Category添加属性与成员变量

Category是Objective-C中常用的语法特性，通过它可以很方便的为已有的类来添加函数。但是Category不允许为已有的类添加新的属性或者成员变量。     

 

一种常见的办法是通过runtime.h中objc_getAssociatedObject / objc_setAssociatedObject来访问和生成关联对象。通过这种方法来模拟生成属性。

头文件声明：

@interface NSObject (Extension)

@property (nonatomic, strong) NSString *userName;
@property (nonatomic, assign) SEL onImageFinish;

@end

实现的.m文件：

#import "NSObject+Extension.h"
#import 

static const void * g_UserNameKey = &g_UserNameKey;
static const void * g_OnImageFinish = &g_OnImageFinish;

@implementation NSObject (Extension)

@dynamic userName;

- (NSString *)userName {
    return objc_getAssociatedObject(self, g_UserNameKey);
}

- (void)setUserName:(NSString *)userName {
    objc_setAssociatedObject(self, g_UserNameKey, userName, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (SEL)onImageFinish {
    SEL action;
    NSValue *value = objc_getAssociatedObject(self, g_OnImageFinish);
    [value getValue:&action];
    return action;
}

- (void)setOnImageFinish:(SEL)action {
    NSValue *value = [NSValue value:&action withObjCType:@encode(SEL)];
    objc_setAssociatedObject(self, g_OnImageFinish, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end