Implementing a Singleton in iOS

While I was at Apple’s Cocoa Camp over summer, one of the speakers talked rather passionately about the proper, thread-safe way to implement a singleton in iOS, which I thought deserves more attention than it currently does on the Web.

The Common Naive Way

According to the first result on my Google search, a singleton should be implemented as follows:

static SingletonClass *sharedInstance = nil;

// Get the shared instance and create it if necessary.
+ (SingletonClass*)sharedInstance {
    if (sharedInstance == nil) {
        sharedInstance = [[super allocWithZone:NULL] init];
    }
 
    return sharedInstance;
}

Although this method is quick and easy, it is not thread-safe and the object constructor might be called multiple times if several threads access the method concurrently. We need a way to lock the constructor and insist that it be executed at most once. This is where Grand Central Dispatch comes in.

The Troublesome Thread-safe Way

GCD provides a neat API that makes concurrency programming in Cocoa convenient. To implement a thread-safe singleton in Cocoa, all you need to do is the following:

static MyObject *singleton = nil;

+(MyObject *) sharedInstance {

    NSLog (@"sharedInstance called.");

    if (nil != singleton) return singleton;
    static dispatch_once_t pred;        // lock
    dispatch_once(&pred, ^{             // this code is at most once
        singleton = [[MyObject alloc] init];
    });

    return singleton;

}

Let’s look at the code in detail. First, notice that the first few lines are the same: we have a static variable for our singleton, and we check whether it is nil whenever we access sharedInstance. If singleton is nil, allocate and initialize it; otherwise, return it.

The real difference is where we use the dispatch_* types and functions. You may think of dispatch_once_t as a type of lock that dispatch_once() uses to ensure that the code is executed at most once. Here, we wrap up whatever code we wish to be executed during the initialization of our singleton in a block. We then pass the block as an argument to dispatch_once. (Here’s a pretty good tutorial on blocks.)

This implementation ensures that singleton is never executed more than once even when multiple threads access sharedInstance concurrently:

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0U);
    
dispatch_apply(10, queue, ^(size_t i){
    [MyObject sharedInstance];
});

The above code results in the following output:

2011-09-05 10:12:05.961 SingletonTest[1602:10c03] sharedInstance called.
2011-09-05 10:12:05.961 SingletonTest[1602:12603] sharedInstance called.
2011-09-05 10:12:05.961 SingletonTest[1602:ef03] sharedInstance called.
2011-09-05 10:12:05.961 SingletonTest[1602:12703] sharedInstance called.
2011-09-05 10:12:05.966 SingletonTest[1602:10c03] init called.
2011-09-05 10:12:05.966 SingletonTest[1602:ef03] sharedInstance called.
2011-09-05 10:12:05.966 SingletonTest[1602:10c03] sharedInstance called.
2011-09-05 10:12:05.966 SingletonTest[1602:12703] sharedInstance called.
2011-09-05 10:12:05.966 SingletonTest[1602:12603] sharedInstance called.
2011-09-05 10:12:05.969 SingletonTest[1602:10c03] sharedInstance called.
2011-09-05 10:12:05.969 SingletonTest[1602:ef03] sharedInstance called.