Not receiving a notification that the contact occurred ?

    //GREENSHAPE
    SKShapeNode *greenShape = [[SKShapeNode alloc] init];

    CGMutablePathRef pathTwo = CGPathCreateMutable();
    CGPathMoveToPoint(pathTwo, NULL, 0, 0);
    CGPathAddLineToPoint(pathTwo, NULL, 70, 40);
    CGPathAddLineToPoint(pathTwo, NULL, 0, -81);
    CGPathAddLineToPoint(pathTwo, NULL, 0, 0);
    greenShape.path = pathTwo;
    greenShape.fillColor = [SKColor greenColor];
    greenShape.strokeColor = [SKColor greenColor];
    greenShape.physicsBody = [SKPhysicsBody bodyWithEdgeLoopFromPath:pathTwo];
    greenShape.physicsBody.affectedByGravity = NO;
    greenShape.physicsBody.dynamic = NO;
    greenShape.physicsBody.categoryBitMask = CollisionCategoryGreenShape;
    greenShape.physicsBody.contactTestBitMask = CollisionCategoryGreenBall;
    greenShape.physicsBody.collisionBitMask = CollisionCategoryGreenBall;

    [square addChild:greenShape];



-(void) didBeginContact:(SKPhysicsContact *)contact {
    SKPhysicsBody *firstBody, *secondBody;


    if (contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask) {
        firstBody = contact.bodyA;
        secondBody = contact.bodyB;
    }
    else {
        firstBody = contact.bodyB;
        secondBody = contact.bodyA;
    }


    if (firstBody.categoryBitMask == CollisionCategoryGreenShape &&
        secondBody.categoryBitMask == CollisionCategoryGreenBall) {

        NSLog(@"Contact");
    }
}
@end

#import "Balls.h"
#import "Utility.h"
@implementation Balls
+(instancetype) ballOfType:(Balltype)type {


    Balls *ball;


    if (type == BalltypeBlue) {
        ball = [self spriteNodeWithImageNamed:@"blueBall"];


    }
    else if (type == BalltypeGreen) {
        ball = [self spriteNodeWithImageNamed:@"greenBall"];
        ball.physicsBody.categoryBitMask = CollisionCategoryGreenBall;
        ball.physicsBody.contactTestBitMask = CollisionCategoryGreenShape;
        ball.physicsBody.collisionBitMask = CollisionCategoryGreenShape;
        ball.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(30, 30)];
        ball.physicsBody.affectedByGravity = NO;




    }
    else {
        ball = [self spriteNodeWithImageNamed:@"orangeBall"];


    }


    ball.zPosition = 1;
    //[ball setUpPhysics];
    ball.name = @"ball";
    return ball;




}

-(void) setUpPhysics {

    self.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(30, 30)];
    self.physicsBody.affectedByGravity = NO;



}


Utility.h 
#import <Foundation/Foundation.h>

typedef NS_OPTIONS(uint32_t, CollisionCategory) {
    CollisionCategoryGreenBall   = 1 << 0,
    CollisionCategoryGreenShape  = 1 << 1,

};

@interface Utility : NSObject
@end