1.推荐单例写法

#pragma mark  - 创建单例
static id _instance;

+ (instancetype)allocWithZone:(struct _NSZone *)zone
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [super allocWithZone:zone];
    });
    return _instance;
}

+ (instancetype)sharedInstance
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [[self alloc] init];
    });
    return _instance;
}

- (id)copyWithZone:(NSZone *)zone
{
    return _instance;
}

- (id)mutableCopyWithZone:(NSZone *)zone {
    return _instance;
}

封装成宏定义

#define SingletonH(name) + (instancetype)shared##name;

#define SingletonM(name) \
static id _instance; \
 \
+ (instancetype)allocWithZone:(struct _NSZone *)zone \
{ \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
        _instance = [super allocWithZone:zone]; \
    }); \
    return _instance; \
} \
 \
+ (instancetype)shared##name \
{ \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
        _instance = [[self alloc] init]; \
    }); \
    return _instance; \
} \
 \
- (id)copyWithZone:(NSZone *)zone \
{ \
    return _instance; \
}\
\
- (id)mutableCopyWithZone:(NSZone *)zone { \
    return _instance; \
}

调用实例:

viewcontroller.h文件
===========================

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

@interface ViewController : UIViewController
SingletonH(viewController)
@end



viewcontroller.m文件
===========================

@interface ViewController ()

@end

@implementation ViewController

SingletonM(ViewController)

- (void)viewDidLoad {
    [super viewDidLoad];

    NSLog(@"%@ %@ %@ %@", [ViewController sharedViewController],[ViewController sharedViewController], [[ViewController alloc] init],[[ViewController alloc] init]);

}

参考链接