iOS异步请求下载图片

    NSURL *url = [NSURL URLWithString:@"http://f.hiphotos.baidu.com/image/w%3D2048/sign=91c1063e1f950a7b753549c43ee963d9/f31fbe096b63f624b6a9640b8544ebf81b4ca3c6.jpg"];  
    NSData *data = [[NSData alloc] initWithContentsOfURL:url];  
    UIImage *img = [UIImage imageWithData:data];  

    @interface ViewController : UIViewController <NSURLConnectionDataDelegate> {  
        NSMutableData* _imageData;//如果图片比较大的话，response会分几次返回相应数据，所以需要用NSMutableData来接受数据  
        float _length;  
    }  
    @end  

    - (void)viewDidLoad

    {

        [super viewDidLoad];

    //初始化图片数据  
    _imageData = [[NSMutableData alloc] init];  

    //请求  
    NSURL *url = [NSURL URLWithString:@"http://f.hiphotos.baidu.com/image/w%3D2048/sign=91c1063e1f950a7b753549c43ee963d9/f31fbe096b63f624b6a9640b8544ebf81b4ca3c6.jpg"];  
    NSURLRequest *request = [NSURLRequest requestWithURL:url];  
    //连接  
    [NSURLConnection connectionWithRequest:request delegate:self];  
}  </pre> 


 4.接受响应头和响应体
    //响应头

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response  
{  
    //清空图片数据  
    [_imageData setLength:0];  
    //强制转换  
    NSHTTPURLResponse *resp = (NSHTTPURLResponse *)response;  
    _length = [[resp.allHeaderFields objectForKey:@"Content-Length"] floatValue];  
    //设置状态栏接收数据状态  
    [UIApplication sharedApplication].networkActivityIndicatorVisible = YES;  
}  

//响应体  
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data  
{  
    [_imageData appendData:data];//拼接响应数据  
}  </pre> 


 5.请求完成之后将图片显示出来，并且设置状态栏
    - (void)connectionDidFinishLoading:(NSURLConnection *)connection  
    {  
        UIImage* image = [UIImage imageWithData:_imageData];  
        self.view.backgroundColor = [UIColor colorWithPatternImage:image];  
        //设置状态栏  
        [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;  
    }