Saturday, July 30, 2011

One singleton to rule them all

One of the most common design patterns is the singleton. However, implementing singletons, especially in Objective-C, can be very repetitive. I often forget how to properly override all the memory management methods and need to look up a previous singleton. One of the goals of programming is to automate the repetetive, mundane tasks. Cocoa With Love has a great synthesized singleton made from a C pre-processor macro. The problem with this is that it's not flexible. So, I made an Objective-C singleton that can be dropped in any project. By subclassing RBSingleton you can gain all the benefits of a singleton without adding any further code and you have full flexibility to customize each subclass.

There are two main parts that make RBSingleton possible. First, it stores all the singleton instances in a class-wide dictionary. A singleton typically keeps a static pointer to the singleton instance. This works fine as long as it's not subclassed. When subclasses become involved, the subclass that allocates first defines the singleton instance for all subclasses. This restriction is normally beneficial to singletons, but breaks with subclassing. I could have required subclasses to provide a pointer to a static class pointer through a method call, but this imposes more on the subclass's implementation than I wanted. By using a class-wide dictionary, every subclass can store its singleton instance under a unique key. To guarantee a unique key, I simply use the subclass's name.

Second, the singleton instance is allocated by dynamically calling NSObject's -allocWithZone: method as shown below.

Method allocMethod = class_getClassMethod([NSObject class], @selector(allocWithZone:));
sharedInstance = [
method_invoke(self, allocMethod, nil) initialize];

The trick here was to allocate the singleton as the class of the RBSingleton subclass but use NSObject's -allocWithZone: method to do so. This is similar to calling [super allocWIthZone:nil];. The difference is that it's jumping more than one level in the inheritance hierarchy. This is known as a grandsuper call. The Objective-C dynamic runtime makes it possible to make such a call. You will also notice that I hardcoded [NSObject class] into the code. I could have dynamically discovered the root class with the following code:

+ (Class)rootClass {

Class rootClass = nil;
Class currentClass = [self class];

while ((currentClass = class_getSuperclass(currentClass)))
                rootClass = currentClass;

return rootClass;

If I instead needed to get a grandsuper class that isn't necessarily the root class and I know a subclass of the grandsuper I could use simpler code like this:

[RBSingleton superclass];

Since I know the specific grandsuper class I need, I hardcoded it for efficiency. If you use RBSingleton and find a need to dynamically discover a particular grandsuper class, you can use the above code to do so. I've uploaded RBSingleton as a Gist and you can find it here.

One last note, to use RBSingleton you need to include libobjc.dylib for the dynamic runtime calls.