在iOS中可以这样获取一张网络图片

    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];  

但是图片比较大的时候程序会卡在这里，所以我们要用异步请求来下载图片

1.新建一个single view工程

2.ViewController.h文件：

    @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  

3.ViewController.m文件：

    - (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];        }  

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        }  

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;        }