BENEDIKT TERHECHTE

This is my research notebook. I'm an OSX / iOS indie developer. After 8 years of Objective-C I really enjoy Swift nowadays. Trying to publish all my research on Development, Swift & other technologies here.

Wed, 7 Dec 2011 #

Fast NSDictionary traversal in Objective-C

Dictionaries in Cocoa are really powerful objects that offer a solid base of useful methods, are easy to expand, and fast - especially due to the framework-wide distinction between mutable and immutable classes. One can really see the benefits of nurturing and enhancing a single API over the course of more than 18 years 1.

Nowadays, as soon as your application receives data from any of the various web APIs, you oftentimes and up with a huge JSON file that contains various properties and subproperties of an entity that you intend to display or process in a specific way. In short, it is more and more the case that you end up with an structure that bears resemblance to well known Russian Dolls.

Take, for example, the data returned from the Instagram API for a media item (I shortened the data a bit)

{
    "data": [{
        "location": {
            "id": "833",
            "latitude": 37.77956816727314,
            "longitude": -122.41387367248539,
            "name": "Civic Center BART"
        },
        "comments": {
            "count": 16,
            "data": [ ... ]
        },
        "caption": null,
        "link": "http://instagr.am/p/BXsFz/",
        "likes": {
            "count": 190,
            "data": [{
                "username": "shayne",
                "full_name": "Shayne Sweeney",
                "id": "20",
                "profile_picture": "..."
            }, {...subset of likers...}]
        }}]
}

Classical Lookup

Now, if you utilize this structure from Instagram in your Objective-C app, and want to access all the ‘likes’ for this entity, you’d need to write something like this 2:

[[[aDictionary objectForKey:@"data"] objectForKey:@"likes"]
 objectForKey: @"data"]

Since sending a message to nil objects is allowed in Objective-C, this would run fine even if one of the keys does not exist. Instead, the main problem here is a visual one: due to Objective-C’s very elaborate syntax, adding one or two more hierarchies quickly results in a very cluttered appearance:

[[[[[aDictionary objectForKey:@"data"] objectForKey:@"likes"]
 objectForKey: @"data"] objectForKey: @"anotherLevel"]
 objectForKey: @"andanotherLevel"]

This certainly looks like it could get out of hand. So, which other options do we have? As it turns out, KVO (Key-Value-Coding) offers a terrific alternative:

KVO Lookup

KVO lets you access any property on an object through a key or keypath. If, for example, you had a Car object with a Trunk property which is a trunk object with a color property, you could access it like this:

[Car valueForKeyPath:@"trunk.color"];

As the Apple Docs state, NSDictionaries contain a KVO extension which registers all dictionary keys as KVO properties. This allows us to lookup our beloved ‘like’ value from Instagram as follows:

[aDictionary valueForKeyPath: @"data.likes.data"];

Fantastic isn’t it? In fact, that’s how I did most of my more complex dictionary lookups in InstaDesk until I recently realized that that could be a terrible performance hog. You see, KVO allows to do fantastic things, but that comes at a certain implementation overhead since much of it’s magic is added dynamically at runtime. So I wondered how much worse KVO access is against raw dictionary access. To find that out, I wrote this benchmark.

Time Implementation
11.93 KVO Access
1.53 Direct Dictionary

Shocking! 12 seconds against 1.5 seconds is roughly 8 times as fast. This is a situation in which the added syntax advantages of KVO loose hands down against the performance disadvantages.

Which is kinda sad though, as the KVO access really looks cleaner and is easier to write. This got me thinking. Would it be possible, using the C preprocessor, to write a bit of macro magic that takes in a list of keys and outputs the complex and cluttered Objective-C NSDictionary code?

C Preprocessor Magic

This is a tough problem. The C preprocessor is not turing complete and thus rather limited if you want to do more than replace code or conduct simple logical branching 3. One could, of course, write a macro that spits out C code that does the nitty gritty. But that was not what I wanted. I wanted the end result to be unrolled ‘objectForKey:’ messages and not calls to a C function that iterates over a string or list.

Even though the preprocessor doesn’t allow us to iterate or loop over lists or strings, it does have support for variadic argument lists (__VA_ARGS__), and, through a neat little hack, allows to count the number of items in such a variadic list. Given these constraints I had this idea on how solve the problem at hand.

As you can see, the C preprocessor code is rather long. But that’s only so it can support up to 8 NSDictionary levels. The usage is incredibly simple. Writing:

KeyPathDictionary(aDictionary, "data", "likes", "data", "friend",
 "voldemort", "wand");

Expands to:

[[[[[aDictionary objectForKey:@"data"] objectForKey:@"likes"]
 objectForKey: @"data"] objectForKey: @"friend"]
 objectForKey: @"voldemort"] objectForKey: @"wand"];

Isn’t that fantastic?

What I did was that I created 8 different main macros, each responsible for one NSDictionary lookup hierarchy (AKeyPathDictionary - HKeyPathDictionary). The ‘KeyPathDictionary’ macro then gets the argument list and counts the amount of entries in there (the PP_ macros). Based on the result of this, it will call one of the aforementioned 8 main macros and will expand the arguments into it.

Category Approach (Update)

Mayoff on HN suggests using a category with a va_list approach to solve this problem. I like his solution, so I’m including it here as a reference. It has the added benefit that one could extend it to also support retrieving individual NSArray entities during traversal. I.e.: ‘data.friends.data.0.name’ to access the first friends name. So, without further ado:

@interface NSDictionary (objectForKeyList)
- (id)objectForKeyList:(id)key, ...;
@end

@implementation NSDictionary (objectForKeyList)

- (id)objectForKeyList:(id)key, ...
{
  id object = self;
  va_list ap;
  va_start(ap, key);
  for ( ; key; key = va_arg(ap, id))
      object = [object objectForKey:key];

  va_end(ap);
  return object;
}
@end

Benchmarks

I also tried these implementations on an eight-level dictionary with the following benchmarks:

Time Implementation
30.69 KVO Access
2.79 Category Approach
2.35 Direct Dictionary
2.28 Preprocessor Macro

As you can see, the gap only widens with more levels in your dictionaries.

Outlook

I switched almost all multilevel dictionary access in the upcoming InstaDesk 1.3.8 1.4 with this code and it increased the overall speed in a tangible manner. Especially during object creation.

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  1. At least the NSDictionary.h header says 1994-2009, though I think that development of it began even earlier at [NeXTSTEP](http://en.wikipedia.org/wiki/NeXTSTEP).

  2. Actually, in many situations you'd probably abstract the data in a real object with ivars and properties which invalidate most of what I'm explaining here since, property access in Objective-C is far faster than dictionary access. However, there're quite a lot of situations where you need to access multilevel dictionaries, and in these cases the solution outlined here will help you a lot (performance-wise).

  3. In C++, there's the [Boost](http://www.boost.org) preprocessor which offers more functionality and is actually turing-complete.