ios block一定会犯的几个错误

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

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

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：

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：

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

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’tactuallygoawayuntilqueueisemp