DemoSwizzle.h

//
//  ViewController+Tracking.h
//  OCLearning
//
//  Created by Manish Verma on 26/07/16.
//  Copyright © 2016 Manish Verma. All rights reserved.
//

#import "ViewController.h"

@interface ViewController (Tracking)

@end

DemoSwizzle.m

//
//  ViewController+Tracking.m
//  OCLearning
//
//  Created by Manish Verma on 26/07/16.
//  Copyright © 2016 Manish Verma. All rights reserved.
//

#import "ViewController+Tracking.h"

#import "NSObject+Swizzling.h"

@implementation ViewController (Tracking)

+ (void)load {
    [self swizzleInstanceMethodInClass:[self class]
                      originalSelector:@selector(viewWillAppear:)
                  withSwizzledSelector:@selector(mv_ViewWillAppear:)];
}

- (void)mv_ViewWillAppear:(BOOL)animated {
    NSLog(@"View controller tracking category: ViewWillAppear");
    
    [self mv_ViewWillAppear:YES];
}

@end

NSObject+Swizzling.h

//
//  NSObject+Swizzling.h
//  OCLearning
//
//  Created by Manish Verma on 26/07/16.
//  Copyright © 2016 Manish Verma. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

@interface NSObject (Swizzling)

+ (void)swizzleInstanceMethodInClass:(Class)class
                                    originalSelector:(SEL)originalSelector
                                    withSwizzledSelector:(SEL)swizzledSelector;

+ (void)swizzleStaticMethodInClass:(Class)class
                                    originalSelector:(SEL)originalSelector
                                    withSwizzledSelector:(SEL)swizzledSelector;

@end

NSObject+Swizzling.m

//
//  NSObject+Swizzling.m
//  OCLearning
//
//  Created by Manish Verma on 26/07/16.
//  Copyright © 2016 Manish Verma. All rights reserved.
//

#import "NSObject+Swizzling.h"

@implementation NSObject (Swizzling)

+ (void)swizzleInstanceMethodInClass:(Class)class
                        originalSelector:(SEL)originalSelector
                        withSwizzledSelector:(SEL)swizzledSelector
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        
        Method originalMethod   = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod   = class_getInstanceMethod(class, swizzledSelector);
        
        BOOL didAddedMethod     = class_addMethod(class,
                                            originalSelector,
                                            method_getImplementation(swizzledMethod),
                                            method_getTypeEncoding(swizzledMethod));
        
        if (didAddedMethod) {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        }else{
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

+ (void)swizzleStaticMethodInClass:(Class)class
                  originalSelector:(SEL)originalSelector
              withSwizzledSelector:(SEL)swizzledSelector
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        
        Method originalMethod   = class_getClassMethod(class, originalSelector);
        Method swizzledMethod   = class_getClassMethod(class, swizzledSelector);
        
        BOOL didAddedMethod     = class_addMethod(class,
                                                  originalSelector,
                                                  method_getImplementation(swizzledMethod),
                                                  method_getTypeEncoding(swizzledMethod));
        
        if (didAddedMethod) {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        }else{
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

@end