Objective-C 单例宏

实现

先贴出代码
也可以直接访问我的github:RWSingleton

#import <objc/runtime.h>

#define RW_DECLARE_SINGLETON_FOR_CLASS_WITH_ACCESSOR(classname, accessorMethodName) \
+ (classname *)accessorMethodName;

#if __has_feature(objc_arc)
    #define RW_SYNTHESIZE_SINGLETON_RETAIN_METHODS
#else
    #define RW_SYNTHESIZE_SINGLETON_RETAIN_METHODS \
    - (id)retain \
    { \
        return self; \
    } \
    \
    - (NSUInteger)retainCount \
    { \
        return NSUIntegerMax; \
    } \
    \
    - (oneway void)release \
    { \
    } \
    \
    - (id)autorelease \
    { \
        return self; \
    }
#endif

#define RW_SYNTHESIZE_SINGLETON_FOR_CLASS_WITH_ACCESSOR(classname, accessorMethodName) \
\
static classname *accessorMethodName##Instance = nil; \
\
+ (classname *)accessorMethodName \
{ \
    static dispatch_once_t onceToken;\
    dispatch_once(&onceToken,^{\
        accessorMethodName##Instance = [super allocWithZone:NULL]; \
        accessorMethodName##Instance = [accessorMethodName##Instance init]; \
        method_exchangeImplementations(\
                class_getInstanceMethod([accessorMethodName##Instance class], @selector(init)),\
                class_getInstanceMethod([accessorMethodName##Instance class], @selector(init_once)));\
    });\
    return accessorMethodName##Instance; \
}\
\
+ (id)allocWithZone:(NSZone *)zone \
{ \
    return [self accessorMethodName]; \
} \
\
- (id)copyWithZone:(NSZone *)zone \
{ \
    return self; \
} \
- (id)init_once\
{ \
    return self; \
} \
RW_SYNTHESIZE_SINGLETON_RETAIN_METHODS

#define RW_DECLARE_SINGLETON_FOR_CLASS(classname) RW_DECLARE_SINGLETON_FOR_CLASS_WITH_ACCESSOR(classname, shared##classname)
#define RW_SYNTHESIZE_SINGLETON_FOR_CLASS(classname) RW_SYNTHESIZE_SINGLETON_FOR_CLASS_WITH_ACCESSOR(classname, shared##classname)

使用

例如你需要一个名为MyOjbect的单例. 在MyOjbect.h中,代码如下：

#import "RWSingletonMacro.h"
@interface MyObject : NSObject
RW_DECLARE_SINGLETON_FOR_CLASS_WITH_ACCESSOR(MyObject, sharedObject)
@end

在MyObject.m中:

#import "MyObject.h"
@implementation MyObject
RW_SYNTHESIZE_SINGLETON_FOR_CLASS_WITH_ACCESSOR(MyObject, sharedObject)
@end

注意

很多时候，单例都会拥有自己的instance varible,所以这里做了method siwwizling, 你可以重载
-(id)init,在其中实现你的初始化逻辑.MyObject.m看起来如下:

#import "MyObject.h"
@implementation MyObject
RW_SYNTHESIZE_SINGLETON_FOR_CLASS_WITH_ACCESSOR(MyObject, sharedObject)

- (id) init
{
  self = [super init];
  //do your things
  return self
}
@end

此外，从实现中可得知，即便是如下代码也可以保证obj是单例。

MyObject* obj = [[MyObject alloc] init];

原作写于segmentfault 链接