Example for method_exchangeImplementations

MethodExhange1.m

#import "ViewController.h"
#import <objc/runtime.h>

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    Class cls = self.class;
    Method m1 = class_getInstanceMethod(cls, @selector(methodA));
    Method m2 = class_getInstanceMethod(cls, @selector(methodB));
    method_exchangeImplementations(m1, m2);

    [self methodA];
    // methodB called
}

- (void)methodA
{
    NSLog(@"methodA called");
}

- (void)methodB
{
    NSLog(@"methodB called");
}

@end

MethodExhange2.m

#import "ViewController.h"
#import <objc/runtime.h>

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    Class cls = self.class;
    Method m1 = class_getInstanceMethod(cls, @selector(methodA));
    Method m2 = class_getInstanceMethod(cls, @selector(methodB));
    method_exchangeImplementations(m1, m2);

    [self methodA];
    // methodB called
    // methodA called
}

- (void)methodA
{
    NSLog(@"methodA called");
}

- (void)methodB
{
    NSLog(@"methodB called");
    [self methodB];
}

@end