IOS開發之－－－非同步下載顯示圖片

下載圖片並非同步顯示更新資料到前台，我們可以有很多種方法，在IOS中提到了兩種方法如下：

需要定義一個ImageView和一個Button如下：

@property (retain, nonatomic) IBOutlet UIImageView *imageView;- (IBAction)download:(id)sender;
第一種方法：
- (IBAction)download:(id)sender {    NSURL *url = [NSURL URLWithString:@"http://img6.cache.netease.com/cnews/2012/6/1/20120601085505e3aba.jpg"];        [NSThread detachNewThreadSelector:@selector(dowork:) toTarget:self withObject:url];    }
其中調用了dowork函數
-(void) dowork: (NSURL*) url{    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];    NSError *error;        NSURLRequest *request = [NSURLRequest requestWithURL:url];    NSHTTPURLResponse *response;        NSData* retData = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];        if (response.statusCode == 200) {        UIImage* img = [UIImage imageWithData:retData];        [self   performSelectorOnMainThread:@selector(refreshUI:) withObject:img waitUntilDone:YES];            }    NSLog(@"d %d", response.statusCode);    [pool drain];    }
dowork方法中下載完成後通知UI更新，函數如下
-(void) refreshUI:(UIImage* ) img{    self.imageView.image = img;}
第二種方法使用GCD來更新UI
- (IBAction)download:(id)sender {    [self blockWork];    }Button調用block方法來下載並回調async來非同步更新UI來顯示圖片-(void)blockWork{    dispatch_queue_t queue;    queue = dispatch_queue_create("com.example.operation", NULL);    dispatch_async(queue, ^{        NSString *urlString = @"http://img6.cache.netease.com/cnews/2012/6/1/20120601085505e3aba.jpg";        NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:urlString]];                UIImage *imge = [UIImage imageWithData:imageData];                dispatch_async(dispatch_get_main_queue(), ^{            self.imageView.image = imge;        });            });
使用Block 文檔：
http://www.devdiv.com/Blocks編程要點【中文完整翻譯版】-article-3205-1.html