オブジェクトを文字列化するときの書式を変える

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

int main(int argc, char *argv[]) {
    @autoreleasepool {
        NSLog(@"%@", [NSDate distantFuture]); // 4001-01-01 00:00:00 +0000

        Class nsdate = [NSDate class];

        // と言いつつ今回差し替えるのは`descriptionWithLocale:`
        // NSDateは他に`description`と`descriptionWithCalendarFormat:timeZone:locale:`がある
        SEL name = @selector(descriptionWithLocale:);

        // 新しい`descriptionWithLocale:`メソッド、Blocksの第一引数が`self`の代わりになる
        IMP description = imp_implementationWithBlock(^NSString*(id obj, id locale) {
            NSDateFormatter* format = [[NSDateFormatter alloc] init];
            [format setDateFormat:@"yyyy/MM/dd HH:mm:ss"];
            return [format stringFromDate:obj];
        });

        class_replaceMethod(nsdate, name, description,
            method_getTypeEncoding(class_getInstanceMethod(nsdate, name)));

        NSLog(@"%@", [NSDate distantFuture]); // 4001/01/01 09:00:00

        return 0;
    }
}