Singleton in Objective-C

Header file

Manager.h

@interface Manager : NSObject
+ (instancetype)sharedInstance;
@end

Method file

• Prevent concurrent access from multiple thread by @synchronized or dispatch_once
• Prevent multiple allocation by overriding allocWithZone

Manager.m

@implementation Manager

static Manager *_manager = nil;

+ (instancetype)sharedInstance {
    @synchronized(self) {
        if(!_manager) {
            (void)[[self alloc] init];
        }
    }
    return _manager;
}

+ (id)allocWithZone:(NSZone *)zone {
    @synchronized(self) {
        if(!_manager) {
            _manager = [super allocWithZone:zone];
        }
    }
    return _manager;
}
@end

or

Manager.m

@implementation Manager

static Manager *_manager = nil;

+ (instancetype)sharedInstance {
    static dispath_once_t onceToken;
    dispatch_once(&onceToken, ^{
        (void)[[self alloc] init];
    });
    return _manager;
}

+ (id)allocWithZone:(NSZone *)zone {
    static dispath_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _manager = [super allocWithZone:zone];
    });
    return _manager;
}
@end

Access

[Manager sharedInstance].property
[[Manager sharedInstance] method];

References
http://qiita.com/yuky_az/items/27031ec5ca55a95d6209
http://programming-ios.com/objective-c-singleton/
http://news.mynavi.jp/column/objc/051/