Best way to cache images on ios app?

Question

Shortly, I have an NSDictionary with urls for images that I need to show in my UITableView. Each cell has a title and an image. I had successfully made this happen, although the scrolling was lagging, as it seemed like the cells downloaded their image every time they came into the screen. I searched for a bit, and found SDWebImage on github. This made the scroll-lagg go away. I am not completely sure what it did, but I believed it did some caching. But! Every time I open the app for the first time, I see NO images, and I have to scroll down, and back up for them to arrive. And if I exit the app with home-button, and open again, then it seemes like the caching is working, because the images on the screen are visible, however, if I scroll one cell down, then the next cell has no image. Until i scroll past it and back up, or if I click on it. Is this how caching is supposed to work? Or what is the best way to cache images downloaded from the web? The images are being updated rarily, so I was close to just import them to the project, but I like to have the possibility to update images without uploading an update..

Is it impossible to load all the images for the whole tableview form the cache(given that there is something in the cache) at launch? Is that why I sometimes see cells without images?

And yes, I'm having a hard time understanding what cache is.

--EDIT--

I tried this with only images of the same size (500x150), and the aspect-error is gone, however when I scroll up or down, there are images on all cells, but at first they are wrong. After the cell has been in the view for some milliseconds, the right image appears. This is amazingly annoying, but maybe how it has to be?.. It seemes like it chooses the wrong index from the cache at first. If I scroll slow, then I can see the images blink from wrong image to the correct one. If I scroll fast, then I believe the wrong images are visible at all times, but I can't tell due to the fast scrolling. When the fast scrolling slows down and eventually stops, the wrong images still appear, but immediately after it stops scrolling, it updates to the right images. I also have a custom UITableViewCell class, but I haven't made any big changes.. I haven't gone through my code very much yet, but I can't think of what may be wrong.. Maybe I have something in the wrong order.. I have programmed much in java, c#, php etc, but I'm having a hard time understanding Objective-c, with all the .h and .m ... I have also `

@interface FirstViewController : UITableViewController{

/**/
NSCache *_imageCache;
}

(among other variables) in FirstViewController.h. Is this not correct?

Here's my cellForRowAtIndexPath.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"hallo";
CustomCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

if (cell == nil)
{
    cell = [[CustomCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
}

NSMutableArray *marr = [hallo objectAtIndex:indexPath.section];
NSDictionary *dict = [marr objectAtIndex:indexPath.row];

NSString* imageName = [dict objectForKey:@"Image"];
//NSLog(@"url: %@", imageURL);

UIImage *image = [_imageCache objectForKey:imageName];

if(image)
{
    cell.imageView.image = image;
}
else
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        NSString* imageURLString = [NSString stringWithFormat:@"example.com/%@", imageName];
        NSURL *imageURL = [NSURL URLWithString:imageURLString];
        UIImage *image = [[UIImage alloc] initWithData:[NSData dataWithContentsOfURL:imageURL]];

        if(image)
        {
            dispatch_async(dispatch_get_main_queue(), ^{
                CustomCell *cell =(CustomCell*)[self.tableView cellForRowAtIndexPath:indexPath];
                if(cell)
                {
                    cell.imageView.image = image;
                }
            });
            [_imageCache setObject:image forKey:imageName];
        }
    });
}

cell.textLabel.text = [dict objectForKey:@"Name"];

return cell;
}

Answer 1

Caching just means keeping a copy of the data that you need so that you don't have to load it from some slower source. For example, microprocessors often have cache memory where they keep copies of data so that they don't have to access RAM, which is a lot slower. Hard disks often have memory caches from which the file system can get much quicker access to blocks of data that have been accessed recently.

Similarly, if your app loads a lot of images from the network, it may be in your interest to cache them on your device instead of downloading them every time you need them. There are lots of ways to do that -- it sounds like you already found one. You might want to store the images you download in your app's /Library/Caches directory, especially if you don't expect them to change. Loading the images from secondary storage will be much, much quicker than loading them over the network.

You might also be interested in the little-known NSCache class for keeping the images you need in memory. NSCache works like a dictionary, but when memory gets tight it'll start releasing some of its contents. You can check the cache for a given image first, and if you don't find it there you can then look in your caches directory, and if you don't find it there you can download it. None of this will speed up image loading on your app the first time you run it, but once your app has downloaded most of what it needs it'll be much more responsive.