首先创建一个初始化工程，创建 HQViewController，在 HQViewController中加入一个 log 方法，如下:

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
}
- (void)log{ 
    NSLog(@"HQtest1");
}

然后创建一个子类HQSubViewController,重写 log方法中并在方法中实现调用原方法代码,如下:

- (void)viewDidLoad {
    [super viewDidLoad];
}

- (void)log{  
    NSLog(@"HQtest2");
    
    struct objc_super superClass = {
        .receiver = self,
        .super_class = class_getSuperclass(object_getClass(self))
    };
    
// 调用原来的方法
    void (*objc_msgSendSuperClass)(void *,SEL) = (void *)objc_msgSendSuper;
    objc_msgSendSuperClass(&superClass,_cmd);
//    _cmd在Objective-C的方法中表示当前方法的selector
    
}

再创建一个 NSObject 的 Category 负责进行方法交换，将原对象的 isa 指针指向该对象类的子类HQSubViewController:

@implementation NSObject (ChangeM)

+ (void)changeInstance:(NSObject *)instance method:(SEL)selector{
    
    Method oldMethod = class_getInstanceMethod([instance class], selector);
    if (!oldMethod) {
        NSLog(@"方法不存在");
        return;
    }
    
    Class newClass = [HQSubViewController class];
    
    // 将原类中的isa指针指向 子类
    object_setClass(instance, newClass);
}

@end

最后在 ViewControlller 中进行测试即可，此时的 ViewController 代码如下:

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor whiteColor];
    
    HQViewController *viewC1 = [[HQViewController alloc]init];
    [viewC1 log];
    
    HQViewController *viewC2 = [[HQViewController alloc]init];
    [HQViewController changeInstance:viewC2 method:@selector(log)];
    [viewC2 log];
    
}

@end

打印结果如下:

2017-03-28 11:51:44.288 HQChangeMethod[10862:625765] HQtest1
2017-03-28 11:51:44.289 HQChangeMethod[10862:625765] HQtest2
2017-03-28 11:51:44.289 HQChangeMethod[10862:625765] HQtest1

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