iOS 运行时添加属性和方法

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;
}
@end
NSString *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：

Steve
Jobs

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);
}