A couple of weeks ago Matt Long was having a problem with an app running out of memory. He had a ginormous data file he needed to load up and process, and that memory hit was more than the app could bear. It would load just fine, into an NSData, but before he could finish with it the app would run short of memory and die.

Until recently the obvious thing would have been to tell NSData to create a memory-mapped instance. Given NSString *path pointing to a file, you could create an NSData with almost no memory hit regardless of file size by creating it as:

NSData *data = [NSData dataWithContentsOfMappedFile:path];

Starting with iOS 5 though, this method has been deprecated. Instead, what you’re supposed to do is:

NSError *error = nil;
NSData *data = [NSData dataWithContentsOfFile:path options:NSDataReadingMappedAlways error:&error];

So, fine, whatever, it’s a different call, so what? Well, it wasn’t working. Instruments was showing that the app was taking the full memory hit when the NSData was created. Mapping wasn’t working despite using NSDataReadingMappedAlways. So what could he do? The wheels of my mind started turning.

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With the March 1st start date approaching when sandboxing becomes a requirement for submissions to the Mac App Store, I’ve been considering my options. (more…)