Scroll view and table view performance when loading images from disk

Question

I\'m trying to improve the performance of my image-intensive iPhone app by using a disk-based image cache instead of going over the network. I\'ve modeled my image cache after S

Answer 1

If you've narrowed it down to network activity I would try encapsulating your request to ensure it is 100% off of the main thread. While you can use NSURLConnection asynchronously and respond to it's delegate methods, I find it easier to wrap a synchronous request in a background operation. You can use NSOperation or grand central dispatch if your needs are more complex. An (relatively) simple example in an imageLoader implementation could be:

// imageLoader.m

// assumes that that imageCache uses kvp to look for images
- (UIImage *)imageForKey:(NSString *)key
{
    // check if we already have the image in memory
     UImage *image = [_images objectForKey:key];

    // if we don't have an image:
    // 1) start a background task to load an image from a file or URL
    // 2) return a default image to display while loading
    if (!image) {
        [self performSelectorInBackground:@selector(loadImageForKey) withObject:key];
        image = [self defaultImage];
    }

    return image;
}

- (void)loadImageForKey:(NSString *)key
{
    NSAutoReleasePool *pool = [[NSAutoReleasePool alloc] init];

    // attempt to load the image from the file cache
    UIImage *image = [self imageFromFileForKey:key];

    // if no image, load the image from the URL
    if (!image) {
        image = [self imageFromURLForKey:key];
    }

    // if no image, return default or imageNotFound image
    if (!image) {
        image = [self notFoundImage];
    }

    if ([_delegate respondsTo:@selector(imageLoader:didLoadImage:ForKey:)]) {
        [_delegate imageLoader:self didLoadImage:image forKey:key];
    }

    [pool release];
}

- (UIImage *)imageFromURLForKey:(NSString *)key
{
    NSError *error = nil;
    NSData *imageData = [NSData dataWithContentsOfURL:[self imageURLForKey:key]
                                              options:0
                                                error:&error];

    UIImage *image;

    // handle error if necessary
    if (error) {
        image = [self errorImage];
    }

    // create image from data
    else {
        image = [UIImage imageWithData:imageData];
    }

    return image;
}