利用类别(Category)为对象添加”实例变量”

主要利用ObjectiveC的C API中函数:

void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
id objc_getAssociatedObject(id object, const void *key)

原理详细参见官方的ObjectiveC Runtime Reference.

好了,现在直接写怎么做了.

1.在定义类别之前, 引入runtime.

#import <objc/runtime.h>

2.定义&实现类别(以UIView为例)
定义:

@interface UIView(AddVariables)

@property (nonatomic, retain) NSString *viewName;

@end

实现:

// 定义存取的Key
static const char *viewNameKey = "viewNameKey";

@implementation UIView(AddVariables)

// get方法
- (NSString *)viewName {
    return (NSString *)objc_getAssociatedObject(self, viewNameKey);
}
// set方法
- (void)setViewName:(NSString *)newViewNameKey {
    objc_setAssociatedObject(self, viewNameKey, newViewNameKey, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

3.调用测试:

- (void)viewDidLoad {

    UIView *testView = [[UIView alloc] initWithFrame:self.view.bounds];
    [self.view addSubview:testView];
    [testView release];

    [testView setViewName:@"Nori's View"];
    NSLog(@"[testView's newPropery]:%@", testView.viewName);

    [super viewDidLoad];
}

输出结果如下:

testView's newPropery:Nori's View

另外有几篇不错的文章关于类别和关联的

iOS学习系列 - 扩展机制category与associative

http://www.cnblogs.com/liping13599168/archive/2012/09/13/2682664.html

利用类别(Category)为对象添加”实例变量”

http://blog.csdn.net/qhexin/article/details/7692846

Objective-C 编程语言(7)关联

http://blog.csdn.net/zhangxingping/article/details/7619444