ios中给分类添加属性

ios中利用类别给已有的类扩展方法是可以的，但是如果直接的添加属性是会报错的。利用runtime可以达到添加属性的目的。

1.先创建一个分类，以下以UIImage为例子。

2.增加需要的属性。

3.导入runtime框架，重写set方法和get方法。

@interface UIImage (NJKUIImage)

@property(nonatomic, assign)NSInteger       tag;
@property(nonatomic, copy)NSString          *name;
@property(nonatomic, strong)NSDate          *data;

@end

#import "UIImage+NJKUIImage.h"
#import <objc/runtime.h>

@implementation UIImage (NJKUIImage)

static const void *njk_view_tag = @"njk_image_tag";
static const void *njk_view_name = @"njk_view_name";
static const void *njk_view_data = @"njk_view_data";

- (void)setTag:(NSInteger)tag{
//常量属性只需要转换一下
    NSString *string = [NSString stringWithFormat:@"%ld",(long)tag];
    objc_setAssociatedObject(self, &njk_view_tag, string, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (NSInteger)tag{
//常量属性读取时也需要添加转换
    return [objc_getAssociatedObject(self, &njk_view_tag)integerValue];
}

- (void)setName:(NSString *)name{
    objc_setAssociatedObject(self, njk_view_name, name, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (NSString *)name{
    return objc_getAssociatedObject(self, njk_view_name);
}

- (void)setData:(NSDate *)data{
    objc_setAssociatedObject(self, &njk_view_data, data, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (NSDate *)data{
    return objc_getAssociatedObject(self, &njk_view_data);
}

@end

//其中注意以下的参数是用来表示创建的属性的类型的

typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
    OBJC_ASSOCIATION_ASSIGN = 0,           /**< Specifies a weak reference to the associated object. */
    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object. 
                                            *   The association is not made atomically. */
    OBJC_ASSOCIATION_COPY_NONATOMIC = 3,   /**< Specifies that the associated object is copied. 
                                            *   The association is not made atomically. */
    OBJC_ASSOCIATION_RETAIN = 01401,       /**< Specifies a strong reference to the associated object.
                                            *   The association is made atomically. */
    OBJC_ASSOCIATION_COPY = 01403          /**< Specifies that the associated object is copied.
                                            *   The association is made atomically. */
};