What is best way to share class instance (create and share singleton)?

I know 2 ways. What is better? And anything better than 2 ways开发者_如何学JAVA?

+ (MyClass *)shared {
    /*
    static MyClass *sharedInstance = nil;

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

    /*
    static dispatch_once_t pred;
    static MyClass *sharedInstance = nil;

    dispatch_once(&pred, ^{
        sharedInstance = [[self alloc] init];
    });

    return sharedInstance;
    */ 
} 

Can also create a your class instance in AppDelegate and use it anywhere in your project.

appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];

appappDelegate.<yourClassInstance>

Here is another way to setup you shared instance. Thread safety is handled by the runtime and the code is very straight forward. This is usually how I setup my singletons. If the singleton object uses lots of resources but may not used then the dispatch_once approach works well.

static MyClass *sharedInstance = nil;

+ (void) initialize 
{
   sharedInstance = [[MyClass alloc] init];
}

+ (MyClass*)sharedInstance   
{
    return sharedInstance;
}

Just use the dispatch_once version - it's reliable and clean. Besides, it will work also with ARC - unlike the approach suggested above.

Here's some Details

+ (YourClass *)sharedInstance 
{
    // structure used to test whether the block has completed or not
    static dispatch_once_t p = 0;

    // initialize sharedObject as nil (first call only)
    __strong static id _sharedObject = nil;

    // executes a block object once and only once for the lifetime of an application
    dispatch_once(&p, ^{
        _sharedObject = [[self alloc] init];
    });

    // returns the same object each time
    return _sharedObject;
}

The second one looks better, but this is still not perfect. Check Apple's recommendation:

http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid/TP40002974-CH4-97333-CJBDDIBI