iOS异步请求下载图片

2015/06/14 34905

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在iOS中可以这样获取一张网络图片
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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：

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@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：
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- (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
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//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
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- (void)connectionDidFinishLoading:(NSURLConnection *)connection 
{ 
    UIImage* image = [UIImage imageWithData:_imageData]; 
    self.view.backgroundColor = [UIColor colorWithPatternImage:image]; 
    //设置状态栏 
    [UIApplication sharedApplication].networkActivityIndicatorVisible = NO; 
}