Method swizzling doesn't work in Objective-C

Question

I want to test method swizzling in Objective-C:

1. Add a method goodName in AViewController

   #import "AViewController.h"
   #import "UIViewController+Tracking.h"
   @interface AViewController ()
   
   @end
   
   @implementation AViewController
    - (void)viewDidLoad {
       [super viewDidLoad];
       [self goodName];
   }
    - (void) goodName {
       NSLog(@"I am a good Name");
   }
   @end
   

2. Add a category for swizzling goodName method.

   #import "UIViewController+Tracking.h"
   #import <objc/runtime.h>
   #import "AViewController.h"
   @implementation UIViewController (Tracking)
   
   + (void) load {
       static dispatch_once_t onceToken;
       dispatch_once(&onceToken, ^{
           Class class = [self class];
           SEL originalSelector = @selector(goodName);
           SEL swizzledSelector = @selector(xxx_xxx_goodName);
   
           Method originalMethod = class_getInstanceMethod(class, originalSelector);
           Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
   
           BOOL didAddMethod = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
   
           if (didAddMethod) {
               class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
           } else {
               method_exchangeImplementations(originalMethod, swizzledMethod);
           }
       });
   }
   
   - (void)xxx_xxx_goodName {
       NSLog(@"i am xxx goodName");
       [self xxx_xxx_goodName];
   }
   
   @end
   

3. If everything works well, the goodName was called will output i am xxx goodName, but it's output I am a good Name.

Any part wrong?

Answer 1

You're trying to swizzle methods in AViewController but you put the code in +(void)load for base UIViewController. Those are two separate class objects so you need to do it in +(void)load for AViewController.