UrlEncoding category for NSDictionary class

I've been writing some Objective-C code that builds URLs with long query strings. Since query strings are almost always associative arrays, I whipped up a simple category for the NSDictionary class.

A category is the Objective-C mechanism for adding additional methods to an existing class. To a C++ programmer, it's like being able to append more virtual methods to a class's vtable. C# has a similar facility called extension methods, and there are proposals to add this to Java. Of course, this is no big deal for scripting languages like Ruby, Python or JavaScript, but I like how Objective-C handles this.

For one thing, Objective-C forces you to name your category; this helps a lot in thinking and talking about a set of additional methods. For another, since the category declaration syntax resembles class declaration syntax, you naturally want to place it in its own pair of .h and .m files, like a class. Apple recommends a naming convention of class+category.h/m, which seems pretty reasonable.

So here's the header file for my UrlEncoding category for NSDictionary:

// file "NSDictionary+UrlEncoding.h"

#import <cocoa/cocoa.h>


@interface NSDictionary (UrlEncoding)

-(NSString*) urlEncodedString;

@end

And here's the module file:

// file "NSDictionary+UrlEncoding.m"

#import "NSDictionary+UrlEncoding.h"


// helper function: get the string form of any object
static NSString *toString(id object) {
return [NSString stringWithFormat: @"%@", object];
}

// helper function: get the url encoded string form of any object
static NSString *urlEncode(id object) {
NSString *string = toString(object);
return [string stringByAddingPercentEscapesUsingEncoding: NSUTF8StringEncoding];
}


@implementation NSDictionary (UrlEncoding)

-(NSString*) urlEncodedString {
NSMutableArray *parts = [NSMutableArray array];
for (id key in self) {
id value = [self objectForKey: key];
NSString *part = [NSString stringWithFormat: @"%@=%@", urlEncode(key), urlEncode(value)];
[parts addObject: part];
}
return [parts componentsJoinedByString: @"&"];
}

@end

The code is pretty simple. Iterate over the keys in the dictionary and build an array of URL encoded "name=value" parts, then join the parts together with ampersands. The urlEncodedString method will handle keys and values of any class as long it has a reasonable description method. The code assumed that you never want a name/key to appear more than once in the query string, which is the most common case.