如何懒惰地从照片库加载图像？

Question

I would like to load images lazily and displaying it in scrollview in which paging is enabled. Loading all images in a single shot leads App to crash, as i am loading fullScreenImage. Its working fine, If I load thumbnail image using aspectRatioThumbnail, but image quality is worse. So I want to load images for only visible pages instead of loading everything at a time. If I know how to load images lazily from photo library, I can display images using only three pages. below are my code...

-(void)loadImagesFromLibrary{

     imageList = [[NSMutableArray alloc] init];
     ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

     CGFloat __block xaxis=05;
     NSUInteger __block images = 0;

     dispatch_queue_t queue = dispatch_get_main_queue();
     dispatch_async(queue, ^(void){

     [library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos
                           usingBlock:^(ALAssetsGroup *group, BOOL *stop) {

                               if(group){

                                   [group setAssetsFilter:[ALAssetsFilter allPhotos]];

                                   [group enumerateAssetsUsingBlock:^(ALAsset *assets, NSUInteger index, BOOL *innerStop){

                                       if (assets && [[assets valueForProperty:ALAssetPropertyType] isEqualToString:ALAssetTypePhoto])
                                       {
                                           //ALAssetRepresentation *representation = [assets defaultRepresentation ];
                                           //UIImage *img = [UIImage imageWithCGImage:[representation fullScreenImage]];

                                           UIImage *img = [UIImage imageWithCGImage:[assets aspectRatioThumbnail]];
                                           if(img){

                                               [imageList addObject:img];

                                               UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(xaxis, 5, 300, HEIGHT)];
                                               [imageView setImage:[self rectSizeFormat:img Size:CGSizeMake(300, HEIGHT)]];      //resizing the image 
                                               [imageView setTag:images++];
                                               [imageView setUserInteractionEnabled:YES];

                                               UITapGestureRecognizer *tapGesture = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(displayImageInMiddleView:)];
                                               [tapGesture setNumberOfTapsRequired:1];
                                               [imageView addGestureRecognizer:tapGesture];
                                               [tapGesture release];

                                               [scrollView addSubview: imageView];
                                               [imageView release];

                                               xaxis+=320;

                                           }  
                                       }
                                   }];

                                  [scrollView setContentSize:CGSizeMake(xaxis, HEIGHT)];
                               }
                               else{

                                   DLog(@"No image");
                               }

                           }

                         failureBlock:^(NSError *err) {

                             DLog(@"%@", [err localizedDescription]);
                         }];

    });
}

I know it can't be explained in few lines. All I am asking is give me a hint to achieve what I want, or suggest me how I can do it in a different way. Many thanks in advance.

Answer 1

I have a similar issue with a "filmstrip" type display. My solution is to enumerate the assets in my target album and for each one

• Create a userImage object and add it to an NSMutableArray.
• Save the assetURL in the object.
• Get the aspect ratio thumbnail and save it in the object.
• Put the thumbnail image into the filmstrip.

When I need a higher-resolution version of each filmstrip image I fire off an async request to load the full-sized image using the saved assetURL. Of course that is asynchronous so you have to have a place in the UI (UIImageView in my case) to put the image when it finally arrives from the asset library. Before the full-sized image is loaded I display the thumbnail in the same UIImageView (with scale-aspect-fit).

The asynchronous nature of the image loads is a real annoyance.