NSAssert()

一.
NSAssert()是一个宏，用于开发阶段调试程序中的Bug，通过为NSAssert()传递条件表达式来断定是否属于Bug，满足条件返回真值，程序继续运行，如果返回假值，则抛出异常，并且可以自定义异常描述。
NSAssert()是这样定义的：

 #define NSAssert(condition, desc)

condition是条件表达式，值为YES或NO；desc为异常描述，通常为NSString。当conditon为YES时程序继续运行，为NO时，则程序异常抛出带有desc描述的异常信息。NSAssert()可以出现在程序的任何一个位置。

二.
在ios开发过程中，有时候会用到第三方的静态库(.a文件)，OC没有为每个函数（或者方法）定义链接符号，它只为每个类创建链接符号。这样当在一个静态库中使用类别来扩展已有类的时候，链接器不知道如何把类原有的方法和类别中的方法整合起来，就会导致你调用类别中的方法时，会出现selector not recognized的错误，从而导致app闪退。使用这段宏定义他可以虚拟新建一个与名字category 相同.h.m 让编译器 编译通过。即可解决上面的问题。

写在.m文件

/**
 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 only
 contain categories and no classes.
 More info: http://developer.apple.com/library/mac/#qa/qa2006/qa1490.html .
 *******************************************************************************

 Example:
      #import "NSObject+YYAdd.h"
      YYSYNTH_DUMMY_CLASS(NSObject_YYAdd)
 */
#ifndef YYSYNTH_DUMMY_CLASS
#define YYSYNTH_DUMMY_CLASS(_name_) \
@interface YYSYNTH_DUMMY_CLASS_ ## _name_ : NSObject @end \
@implementation YYSYNTH_DUMMY_CLASS_ ## _name_ @end
#endif

三.关于block,weak&strong

/**
 Synthsize a weak or strong reference.

 Example:
    @weakify(self)
    [self doSomething^{
        @strongify(self)
        if (!self) return;
        ...
    }];

 */
#ifndef weakify
    #if DEBUG
        #if __has_feature(objc_arc)
        #define weakify(object) autoreleasepool{} __weak __typeof__(object) weak##_##object = object;
        #else
        #define weakify(object) autoreleasepool{} __block __typeof__(object) block##_##object = object;
        #endif
    #else
        #if __has_feature(objc_arc)
        #define weakify(object) try{} @finally{} {} __weak __typeof__(object) weak##_##object = object;
        #else
        #define weakify(object) try{} @finally{} {} __block __typeof__(object) block##_##object = object;
        #endif
    #endif
#endif

#ifndef strongify
    #if DEBUG
        #if __has_feature(objc_arc)
        #define strongify(object) autoreleasepool{} __typeof__(object) object = weak##_##object;
        #else
        #define strongify(object) autoreleasepool{} __typeof__(object) object = block##_##object;
        #endif
    #else
        #if __has_feature(objc_arc)
        #define strongify(object) try{} @finally{} __typeof__(object) object = weak##_##object;
        #else
        #define strongify(object) try{} @finally{} __typeof__(object) object = block##_##object;
        #endif
    #endif
#endif

四. 实现动态给category 添加属性
具体参见http://www.jianshu.com/p/4a1531bac39f
普及一点宏定义 的关键字 一个# 表示要替换变量 ，两个 ##要拼接字符串 ，\表示多行 cmd 是iOS 是一种特定宏 ，表示本地方法， 在这里表示 @selector(_setter:)

/**
 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 
 *******************************************************************************
 Example:
     @interface NSObject (MyAdd)
     @property (nonatomic, retain) UIColor *myColor;
     @end
     
     #import 
     @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

/**
 Synthsize a dynamic c type property in @implementation scope.
 It allows us to add custom properties to existing classes in categories.
 
 @warning #import 
 *******************************************************************************
 Example:
     @interface NSObject (MyAdd)
     @property (nonatomic, retain) CGPoint myPoint;
     @end
     
     #import 
     @implementation NSObject (MyAdd)
     YYSYNTH_DYNAMIC_PROPERTY_CTYPE(myPoint, setMyPoint, CGPoint)
     @end
 */
#ifndef YYSYNTH_DYNAMIC_PROPERTY_CTYPE
#define YYSYNTH_DYNAMIC_PROPERTY_CTYPE(_getter_, _setter_, _type_) \
- (void)_setter_ : (_type_)object { \
    [self willChangeValueForKey:@#_getter_]; \
    NSValue *value = [NSValue value:&object withObjCType:@encode(_type_)]; \
    objc_setAssociatedObject(self, _cmd, value, OBJC_ASSOCIATION_RETAIN); \
    [self didChangeValueForKey:@#_getter_]; \
} \
- (_type_)_getter_ { \
    _type_ cValue = { 0 }; \
    NSValue *value = objc_getAssociatedObject(self, @selector(_setter_:)); \
    [value getValue:&cValue]; \
    return cValue; \
}
#endif

不用宏定义的写法

#import 
#import 

@interface NSObject (Extension)
@property (nonatomic, strong) NSArray *books;
@end

#import "NSObject+Extension.h"
@implementation NSObject (Extension)
-(void)setBooks:(NSArray *)books{
    objc_setAssociatedObject(self, @selector(setBooks:), books, OBJC_ASSOCIATION_RETAIN);
}
-(NSArray *)books{
    return objc_getAssociatedObject(self, @selector(setBooks:));
}
@end

使用：

#import "NSObject+Extension.h"
@implementation NSObject (Extension)
YYSYNTH_DYNAMIC_PROPERTY_OBJECT(books,setBooks,RETAIN, NSArray *);
@end

五. 写带有返回值的宏定义

/**
 Convert CFRange to NSRange
 @param range CFRange @return NSRange
 */
static inline NSRange YYNSRangeFromCFRange(CFRange range) {
    return NSMakeRange(range.location, range.length);
}