单例模式简单实现

 1 @interface HMMusicTool : NSObject
 2 + (instancetype)sharedMusicTool;
 3 @end
 4 
 5 @implementation HMMusicTool
 6 static id _instance;
 7 
 8 /**
 9  *  alloc方法内部会调用这个方法
10  */
11 + (id)allocWithZone:(struct _NSZone *)zone
12 {
13     if (_instance == nil) { // 防止频繁加锁
14         @synchronized(self) {
15             if (_instance == nil) { // 防止创建多次
16                 _instance = [super allocWithZone:zone];
17             }
18         }
19     }
20     return _instance;
21 }
22 
23 + (instancetype)sharedMusicTool
24 {
25     if (_instance == nil) { // 防止频繁加锁
26         @synchronized(self) {
27             if (_instance == nil) { // 防止创建多次
28                 _instance = [[self alloc] init];
29             }
30         }
31     }
32     return _instance;
33 }
34 
35 - (id)copyWithZone:(NSZone *)zone
36 {
37     return _instance;
38 }
39 @end