ViewController.h

ViewController.h

//
//  ViewController.h
//  runtime_special_case
//
//  Created by tonyli on 4/10/18.
//  Copyright © 2018 tonyli. All rights reserved.
//

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

@interface ViewController : UIViewController

@end

@interface UIImage (UIImageExtension)
- (void)swizzle_imageNamed:(NSString *)name;
@end

@interface SubUIImage : UIImage
- (void)hello_imageNamed:(NSString *)name;
- (void)swizzle_imageNamed:(NSString *)name;
@end

ViewController.m

//
//  ViewController.m
//  runtime_special_case
//
//  Created by tonyli on 4/10/18.
//  Copyright © 2018 tonyli. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

//    UIImage*image= [SubUIImage imageNamed:@"trump180402.jpg"];
//    UIImageView*imageview = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
//    imageview.image = image;
//
//    [self.view addSubview:imageview];
    
    // should print `swizzled method hello!`
    [[SubUIImage new] swizzle_imageNamed:@"trump180402.jpg"];
    // should print `Value of original method = trump180402.jpg`
    [[UIImage new] swizzle_imageNamed:@"trump180402.jpg"];
}


- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

@implementation SubUIImage: UIImage

+ (void)load
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        
        SEL originalSelector = @selector(hello_imageNamed:);
        SEL swizzledSelector = @selector(swizzle_imageNamed:);
        
        // Use class_getInstanceMethod for "normal" methods
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        // Swap the two methods.
        BOOL success = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
        if (success) {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        }else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

- (void)hello_imageNamed:(NSString *)name
{
    NSLog(@"swizzled method hello!");
}

- (void)swizzle_imageNamed:(NSString *)name
{
    NSLog(@"Value of override method = %@", name);
}

@end

@implementation UIImage (UIImageExtension)
- (void)swizzle_imageNamed:(NSString *)name
{
    NSLog(@"Value of original method = %@", name);
}
@end