Monkey-Patching iOS with Objective-C Categories Part III: Swizzling

NSObject_Swizzle.h

#import <Foundation/Foundation.h>

@interface NSObject (Swizzle)

+ (void)swizzleInstanceSelector:(SEL)originalSelector 
                withNewSelector:(SEL)newSelector;

@end

NSObject_Swizzle.m

#import "NSObject+Swizzle.h"
#import <objc/runtime.h>

@implementation NSObject (Swizzle)

+ (void) swizzleInstanceSelector:(SEL)originalSelector 
                 withNewSelector:(SEL)newSelector
{
  Method originalMethod = class_getInstanceMethod(self, originalSelector);
  Method newMethod = class_getInstanceMethod(self, newSelector);

  BOOL methodAdded = class_addMethod([self class],
                                     originalSelector,
                                     method_getImplementation(newMethod),
                                     method_getTypeEncoding(newMethod));
  
  if (methodAdded) {
    class_replaceMethod([self class], 
                        newSelector, 
                        method_getImplementation(originalMethod),
                        method_getTypeEncoding(originalMethod));
  } else {
    method_exchangeImplementations(originalMethod, newMethod);
  }
}

@end

SwizzleExample.m

SEL firstMethodSelector = @selector(firstMethod);
SEL secondMethodSelector = @selector(secondMethod);
Method firstMethod = class_getInstanceMethod(self, firstMethodSelector);
Method secondMethod = class_getInstanceMethod(self, secondMethodSelector);
 
BOOL methodAdded = class_addMethod([self class],
                                   firstMethodSelector,
                                   method_getImplementation(secondMethod),
                                   method_getTypeEncoding(secondMethod));
  
if (methodAdded) {
class_replaceMethod([self class], 
                      secondMethodSelector, 
                      method_getImplementation(firstMethod),
                      method_getTypeEncoding(firstMethod));
} else {
  method_exchangeImplementations(firstMethod, secondMethod);
}

SwizzleUIVIewController.m

@implementation UIViewController (TourGuide)

+ (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
   SEL viewWillAppearSelector = @selector(viewWillAppear:);
   SEL tourGuideWillAppearSelector =  @selector(tourGuideWillAppear:);
   Method originalMethod = class_getInstanceMethod(self, viewWillAppearSelector);
   Method newMethod = class_getInstanceMethod(self, tourGuideWillAppearSelector);
    
   BOOL methodAdded = class_addMethod([self class],
                                      viewWillAppearSelector,
                                      method_getImplementation(newMethod),
                                      method_getTypeEncoding(newMethod));
     
   if (methodAdded) {
   class_replaceMethod([self class], 
                         tourGuideWillAppearSelector, 
                         method_getImplementation(originalMethod),
                         method_getTypeEncoding(originalMethod));
   } else {
     method_exchangeImplementations(originalMethod, newMethod);
   }
  });
}

UIViewController_TourGuide.h

#import <UIKit/UIKit.h>

@interface UIViewController (TourGuide)

@property (nonatomic, retain) NSArray* tourSteps;

@end

UIViewController_TourGuide.m

#import "UIViewController_TourGuide.h"
#import "NSObject_Swizzle.h"

static char tourStepsKey;

@implementation UIViewController (TourGuide)

+ (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    [self swizzleInstanceSelector:@selector(viewWillAppear:) 
                  withNewSelector:@selector(tourGuideWillAppear:)];
  });
}

- (NSArray*) getTourSteps {
  return objc_getAssociatedObject(self, &tourStepsKey);
}

- (void) setTourSteps:(NSArray*)tourSteps {
  objc_setAssociatedObject(self, &tourStepsKey,
    tourSteps, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (void) displayTour {
  // run the tour, and save the fact it was run
}

- (BOOL) isTourRan {
  // return TRUE if tour has been run
}

- (void) tourGuideWillAppear:(BOOL)animated {
  // call the old implementation, now under the new message signature
  [self tourGuideWillAppear:animated];
  
  if (![self isTourRan]) {
    [self displayTour];
  }
}

@end

UIViewController_TourGuide_Override.m

#import "UIViewController_TourGuide.h"

@implementation UIViewController (TourGuide)

//...

- (void) tourGuideWillAppear:(BOOL)animated {
  // call the old implementation, now under the new message signature
  [self tourGuideWillAppear:animated];
  
  if (![self isTourRan]) {
    [self displayTour];
  }
}

//...

@end