How to call blocks?

block copy.m

#import <Foundation/Foundation.h>


// 2nd class
@interface DownTheLine : NSObject
@end

@implementation DownTheLine
-(void)deepDown:(void (^)(int var))completionHandler {
    completionHandler(4);
}
@end


// 1st class
@interface FirstRecv : NSObject
@end

@implementation FirstRecv
-(void)first:(void (^)(int var))completionHandler {
    DownTheLine *down = [[DownTheLine alloc] init];
    
    [down deepDown:completionHandler];
}

-(void)second:(void (^)(int var))completionHandler {
    DownTheLine *down = [[DownTheLine alloc] init];
    
    [down deepDown:^(int var) {
        completionHandler(var);
    }];
}
@end

#define NUM 2000
// main
int main(int argc, char *argv[]) {  
    @autoreleasepool {
        FirstRecv *a = [[FirstRecv alloc] init];

        CFAbsoluteTime startTime;
        
        __block int counter;
        
        startTime = CFAbsoluteTimeGetCurrent();
        counter = 0;
        for (int i = 0; i < NUM; ++i) {
            [a first:^(int var) {
                //NSLog(@"1st run: %d", var);
                counter += i;
            }];
        }
        NSLog(@"%.2fms", 1000.0*(CFAbsoluteTimeGetCurrent() - startTime));

        
        startTime = CFAbsoluteTimeGetCurrent();
        counter = 0;
        for (int i = 0; i < NUM; ++i) {
            [a second:^(int var) {
                //NSLog(@"2nd run: %d", var);
                counter += i;
            }];
        }
        NSLog(@"%.2fms", 1000.0*(CFAbsoluteTimeGetCurrent() - startTime));
        
    }
}