iOS_单例

 1 @implementation MAShareTool
 2 /**
 3  步骤:
 4  1.一个静态变量_inastance
 5  2.重写allocWithZone, 在里面用dispatch_once, 并调用super allocWithZone
 6  3.自定义一个sharedXX, 用来获取单例. 在里面也调用dispatch_once, 实例化_instance
 7  -----------可选------------
 8  4.如果要支持copy. 则(先遵守NSCopying协议)重写copyWithZone, 直接返回_instance即可.
 9  
10  
11  */
12 /**第1步: 存储唯一实例*/
13 static MAShareTool *_instance;
14 
15 /**第2步: 分配内存时都会调用这个方法. 保证分配内存alloc时都相同*/
16 +(id)allocWithZone:(struct _NSZone *)zone{
17     //调用dispatch_once保证在多线程中也只被实例化一次
18     static dispatch_once_t onceToken;
19     dispatch_once(&onceToken, ^{
20         _instance = [super allocWithZone:zone];
21     });
22     return _instance;
23 }
24 
25 /**第3步: 保证init初始化时都相同*/
26 +(instancetype)sharedTool{
27     static dispatch_once_t onceToken;
28     dispatch_once(&onceToken, ^{
29         _instance = [[MAShareTool alloc] init];
30     });
31     return _instance;
32 }
33 
34 /**第4步: 保证copy时都相同*/
35 -(id)copyWithZone:(NSZone *)zone{
36     return _instance;
37 }
38 
39 @end

-(void)viewDidLoad{
    //实例化一个类的几种方法. 单例就是要保证实例化出来的类是同一个类
    //1.alloc init方法. 一般不这么来调用单例.
    MAShareTool *t1 = [[MAShareTool alloc] init];
    MAShareTool *t2 = [[MAShareTool alloc] init];
    
    //2.类方法
    MAShareTool *t3 = [MAShareTool sharedTool];
    
    //3.copy
    MAShareTool *t4 = [t3 copy];
    
    NSLog(@"%@ %@ %@ %@", t1, t2, t3, t4);
}

#import "MAShareTool.h"

@implementation MAShareTool

static MAShareTool *_instance;

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

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

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

#pragma mark - MRC中需要覆盖的方法
//不需要计数器+1
- (id)retain {
    return self;
}

//不需要. 堆区的对象才需要
- (id)autorelease {
    return self;
}

//不需要
- (oneway void)release {
}

//不需要计数器个数. 直接返回最大无符号整数
- (NSUInteger)retainCount {
    return UINT_MAX;  //参照常量区字符串的retainCount
}

@end

//=============================ARC/MRC整合=======================================
#pragma mark - MRC中需要覆盖的方法, ARC与MRC的整合
#if !__has_feature(objc_arc)
- (id)retain {
    return self;
}

- (id)autorelease {
    return self;
}

- (oneway void)release {
}

- (NSUInteger)retainCount {
    return UINT_MAX;
}

#endif
//============================================================================