Solving the warning in compiler: Sending 'BarCodeViewController *_strong' to parameter of incompatible type 'id<PerksDetailsDelegate>'
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"showBarCode"]) {
        BarcodeViewController *barCodeVC = (BarcodeViewController *)segue.destinationViewController;

        [self setDelegate:barCodeVC];

    }
}

#import Card.h
#import PerksDetailsViewController.h
#import BarcodeViewController.h

@interface PerksDetailsViewController ()

@end

@implementation
@synthesize delegate = _delegate;
@synthesize myCard = _myCard;

- (IBAction)redeemPressed:(id)sender {
    // get required points of a perk selected

     NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
     [f setNumberStyle:NSNumberFormatterDecimalStyle];

     self.pointsRequired = [f numberFromString: (self.pointsLabel.text)];

    NSLog(@"points required by the perk %@", self.pointsRequired);

    [self.delegate perksDetailsViewController:self didPassRequiredPoints:self.pointsRequired withCard:self.myCard];
}


- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"showBarCode"]) {
        BarcodeViewController *barCodeVC = (BarcodeViewController *)segue.destinationViewController;

        [self setDelegate:barCodeVC];

    }
}

@end

#import <UIKit/UIKit.h>
#import "Card.h"
#import "BarcodeViewController.h"

@class PerksDetailsViewController;

@protocol PerksDetailsDelegate <NSObject>

- (void)perksDetailsViewController:(PerksDetailsViewController *)sender
             didPassRequiredPoints: (NSNumber *) requiredPoints
                          withCard: (Card *) selectedCard;

@end

@interface PerksDetailsViewController : UIViewController

@property (nonatomic, strong) id <PerksDetailsDelegate> delegate;

@property (nonatomic, strong) Card *myCard;

@end

#import "PerksDetailsViewController.h"

@interface BarcodeViewController () <PerksDetailsDelegate>
@end

@implementation BarcodeViewController
@synthesize myCard = _myCard;
@synthesize resultingPoints = _resultingPoints;

- (void)perksDetailsViewController:(PerksDetailsViewController *)sender didPassRequiredPoints:(NSNumber *)requiredPoints withCard:(Card *)selectedCard
{

    double perksPoints = [requiredPoints doubleValue];

        self.myCard = selectedCard;
        self.resultingPoints = [NSNumber numberWithDouble:[selectedCard subtractPoints:perksPoints] ];
}

@end

@interface BarCodeViewController : UIViewController <PerksDetailsDelegate> {
...
}