[Objective-C] Swizzling（ランタイムAPI）を使ってDebugをしやすくする ref: http://qiita.com/edo_m18/items/9c57f4714d5c14a990c4

file0.m

@interface NSString (sample)

- (NSString *)swapLowercaseString;

@end

////////////////////////////////////////

@implementation NSString (sample)

- (NSString *)swapLowercaseString
{
    NSString *lowercase = [self swapLowercaseString];
    NSLog(@"Called. %@", lowercase);
    return lowercase;
}

@end

file2.m

NSString *sample1 = @"This is Sample";
NSString *sample2 = [sample1 lowercaseString]; // => "Called. this is sample"

NSLog(@"%@", sample2); // => "this is sample"

main.m

#import <objc/runtime.h>
#import "NSString+sample.h"

int main(int argc, char * argv[])
{
    // NSString#lowercaseStringの実装を取り出す
    Method lowercaseString     = class_getInstanceMethod([NSString class], @selector(lowercaseString));

    // カテゴリで追加した`swapLowercaseString｀の実装を取り出す
    Method swapLowercaseString = class_getInstanceMethod([NSString class], @selector(swapLowercaseString));

    // 取り出した実装を入れ替える
    method_exchangeImplementations(lowercaseString, swapLowercaseString);

    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([SampleAppDelegate class]));
    }
}