如何从服务器异步获取图像

Question

我正在做以下事情从服务器加载我的图像。但问题是，当alertview关闭时，imageview在5-7秒后显示。

如何解决这个问题？

- (void)showFullImage :(id) sender 
    {  
    // for loading view 
    self.loading_alert = [[UIAlertView alloc] initWithTitle:NSLocalizedString(@"LOADING",nil) message:nil delegate:self cancelButtonTitle:nil otherButtonTitles: nil]; 
    [self.loading_alert show]; 
    UIActivityIndicatorView *indicator = [[UIActivityIndicatorView alloc] initWithFrame: CGRectMake(125, 65, 30, 30)]; 
    indicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyleWhiteLarge; 

    //indicator.center = CGPointMake(loading_alert.bounds.size.width/2, loading_alert.bounds.size.height - 50); 
    [indicator startAnimating]; 
    [self.loading_alert addSubview:indicator];  

    self.image_url = [self.question_set valueForKey:@"image_url"]; 

//--------Asyncronously fetch image---------------------------------------- 
    NSURL *URL = [NSURL URLWithString:image_url]; 
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL cachePolicy:NSURLCacheStorageAllowedInMemoryOnly 
                timeoutInterval:60.0]; 
    [NSURLConnection sendAsynchronousRequest:request queue:[[NSOperationQueue alloc] init] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error){ 

    NSLog(@"1",nil); 
    imgView= [[UIView alloc] initWithFrame:CGRectMake(0,0,320,480) ]; 

    UIImage* queImage = [UIImage imageWithData:data]; 

UIImageView *iv = [[UIImageView alloc] initWithFrame:CGRectMake(0,42,320,420)]; 
iv.image = queImage; 

[imgView addSubview:iv];  
[self.view addSubview:imgView]; 

    //dismiss alert view on main thread 
    dispatch_async(dispatch_get_main_queue(), ^(void) { 
     // dismiss alert view... 
     NSLog(@"2",nil); 
     [self.loading_alert dismissWithClickedButtonIndex:0 animated:YES]; 
    }); 

}];  
} 

帮助将不胜感激。

Answer 1

您需要在主队列中的后台队列和UI更新中下载图像。

使用GCD这样，

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{ 

     NSURL *URL = [NSURL URLWithString:image_url]; 
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL cachePolicy:NSURLCacheStorageAllowedInMemoryOnly 
                timeoutInterval:60.0]; 
    [NSURLConnection sendAsynchronousRequest:request queue:[[NSOperationQueue alloc] init] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error){ 

    NSLog(@"1",nil); 


    UIImage* queImage = [UIImage imageWithData:data]; 

      dispatch_async(dispatch_get_main_queue(), ^{ 

       imgView= [[UIView alloc] initWithFrame:CGRectMake(0,0,320,480) ]; 
       if([receivedView isKindOfClass:[UIImageView class]]) 
       { 
UIImageView *iv = [[UIImageView alloc] initWithFrame:CGRectMake(0,42,320,420)]; 
iv.image = queImage; 

[imgView addSubview:iv];  
[self.view addSubview:imgView]; 
       } 
// dismiss alert view... 
     NSLog(@"2",nil); 
     [self.loading_alert dismissWithClickedButtonIndex:0 animated:YES]; 

      }); 


     } 


    }); 

Answer 2

SDWebmage - >这是一个加载图像一个很好的第三方库。

Answer 3

忘记GCD，您可以在NSURLConnection的的sendAsynchronousRequest的队列参数传递[NSOperationQueue mainQueue]：队列：completionHandler：方法，所以你不需要考虑太多关于主线程和后台线程，它会自动处理呼叫在后台请求并在主线程上返回数据。

及以下代码将如何看起来像：

NSURL *URL = [NSURL URLWithString:image_url]; 
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL cachePolicy:NSURLCacheStorageAllowedInMemoryOnly timeoutInterval:60.0]; 

// request is called asynchronously and response completion handler will be called on main thread 
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error){ 

    // do your whatever work here as you do normally, because it's in main thread 
    NSLog(@"1",nil); 
    imgView= [[UIView alloc] initWithFrame:CGRectMake(0,0,320,480) ]; 

    UIImage* queImage = [UIImage imageWithData:data]; 

    UIImageView *iv = [[UIImageView alloc] initWithFrame:CGRectMake(0,42,320,420)]; 
    iv.image = queImage; 

    [imgView addSubview:iv]; 
    [self.view addSubview:imgView]; 
}