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);}