单例的实现（完整版代码）

#import "XMGTool.h"

 

static XMGTool * _instance;//静态变量保证了单例的唯一性，静态变量是程序一开始就存在的

 

@interface XMGTool ()<NSCopying, NSMutableCopying>

 

@end

 

@implementation XMGTool

 

+(instancetype)allocWithZone:(struct _NSZone *)zone

{

    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{

        _instance = [super allocWithZone:zone];

    });

    return _instance;

}

 

+(instancetype)shareTool

{

    return [[self alloc]init];

}

 

- (nonnull id)copyWithZone:(nullable NSZone *)zone {

    return _instance;

}

 

- (nonnull id)mutableCopyWithZone:(nullable NSZone *)zone {

    return _instance;

}

 

@end

 

 

外界的调用：

 

　　XMGTool *t1 = [[XMGTool alloc]init];

   XMGTool *t2 = [[XMGTool alloc]init];

   XMGTool *t3 = [XMGTool shareTool];

   XMGTool *t4 = [t1 mutableCopy];

    

    NSLog(@" %@-- %@-- %@--- %@",t1,t2,t3,t4);

 

打印结果

<XMGTool: 0x6000011b2e30>--

<XMGTool: 0x6000011b2e30>--

<XMGTool: 0x6000011b2e30>---

<XMGTool: 0x6000011b2e30>