Objective-C

/** 原文件 */
// Person.h
#import <Foundation/Foundation.h>

@interface Person : NSObject
@property (nonatomic, copy) NSString *name;
@end



// Person.m
#import "Person.h"

@implementation Person
- (instancetype)init {
    if (self = [super init]) {
        _name = @"";
        
        NSLog(@"%@", [self class]); // Person
        NSLog(@"%@", [super class]); // Person
    }
    return self;
}

- (void)dealloc {
    NSLog(@"Call dealloc in Person.m");
}
@end

/** Category文件*/
// Person+Category.h
#import "Person.h"

@interface Person (Category)
// 添加属性address
@property (nonatomic, copy) NSString *address;
@end


// Person+Category.m
#import "Person+Category.h"
#import <objc/runtime.h>

@implementation Person (Category)

static void *addressKey = &addressKey;

#pragma mark - OC 在Category关联属性
- (void)setAddress:(NSString *)address {
    objc_setAssociatedObject(self, &addressKey, address, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

- (NSString *)address {
    return objc_getAssociatedObject(self, &addressKey);
}

/** 在category里面执行dealloc方法后，Person类文件里的dealloc会不执行 */
- (void)dealloc {
    objc_removeAssociatedObjects(self);
    NSLog(@"remove associated");
}
@end

// test
#import "Person+Category.h"

Person *p = [Person new];
p.name = @"xiaoMing";
p.address = @"gz";
NSLog(@"name is: %@, address is: %@", p.name, p.address);

Swift

// .swift文件
import Cocoa

class Car {
    var name = ""
}


private var priceKey: Void?
extension Car {
    /// car's price
    var price: Double? {
        get {
            return objc_getAssociatedObject(self, &priceKey) as? Double
        }
        set {
                objc_setAssociatedObject(self, &priceKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}

let c1 = Car()
c1.name = "su"
c1.price = 110000.1
print("c1's name is: \(c1.name) and price is: \(c1.price!)")

iOS之Category关联属性