Question

我想从不同的网址中获取许多图像并将其设置为图像的按钮。我试图这样做，就像上面所示，但没有任何事情发生。当我访问此视图控制器时，它没有任何按钮图像，也没有显示横幅视图....

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){

            NSURL *url = [NSURL URLWithString: [pictureUrl objectAtIndex:i]];
            NSData *data = [[NSData alloc] initWithContentsOfURL:url];

            dispatch_async(dispatch_get_main_queue(), ^(void){

                UIImage *img1 = [[UIImage alloc]initWithData:data];
                img2.image = img1;
                [bt setBackgroundImage:img2.image forState:UIControlStateNormal];
  });
});

Answer 1

我建议您使用支持图像缓存的库。例如，我使用AFNetworking作为我的一个项目，它很棒。它会自动为您处理背景。在我的情况下，我需要一个库，当我开始新的请求时它自动取消请求，它对我有用。您可以看到代码here。您可以从另一个线程中看到类似的solution，如下所示：

AFHTTPRequestOperation *requestOperation = [[AFHTTPRequestOperation alloc] initWithRequest:urlRequest];
requestOperation.responseSerializer = [AFImageResponseSerializer serializer];
[requestOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
    NSLog(@"Response: %@", responseObject);
    _imageView.image = responseObject;

} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    NSLog(@"Image error: %@", error);
}];
[requestOperation start];

希望它对你有所帮助。

Answer 2

恕我直言，您应该使用 async和sync对，内部块应与第一个同步，以便在下载和提供数据时拍摄图片。另外，不要忘记处理错误：

试试这样：

// Show HUD here
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void)
{

      NSError* error;
      NSURL *url = [NSURL URLWithString: [pictureUrl objectAtIndex:i]];
      NSData *data = [[NSData alloc] initWithContentsOfURL:url options:0 error:&error];

      dispatch_sync(dispatch_get_main_queue(), ^(void){

        if(!error && data)
        {
            UIImage *img1 = [[UIImage alloc] initWithData:data];
            img2.image = img1;
            [bt setBackgroundImage:img2.image forState:UIControlStateNormal]; 
             /*
                //----OR----
               [bt setBackgroundImage:img1 forState:UIControlStateNormal]; 
              */           
        }
        else
        {
            // Do error handling here
        }
        // Hide HUD here
  });
});

希望它有所帮助！

Answer 3

 imageView.image = [UIImage imageWithData:yourDefaultImgUrl];

   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSData *imageData = [NSData dataWithContentsOfURL:yourImageUrl];      
        if (imageData){
            dispatch_async(dispatch_get_main_queue(), ^{               
                 imageView.image = [UIImage imageWithData:imageData];
            });
        }
    });

希望有所帮助。

Answer 4

尝试此修改，

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){

       NSURL *url = [NSURL URLWithString: [pictureUrl objectAtIndex:i]];
       NSError *error;
       NSData *data = [NSData dataWithContentsOfURL:url options:0 error:&error];   
       if(error)
        {
           NSLog(@"Error: %@", [error localizedDescription]);
        }
        else
        {
          UIImage *img1 = [[UIImage alloc]initWithData:data];
          [bt setBackgroundImage:img1 forState:UIControlStateNormal];
        }
       // dispatch_async(dispatch_get_main_queue(), ^(void){
            //img2.image = img1;
       // });
});

希望有所帮助。

Answer 5

试试这个

dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
            dispatch_async(myQueue, ^{
                NSString *ImageURL = @"yourURL.jpg";
                NSData *imageData;
                if (ImageURL == nil || ImageURL == (id)[NSNull null] || [[NSString stringWithFormat:@"%@",ImageURL] length] == 0 || [[[NSString stringWithFormat:@"%@",ImageURL] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] == 0)
                {
                }

                else
                {
                    imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:ImageURL]];

                }
                dispatch_async(dispatch_get_main_queue(), ^{

                    if (ImageURL == nil || ImageURL == (id)[NSNull null] || [[NSString stringWithFormat:@"%@",ImageURL] length] == 0 || [[[NSString stringWithFormat:@"%@",ImageURL] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] == 0)
                    {
                        imageView.image=[UIImage imageNamed:@"photo_frame_noimage.png"];
                    }
                    else if (imageData == nil || imageData == (id)[NSNull null] || [[NSString stringWithFormat:@"%@",imageData] length] == 0 || [[[NSString stringWithFormat:@"%@",imageData] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] == 0)
                    {
                        imageView.image=[UIImage imageNamed:@"photo_frame_noimage.png"];
                    }
                    else
                    {
                        imageView.image=[UIImage imageWithData:imageData];
                    }
                });

            });

dispatch_async用于从url获取许多图像

5 个答案: