单例里你不知道的事儿

一位开发前辈玉令天下的博客里对单例的介绍很不错，原文iOS的单例模式在这里作两点补充。

• 即使用GCD的dispatch_once 创建的单例，最好还是遵循<NSCopying,NSMutableCopying>的协议，并实现相应的方法，防止别人误调copy方法而崩溃

在这里我写了个demo
Singleton.h文件如下

#import <Foundation/Foundation.h>

@interface Singleton : NSObject

+(instancetype) shareInstance;

@property (copy, nonatomic) NSString *name;
@property (copy, nonatomic) NSString *title;

@end

Singleton.m文件如下

#import "Singleton.h"

@interface Singleton () <NSCopying,NSMutableCopying>

@end

@implementation Singleton

static Singleton* _instance = nil;

+(instancetype) shareInstance {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [[super allocWithZone:NULL] init];//调用[[super allocWithZone:NULL] init],因为本类已经重载allocWithZone基本的对象分配方法,要借用父类(NSObject)的功能来帮助处理底层内存分配
        _instance.name = @"";
        _instance.title = @"";
    });
    
    return _instance;
}

+(id)allocWithZone:(struct _NSZone *)zone {
//    return [Singleton shareInstance];
    return _instance;//两种写法效果是一样的
}

- (id)copyWithZone:(nullable NSZone *)zone {
//    return [Singleton shareInstance];
    return _instance;
}

- (id)mutableCopyWithZone:(nullable NSZone *)zone {
//    return [Singleton shareInstance];
    return _instance;
}

- (NSString *)description {
    return [NSString stringWithFormat:@"_name====%@,_title=====%@",_name,_title];
}

测试方法如下

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    Singleton *singleton = [Singleton shareInstance];
    singleton.name = @"singleton";
    singleton.title = @"singletonDesc";
    NSLog(@"singleton------%p,description------%@",singleton,singleton);
    
    Singleton *singletonCopy = [singleton copy];
    NSLog(@"singletonCopy------%p,description------%@",singletonCopy,singletonCopy);
    
    Singleton *singletonMutableCopy = [singleton mutableCopy];
    NSLog(@"singletonMutableCopy------%p,description------%@",singletonMutableCopy,singletonMutableCopy);
    
    Singleton *singletonShareInstanceAgain = [Singleton shareInstance];
    NSLog(@"singletonShareInstanceAgain------%p,singletonShareInstanceAgain------%@",singletonShareInstanceAgain,singletonShareInstanceAgain);

    Singleton *singletonAlloc = [[Singleton alloc] init];
    NSLog(@"singletonAlloc------%p,description------%@",singletonAlloc,singletonAlloc);
}

打印结果如下

2018-08-15 15:00:31.441223+0800 SingletonTest[65282:7781841] singleton------0x60400042c800,description------_name====singleton,_title=====singletonDesc
2018-08-15 15:00:31.441499+0800 SingletonTest[65282:7781841] singletonCopy------0x60400042c800,description------_name====singleton,_title=====singletonDesc
2018-08-15 15:00:31.441726+0800 SingletonTest[65282:7781841] singletonMutableCopy------0x60400042c800,description------_name====singleton,_title=====singletonDesc
2018-08-15 15:00:31.441855+0800 SingletonTest[65282:7781841] singletonShareInstanceAgain------0x60400042c800,singletonShareInstanceAgain------_name====singleton,_title=====singletonDesc
2018-08-15 15:00:31.442457+0800 SingletonTest[65282:7781841] singletonAlloc------0x60400042c800,description------_name====singleton,_title=====singletonDesc

可以看出shareInstance、copy、mutableCopy、alloc创建出来是同一个单例，内存地址相同

• 单例对象的释放：在所有使用该单例的对象都释放后，单例对象本身也会自己释放 原文地址

+(instancetype)shareInstance {
    static __weak SingletonClass *instance;
    SingletonClass *strongInstance = instance;
    @synchronized(self) {
        if (strongInstance == nil) {
            strongInstance = [[[self class] alloc] init];
            instance = strongInstance;
        }
    }
    return strongInstance;
}

写了个demo验证了，这样创建的单例确实会随着持有对象的销毁而销毁

2018-08-15 15:45:26.197552+0800 SingletonTest[65879:7837310] instanceAddress=0x600000000ef0,_content====the VC Hold Instance
2018-08-15 15:45:28.625037+0800 SingletonTest[65879:7837310] instanceAddress=0x600000000f10,_content====the Instance after VC dealloc Information

大概的思路是，从A控制器进入B控制器，B控制器创建一个单例，打印此单例的信息，然后B控制器回到A控制器，在A控制器创建单例并打印单例的信息，两次打印的信息对比地址不同。

如果有错误的地方，欢迎指正。