iOS：单例模式

目录
一，基本概念
二，优缺点
三，如何实现
四，封装成宏
五，实现可继承

一，基本概念

保证一个类全局只有一个实例，并且提供一个统一的入口访问该实例

二，优缺点

1，优点：全局只创建一个实例，节省内存资源
2，缺点：不能继承（下面会介绍如何实现）

三，如何实现

• 第一步：实现shared方法来创建实例

方法一：线程不安全，多线程情况下可能会创建多个实例

static id _instance = nil;
+ (instancetype)sharedInstance {
   if(!_instance) {
       _instance = [[self alloc] init];
   }
   return _instance;
}

方法二：加锁可以保证线程安全，但是会消耗性能

static id _instance = nil;
+ (instancetype)sharedInstance {
   @synchronized(self) { // 加锁
       if(!_instance) {
           _instance = [[self alloc] init];
       }
   }
   return _instance;
}

方法三：GCD是线程安全的，也能保证性能(官方推荐)

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

• 第二步：避免外部直接调用alloc/new/copy/mutableCopy来创建实例

方法一：重写alloc/copy/mutableCopy方法

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

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

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

方法二：禁用init/new/copy/mutableCopy方法

- (instancetype)init OBJC_UNAVAILABLE("use sharedInstance instead");
+ (instancetype)new OBJC_UNAVAILABLE("use sharedInstance instead");
- (instancetype)copy OBJC_UNAVAILABLE("use sharedInstance instead");
- (instancetype)mutableCopy OBJC_UNAVAILABLE("use sharedInstance instead");

四，封装成宏

作用：避免每个类都写一遍实现代码

1，重写方法的宏

// .h文件
#define singletonH \
+ (instancetype)sharedInstance;

// .m文件
#define singletonM \
static id _instance = nil; \
+ (instancetype)sharedInstance { \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
        _instance = [[self alloc] init]; \
    }); \
    return _instance; \
} \
+ (instancetype)allocWithZone:(struct _NSZone *)zone { \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
        _instance = [super allocWithZone:zone]; \
    }); \
    return _instance; \
} \
- (id)copyWithZone:(NSZone *)zone { \
    return _instance; \
} \
- (id)mutableCopyWithZone:(NSZone *)zone { \
    return _instance; \
}

2，禁用方法的宏

// .h文件
#define singletonH \
+ (instancetype)sharedInstance; \
- (instancetype)init OBJC_UNAVAILABLE("use sharedInstance instead"); \
+ (instancetype)new OBJC_UNAVAILABLE("use sharedInstance instead"); \
- (instancetype)copy OBJC_UNAVAILABLE("use sharedInstance instead"); \
- (instancetype)mutableCopy OBJC_UNAVAILABLE("use sharedInstance instead");

// .m文件
#define singletonM \
static id _instance = nil; \
+ (instancetype)sharedInstance { \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
        _instance = [[self alloc] init]; \
    }); \
    return _instance; \
}

3，如何使用

// .h文件
@interface HttpManager : NSObject
    singletonH
@end

// .m文件
@implementation HttpManager
    singletonM
@end

五，实现可继承

1，为什么不能继承：子类和父类共用一个实例，导致子类对象无法访问子类的属性和方法

// Person实现了单例，Man继承自Person，在Man中定义run方法
Person *p1 = [Person sharedInstance];
Person *p2 = [[Person alloc] init];
NSLog(@"p1: %@", p1);
NSLog(@"p2: %@", p2);
    
Man *m1 = [Man sharedInstance];
Man *m2 = [[Man alloc] init];
NSLog(@"m1: %@", m1);
NSLog(@"m2: %@", m2);
    
[m1 run];

// 打印结果
p1: <Person: 0x600000b95650>
p2: <Person: 0x600000b95650>
m1: <Person: 0x600000b95650>
m2: <Person: 0x600000b95650>
-[Person run]: unrecognized selector sent to instance 0x600000b95650

2，如何实现继承：使用runtime的关联函数

// Person.m的实现代码
+ (instancetype)sharedInstance {
    id _instance = objc_getAssociatedObject(self, @"instance");
    // 每个类都需要创建一次，所以不能用GCD
    @synchronized(self) {
        if(!_instance) {
            _instance = [[self alloc] init];
            objc_setAssociatedObject(self, @"instance", _instance, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
        }
    }
    return _instance;
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone {
    id _instance = objc_getAssociatedObject(self, @"instance");
    @synchronized(self) {
        if(!_instance) {
            _instance = [super allocWithZone:zone];
            objc_setAssociatedObject(self, @"instance", _instance, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
        }
    }
    return _instance;
}

- (id)copyWithZone:(NSZone *)zone {
    return objc_getAssociatedObject(self, @"instance");
}

- (id)mutableCopyWithZone:(NSZone *)zone {
    return objc_getAssociatedObject(self, @"instance");
}

// 打印结果
p1: <Person: 0x600000eeb4a0>
p2: <Person: 0x600000eeb4a0>
m1: <Man: 0x600000ef0ce0>
m2: <Man: 0x600000ef0ce0>
man is running