iOS系统GCD学习(10):常见的错误

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;
});
});