imageNamed is evil[转]

imageNamed is evil

So a little while back we wrote about the confusing crashes on the device that we eventually figured out were caused by [UIImage imageNamed:] running out of memory when it tried to cache lots of big images. So we apparently solved the problem by making a thumbnail-sized image set and caching those, some 2.9 meg worth, whilst loading in the full sized ones uncached and only as needed.

Well, apparently turned out to be not good enough. See, it’s a few days later and we’ve finalized the design and got everything implemented and all seems to be good … except that it gets terminated without warning sooner or later. And quite often, Springboard terminates as well. Absolutely no correlation to any action or sequence in the program, no leaks, no out of bounds memory accesses, memory usage of the program barely a pittance. Yet, somehow, look at the console and you see system memory warnings scrolling by, you see it quitting background processes, and eventually quitting Springboard and/or the active application. Whilst that active application hums merrily along in blithe ignorance of the system crashing to the ground behind its back.

So apparently not only is 2.9 meg of images cached with +imageNamed enough to bring the iPhone OS to its knees, it’s not smart enough to, you know, actually do anything about it, like oh I don’t know, empty the cache or something?

It’s not like this is hard or anything, you can replicate the caching functionality precisely by declaring yourself an  NSMutableDictionary *thumbnailCache and populating it like

- (UIImage*)thumbnailImage:(NSString*)fileName
{
   UIImage *thumbnail = [thumbnailCache objectForKey:fileName];

   if (nil == thumbnail)
   {
      NSString *thumbnailFile = [NSString stringWithFormat:@"%@/thumbnails/%@.jpg", [[NSBundle mainBundle] resourcePath], fileName];
      thumbnail = [UIImage imageWithContentsOfFile:thumbnailFile];
      [thumbnailCache setObject:thumbnail forKey:fileName];
   }
   return thumbnail;
}

and if you do get a low memory issue, just [thumbnailCache removeAllObjects] and you’re good. But you know what? After replacing the various +imageNamed calls with this … not a single quibble anywhere, the whole 2.9 meg worth of cached thumbnails go right in there and not a single problem for, literally, hours.

(Not that there were any problems after those hours, we hasten to add; simply that, you know, if you think about it, there probably really isn’t a lot of point extending testing of an iPhone application much past the lifespan of a full battery charge…)

So the moral of the story is: DO NOT USE [UIImage imageNamed] for any significant amount of images. It is EVIL. It WILL bring down your application and/or Springboard, even when your application is putting along using just barely a nibble of memory on its own. Take the handful of lines above and implement your own cache!

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