UITableView 或 UICollectionView在有照片的時候可以 asynch 圖片 asynch photo image

 在顯示UITableView 或 UICollectionView 要一起帶入有圖檔的時候，可以先把畫面顯示出來之後，在去下載圖片並顯示出來，方法如下：

要先新增一個class ImageRequest 繼承NSMutableURLRequest

ImageRequest.h

#import 

#define kIMAGE_REQUEST_CACHE_LIMIT  100
typedef void (^CompletionBlock) (UIImage *, NSError *);

@interface ImageRequest : NSMutableURLRequest

- (UIImage *)cachedResult;
- (void)startWithCompletion:(CompletionBlock)completion;
@end

ImageRequest.m

#import "ImageRequest.h"

NSMutableDictionary *_inflight;
NSCache *_imageCache;

@implementation ImageRequest

- (NSMutableDictionary *)inflight {
    
    if (!_inflight) {
        _inflight = [NSMutableDictionary dictionary];
    }
    return _inflight;
}

- (NSCache *)imageCache {
    
    if (!_imageCache) {
        _imageCache = [[NSCache alloc] init];
        _imageCache.countLimit = kIMAGE_REQUEST_CACHE_LIMIT;
    }
    return _imageCache;
}

- (UIImage *)cachedResult {
    
    return [self.imageCache objectForKey:self];
}

- (void)startWithCompletion:(CompletionBlock)completion {
    
    UIImage *image = [self cachedResult];
    if (image) return completion(image, nil);
    
    NSMutableArray *inflightCompletionBlocks = [self.inflight objectForKey:self];
    if (inflightCompletionBlocks) {
        // a matching request is in flight, keep the completion block to run when we're finished
        [inflightCompletionBlocks addObject:completion];
    } else {
        [self.inflight setObject:[NSMutableArray arrayWithObject:completion] forKey:self];
        
        [NSURLConnection sendAsynchronousRequest:self queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
            if (!error) {
                // build an image, cache the result and run completion blocks for this request
                UIImage *image = [UIImage imageWithData:data];
                [self.imageCache setObject:image forKey:self];
                
                id value = [self.inflight objectForKey:self];
                [self.inflight removeObjectForKey:self];
                
                for (CompletionBlock block in (NSMutableArray *)value) {
                    block(image, nil);
                }
            } else {
                [self.inflight removeObjectForKey:self];
                completion(nil, error);
            }
        }];
    }
}


@end

在UICollectionView上使用的時候

  
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
                  cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];

    NSURL *url = [NSURL URLWithString:@"http:// some url from your model"];
    // note that this can be a web url or file url

    ImageRequest *request = [[ImageRequest alloc] initWithURL:url];

    UIImage *image = [request cachedResult];
    if (image) {
        UIImageView *imageView = (UIImageView *)[cell viewWithTag:127];
        imageView.image = image;
    } else {
        [request startWithCompletion:^(UIImage *image, NSError *error) {
            if (image && [[collectionView indexPathsForVisibleItems] containsObject:indexPath]) {
                [collectionView reloadItemsAtIndexPaths:@[indexPath]];
            }
        }];
    }
    cell.photoView.image = image;

    return cell;
}

出處：
http://stackoverflow.com/questions/15799432/poor-uicollectionview-scrolling-performance-with-uiimage/15799771#15799771