I have seen various implementations of Singleton pattern in Objective-C in the past. The most common template I have seen is the following.

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static MyClass *instance = nil;

+ (MyClass *)sharedInstance {
    @synchronized(self) {
        if (instance == nil) {
            instance = [[self alloc] init];
        }
    }
    return instance;
}

As you can see, it locks the method using self. The issue with this code is that it can get very slow if this method is called hundreds of times, because the code to instantiate the shared instance must be thread-safe. I was researching for a better way to make this faster, then I found this article. The comment section of the article details the use of “Method Swizzling” to replace the method to access the shared instance with more optimised code (simply returning the instance without a check). Method swizzling allows us to modify the mapping from a selector to an implementation.

I’ve been working on optimising my Twitter client using Grand Central Dispatch (GCD) recently. Grand Central Dispatch is an Apple technology to optimise application with multicore processor. It was released with Mac OS 10.6 (Snow Leopard).

GCD makes it easier for programmers to perform tasks on different threads to optimise its algorithm performance. There are other interesting usages, which I will probably cover later. The GCD uses the new language feature of Objective-C 2.1 (also available on C and C++), called Blocks. Blocks lets us create closure-like objects to make it easy to execute a block of code parallel to the main thread. Blocks can also be used in C or C++. I’m not going to cover how to write blocks in this post, so it may be good to have a read about it if you don’t already know how it works.