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;  
    }