单例模式-用GCD实现

用GCD实现单例模式的步骤：

步骤1. 创建头文件 XZSingleton.h，里面代码如下：

// .h文件
#define XZSingletonH(name) + (instancetype)shared##name;

// .m文件
#if __has_feature(objc_arc)

    #define XZSingletonM(name) \
    static id _instace; \
 \
    + (id)allocWithZone:(struct _NSZone *)zone \
    { \
        static dispatch_once_t onceToken; \
        dispatch_once(&onceToken, ^{ \
            _instace = [super allocWithZone:zone]; \
        }); \
        return _instace; \
    } \
 \
    + (instancetype)shared##name \
    { \
        static dispatch_once_t onceToken; \
        dispatch_once(&onceToken, ^{ \
            _instace = [[self alloc] init]; \
        }); \
        return _instace; \
    } \
 \
    - (id)copyWithZone:(NSZone *)zone \
    { \
        return _instace; \
    }

#else

    #define XZSingletonM(name) \
    static id _instace; \
 \
    + (id)allocWithZone:(struct _NSZone *)zone \
    { \
        static dispatch_once_t onceToken; \
        dispatch_once(&onceToken, ^{ \
            _instace = [super allocWithZone:zone]; \
        }); \
        return _instace; \
    } \
 \
    + (instancetype)shared##name \
    { \
        static dispatch_once_t onceToken; \
        dispatch_once(&onceToken, ^{ \
            _instace = [[self alloc] init]; \
        }); \
        return _instace; \
    } \
 \
    - (id)copyWithZone:(NSZone *)zone \
    { \
        return _instace; \
    } \
 \
    - (oneway void)release { } \
    - (id)retain { return self; } \
    - (NSUInteger)retainCount { return ;} \
    - (id)autorelease { return self;}

#endif

步骤2. 要实现的单例类，比如 XZDataTool，XZDataTool.h   XZDataTool.m代码分别 如下：

#import <Foundation/Foundation.h>

@interface XZDataTool : NSObject
XZSingletonH(DataTool)
@end

#import "XZDataTool.h"

@implementation XZDataTool
XZSingletonM(DataTool)
@end