Question

I am implementing a Cache in my iOS app, that would keep images downloaded in RAM.

I did some research and found some code but most of them were for caching images to permanent storage.

I tried NSCache but couldn’t work it around for my need.

The requirements are:

• Limit on saving images. e.g. 100.
• As the Cache limit is reached, it should remove most older image inserted before adding a new one.

I’m not sure about the exact word but I think it should be called FIFO cache (First in first out).

After some research, I did the following implementation.

static NSMutableDictionary *thumbnailImagesCache = nil;

+ (UIImage *)imageWithURL:(NSString *)_imageURL
{
    if (thumbnailImagesCache == nil) {
        thumbnailImagesCache = [NSMutableDictionary dictionary];
    }

    UIImage *image = nil;

    if ((image = [thumbnailImagesCache objectForKey:_imageURL])) {
        DLog(@"image found in Cache")
        return image;
    }

    /*  the image was not found in cache - object sending request for image is responsible to download image and save it to cache */
    DLog(@"image not found in cache")
    return nil;
}

+ (void)saveImageForURL:(UIImage *)_image URLString:(NSString *)_urlString
{
    if (thumbnailImagesCache == nil) {
        thumbnailImagesCache = [NSMutableDictionary dictionary];
    }

    if (_image && _urlString) {
        DLog(@"adding image to cache")

        if (thumbnailImagesCache.count > 100) {
            NSArray *keys = [thumbnailImagesCache allKeys];
            NSString *key0 = [keys objectAtIndex:0];
            [thumbnailImagesCache removeObjectForKey:key0];
        }

        [thumbnailImagesCache setObject:_image forKey:_urlString];
        DLog(@"images count in cache = %d", thumbnailImagesCache.count)
    }
}

Now the problem is that I’m not sure weather this is the correct/efficient solution. Any one have any better idea/solution?

Answer 1

Your assumption about the order of the keys is certainly incorrect. The order of the keys in an NSDictionary is unspecified, the key and value at index 0 need not be the oldest one. You shall store the creation date of each image in the method where you put them in the cache dictionary.

Apart from that, the rest of the code seems valid.