Q : #if , #ifdef , #ifndef 的使用方法?A : #1,#2,#3.#1
#if condition(表达式1)
//程序段1
(#elif condition(表达式n)
//程序段n
)
...
(#else
//程序段3
)
#endif
说明 : 如果 表达式1(condition : true),则执行 程序段1,(否则如果 表达式2(condition : true),则执行 程序段n),(否则执行 程序段3). #2 #ifdef macro(标识符1) //程序段1 (#else //程序段2 ) #endif 说明 : 如果 有定义标识符1(#define macro),则执行 程序段1,(否则执行 程序段2). #3 #ifndef macro(标识符1) //程序段1 (#else //程序段2 ) #endif 说明 : 如果 没有定义标识符1(#define macro),则执行 程序段1,(否则执行 程序段2).与#2相反!

#ifdef __cplusplus
#define YY_EXTERN_C_BEGIN  extern "C" {#define YY_EXTERN_C_END  }
#else
#define YY_EXTERN_C_BEGIN#define YY_EXTERN_C_END/*
功能在C++环境中有定义 __cplusplus 这个宏,如果在C++环境中, YY_EXTERN_C_BEGIN和 YY_EXTERN_C_END中间的代码,编译器用C语言的编译格式来编译.因为 C++ 为了实现函数重载会把函数名和参数等联合起来合成一个中介的函数名，如果 C 函数也被这样编译会出问题.*/

/**Add this macro before each category implementation, so we don't have to use-all_load or -force_load to load object files from static libraries that onlycontain categories and no classes.More info: http://developer.apple.com/library/mac/#qa/qa2006/qa1490.html .*******************************************************************************Example:YYSYNTH_DUMMY_CLASS(NSString_YYAdd)*/
#ifndef YYSYNTH_DUMMY_CLASS
#define YYSYNTH_DUMMY_CLASS(_name_) \
@interface YYSYNTH_DUMMY_CLASS_ ## _name_ : NSObject @end \
@implementation YYSYNTH_DUMMY_CLASS_ ## _name_ @end
#endif

使用方法：
YYSYNTH_DUMMY_CLASS(NSArray_YYAdd)

/**Synthsize a dynamic object property in @implementation scope.It allows us to add custom properties to existing classes in categories.@param association  ASSIGN / RETAIN / COPY / RETAIN_NONATOMIC / COPY_NONATOMIC@warning #import <objc/runtime.h>*******************************************************************************Example:@interface NSObject (MyAdd)@property (nonatomic, retain) UIColor *myColor;@end#import <objc/runtime.h>@implementation NSObject (MyAdd)YYSYNTH_DYNAMIC_PROPERTY_OBJECT(myColor, setMyColor, RETAIN, UIColor *)@end*/
#ifndef YYSYNTH_DYNAMIC_PROPERTY_OBJECT
#define YYSYNTH_DYNAMIC_PROPERTY_OBJECT(_getter_, _setter_, _association_, _type_) \
- (void)_setter_ : (_type_)object { \[self willChangeValueForKey:@#_getter_]; \objc_setAssociatedObject(self, _cmd, object, OBJC_ASSOCIATION_ ## _association_); \[self didChangeValueForKey:@#_getter_]; \
} \
- (_type_)_getter_ { \return objc_getAssociatedObject(self, @selector(_setter_:)); \
}
#endif

.h中
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>#ifndef YYSYNTH_DYNAMIC_PROPERTY_OBJECT
#define YYSYNTH_DYNAMIC_PROPERTY_OBJECT(_getter_, _setter_, _association_, _type_) \
- (void)_setter_ : (_type_)object { \
[self willChangeValueForKey:@#_getter_]; \
objc_setAssociatedObject(self, _cmd, object, OBJC_ASSOCIATION_ ## _association_); \
[self didChangeValueForKey:@#_getter_]; \
} \
- (_type_)_getter_ { \
return objc_getAssociatedObject(self, @selector(_setter_:)); \
}
#endif@interface NSObject (MyAdd)@property (nonatomic, retain) NSString *myName;@end.m中
#import "NSObject+MyAdd.h"
#import <objc/runtime.h>@implementation NSObject (MyAdd)YYSYNTH_DYNAMIC_PROPERTY_OBJECT(myName, setMyName, RETAIN, NSString *);@end