第一种：runtime.h里的方法BOOL class_addProperty(Class cls, const char *name,

const objc_property_attribute_t *attributes, unsigned int attributeCount)

#include <objc/runtime.h>

#import <Foundation/Foundation.h>

@interface SomeClass : NSObject {

NSString *_privateName;}@end@implementation SomeClass- (id)init

{

self = [super init];

if (self) _privateName = @"Steve";

return self;}@endNSString *nameGetter(id self, SEL _cmd)

{

Ivar ivar = class_getInstanceVariable([SomeClass class], "_privateName");

return object_getIvar(self, ivar);}

void nameSetter(id self, SEL _cmd, NSString *newName)

{

Ivar ivar = class_getInstanceVariable([SomeClass class], "_privateName");

id oldName = object_getIvar(self, ivar);

if (oldName != newName) object_setIvar(self, ivar, [newName copy]);}int main(void)

{

@autoreleasepool { objc_property_attribute_t type = { "T", "@/"NSString/"" };

objc_property_attribute_t ownership = { "C", "" }; // C = copy

objc_property_attribute_t backingivar = { "V", "_privateName" };

objc_property_attribute_t attrs[] = { type, ownership, backingivar };

class_addProperty([SomeClass class], "name", attrs, 3);

class_addMethod([SomeClass class], @selector(name), (IMP)nameGetter, "@@:");

class_addMethod([SomeClass class], @selector(setName:), (IMP)nameSetter, "v@:@");

id o = [SomeClass new];

NSLog(@"%@", [o name]);

[o setName:@"Jobs"];

NSLog(@"%@", [o name]);

}}输出：SteveJobs

第二种：- (id)valueForUndefinedKey:(NSString *)key

第三种：static char const * const ObjectTagKey;@implementation NSObject

(ExampleCategoryWithProperty)@dynamic objectTag;- (id)objectTag

{

return objc_getAssociatedObject(self, ObjectTagKey);

}

- (void)setObjectTag:(id)newObjectTag

{

objc_setAssociatedObject(self,

ObjectTagKey, newObject,

OBJC_ASSOCIATION_RETAIN_NONATOMIC);}

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