1 #import "Singleton.h"
 2 
 3 @implementation Singleton
 4 static id _instance;
 5 
 6 /**
 7  *  alloc方法内部会调用这个方法
 8  */
 9 + (instancetype)allocWithZone:(struct _NSZone *)zone{
10     if (_instance == nil) { // 防止频繁加锁
11         @synchronized(self) {
12             if (_instance == nil) { // 防止创建多次
13                 _instance = [super allocWithZone:zone];
14             }
15         }
16     }
17     return _instance;
18 }
19 
20 + (instancetype)sharedSingleton{
21     if (_instance == nil) { // 防止频繁加锁
22         @synchronized(self) {
23             if (_instance == nil) { // 防止创建多次
24                 _instance = [[self alloc] init];
25             }
26         }
27     }
28     return _instance;
29 }
30 
31 - (id)copyWithZone:(NSZone *)zone{
32     return _instance;
33 }
34 @end

 1 #import "HMSingleton.h"
 2 
 3 @implementation Singleton
 4 static id _instance;
 5 
 6 /**
 7  *  当类加载到OC运行时环境中（内存），就会调用一次（一个类只会加载1次）
 8  */
 9 + (void)load{
10     _instance = [[self alloc] init];
11 }
12 
13 + (instancetype)allocWithZone:(struct _NSZone *)zone{
14     if (_instance == nil) { // 防止创建多次
15         _instance = [super allocWithZone:zone];
16     }
17     return _instance;
18 }
19 
20 + (instancetype)sharedSingleton{
21     return _instance;
22 }
23 
24 - (id)copyWithZone:(NSZone *)zone{
25     return _instance;
26 }
27 @end

 1 @implementation Singleton
 2 static id _instance;
 3 
 4 + (instancetype)allocWithZone:(struct _NSZone *)zone{
 5     static dispatch_once_t onceToken;
 6     dispatch_once(&onceToken, ^{
 7         _instance = [super allocWithZone:zone];
 8     });
 9     return _instance;
10 }
11 
12 + (instancetype)sharedSingleton{
13     static dispatch_once_t onceToken;
14     dispatch_once(&onceToken, ^{
15         _instance = [[self alloc] init];
16     });
17     return _instance;
18 }
19 
20 - (id)copyWithZone:(NSZone *)zone{
21     return _instance;
22 }
23 @end

1 #if __has_feature(objc_arc)
2 
3 //ARC环境
4 
5 #else
6 
7 //MRC环境
8 
9 #endif