System Programming


Extending NSData and (not) Overriding dealloc

by Tom Harrington

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.



Handling incoming JSON redux

by Tom Harrington

A few months ago I wrote here about a generic approach to safely take incoming JSON and save values to Core Data object. The goals of that code were twofold:

  1. Provide a safe, generic alternative to Cocoa’s -setValuesForKeysWithDictionary: for use with NSManagedObject and its subclasses
  2. Handle cases where JSON data didn’t match up with what the managed objects expected. Getting a string where you expect a numeric value, or vice versa, for example, or getting a string representation of a date when you want a real NSDate object.

The first item was the most important. It’s tempting to use -setValuesForKeysWithDictionary: to transfer JSON to a model object in one step. The method runs through the dictionary and calls -setValue:forKey: on the target object for every entry. It has a fatal flaw though, in that it doesn’t check to see if the target object actually has a key before trying to set it. Using this method when you don’t have absolute control over the dictionary contents is an invitation to unknown key exceptions and nasty app crashes.

Fixing this for managed objects was relatively easy because Core Data provides convenient Objective-C introspection methods. The general approach was:

  • Get a list of the target object’s attributes
  • For each attribute, see if the incoming dictionary has an entry. If so,
  • Compare the incoming type to the expected type, and convert if necessary.
  • Call -setValue:forKey: with that key and its value.

And then just last week I had the thought, wouldn’t it be nice if this worked for any object, not just for managed objects?



Cocoa Tutorial: Custom Folder Icons

by Matt Long

This is just another one of those things that seems like it ought to be a simple little code snippet and you’re there, but in actuality it’s just not the case. I am building an exporter for my application that will export a movie project in iMovie HD format. I want to mimic the file format exactly and one of the things I noticed about the directories that are stored inside iMovie HD‘s custom format (select ‘Show Package Contents’ from the context menu in the Finder) is that they have custom icons assigned to them. So the problem to solve was how to do that programatically. Here is what I’ve found.


Cocoa Tutorial: Introduction To Identity Services

by Matt Long

Morpheus, Badass!System programming is one of the more interesting topics to me because it seems like it’s the hackers domain. When I first got into programming I always wanted to see if I could make the computer do something that nobody else anticipated it could do. I think it was probably just from watching too many hacker movies. Even the Matrix (the first one, not those other two contorted kill joys) had this affect on me that somehow I could beat the system because as Morpheus put it,

I’ve seen an agent punch through a concrete wall. Men have emptied entire clips at them and hit nothing but air, yet their strength and their speed are still based in a world that is built on rules. Because of that, they will never be as strong or as fast as you can be.

Man! I loved that movie the first time I watched it. A world without rules? Or more specifically a world where you set the rules by your most awesome hacker powers. Sweet!

But alas, as I’ve proceeded and matured (I hope) in my programming abilities, this whole area has become much less glamorous as I realize that hacking is really about finding vulnerabilities in other people’s code. That’s really not that exciting to me, but system level programming remains interesting and a good place to start is to look into who the users are on my system. Time to use Identity Services. And in this intro I’ve written a very basic password cracking system.