A basic example of method swizzle

Method Swizzle

#import <objc/runtime.h>

@implementation UIViewController (Tracking)

+ (void)load {

  static dispatch_once_t onceToken;

  dispatch_once(&onceToken, ^{
    Class class = [self class];
    
    SEL originalSelector = @selector(viewWillAppear:);
    SEL swizzledSelector = @selector(xxx_viewWillAppear:);
    Method originalMethod = class_getInstanceMethod(class, originalSelector);
    Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
    
    // When swizzling a class method, use the following:
    
    // Class class = object_getClass((id)self);
    // ...
    // Method originalMethod = class_getClassMethod(class, originalSelector);
    // Method swizzledMethod = class_getClassMethod(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);
    }
  });
}

#pragma mark - Method Swizzling

- (void)xxx_viewWillAppear:(BOOL)animated {
  NSLog(@"XXXXXXXXXXXXXX viewWillAppear: %@ at %f", self, [[NSDate date] timeIntervalSince1970]);
  [self xxx_viewWillAppear:animated];
  NSLog(@"XXXXXXXXXXXXXX viewWillAppear: %@ at %f", self, [[NSDate date] timeIntervalSince1970]);
}

@end