单例模式

---

Singleton.h

#import <Foundation/Foundation.h>

@interface Singleton : NSObject

+ (Singleton *)sharedInstance;
- (void)print;
@end

---

Singleton.m

#import "Singleton.h"

@implementation Singleton

+ (Singleton *)sharedInstance {
    static Singleton *_sharedInstance = nil;
    static dispatch_once_t oncePredicate;
    
    dispatch_once(&oncePredicate, ^{
        _sharedInstance = [[self alloc] init];
    });
    
    return _sharedInstance;
}

- (void)print{
    NSLog(@"The Singleton Pattern");
}

@end

---

main.m

#import <Foundation/Foundation.h>
#import "Singleton.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        [[Singleton sharedInstance] print];
    }
    return 0;
}

注意:因为Objective-C的方法并没有private和public的概念，在任何时间任何对象之间消息都能被传递。因此上面的例子Singleton对象仍然能用init方法创建。

如果一定要使得init方法失效，来至http://stackoverflow.com/questions/195078/is-it-possible-to-make-the-init-method-private-in-objective-c 的解答方案。