async(dispatch_get_main_queue(), completion);  
        }  
        else {  
            NSLog(@"-- impossible download: %@", urlString);  
        }  
    });     
}  
@end

//************************************************************************//

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
view plain

- (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
view plain

- (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：
view plain

- (void)viewWillAppear:(BOOL)animated  
{  
    dispatch_queue_t downloadQueue = dispatch_queue_create(“Flickr downloader”, NULL);  
    dispatch_async(downloadQueue, ^{  
         NSData *imageData = [FlickrFetcher imageDataForPhotoWithURLString:photo.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;  
         });  
    }); 
}  

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：
view plain

- (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;  
        }); 
    });  
}  

5。第三个错误，队列创建后没有释放，内存泄露
view plain

- (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;  
        }); 
    });  
   dispatch_release(downloadQueue); //won’t actually go away until queue is empty 
}