As a follow up for https://gist.github.com/michaelochs/f106b5c42aafe6bed74ac5dab82281c4 this is what's going on under the hood!

DynamicNSStringEvaluation.m

@interface MyStringProxy : NSProxy

@property (nonatomic) NSString *target;

@end


@implementation MyStringProxy

- (BOOL)respondsToSelector:(SEL)aSelector {
    if (aSelector == NSSelectorFromString(@"_dynamicContextEvaluation:patternString:")) {
        return YES;
    } else {
        return [self.target respondsToSelector:aSelector];
    }
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [self.target methodSignatureForSelector:sel];
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    [invocation invokeWithTarget:self.target];
}

- (NSString *)_dynamicContextEvaluation:(NSString *)context patternString:(NSString *)pattern {
    // Do a FORWARD search as it seems the same pattern is used for all the replacements, one by one.
    // So we are looking for the first appearance of `pattern` and return our replacement. The next
    // placeholder will see the partially replaced string and again looks for the first appearance of
    // `pattern` and so on...
    NSRange range = [context rangeOfString:pattern];
    if (range.location == 0) {
        return [NSString stringWithFormat:@"First(%@)", self.target];
    } else if (range.location + range.length == context.length) {
        return [NSString stringWithFormat:@"last(%@)", self.target];
    } else {
        return [NSString stringWithFormat:@"middle(%@)", self.target];
    }
}

@end


@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    MyStringProxy *proxy = [MyStringProxy alloc];
    proxy.target = @"something";

    NSString *test = [NSString stringWithFormat:@"%@ %@ %@", proxy, proxy, proxy];
    NSLog(@"%@", test); // "First(something) middle(something) last(something)"

    return YES;
}

@end