How to make balls ignore collisions with each other - ios

Question

With sprite kit, I'm trying to make series of balls drop from the air through physics simulation, and I want them to drop without colliding with each other and bounce off. I just want them to go straight down and pass through each other. How should I make this happen with collisionBitMask and categoryBitMask ?
// Common.h
#ifndef Rainy_Poops_common_h
#define Rainy_Poops_common_h
static int poopSize_x = 20;
static int poopSize_y = 20;
static const uint32_t poopCategory = 0x1 << 1;
#endif
// MyScene.h
-(void)update:(CFTimeInterval)currentTime {
/* Called before each frame is rendered */
_timer++;
if (_timer % 3 == 0) {
SKSpriteNode * p = [[Poop alloc] init];
NSInteger random_x = arc4random_uniform([[UIScreen mainScreen]applicationFrame].size.width);
p.position = CGPointMake(random_x, [[UIScreen mainScreen]applicationFrame].size.height - 5);
[self addChild: p];
[self enumerateChildNodesWithName:#"poop" usingBlock:
^(SKNode *node, BOOL *stop) {
Poop *poop = (Poop *) node;
if (!poop.isDropping) {
poop.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(poopSize_x, poopSize_y)];
poop.isDropping = YES;
}
if (poop.position.y < 0) {
[poop removeFromParent];
}
}];
}
}
#import "Poop.h"
#implementation Poop
- (id)init {
self = [super initWithImageNamed:#"poop2.png"];
self.isDropping = NO;
self.name = #"poop";
self.size = CGSizeMake(poopSize_x, poopSize_y);
SKPhysicsBody *physicsPoop = self.physicsBody;
physicsPoop.collisionBitMask = 0;
physicsPoop.categoryBitMask = poopCategory;
physicsPoop.affectedByGravity = YES;
physicsPoop.mass = 100;
physicsPoop.allowsRotation = NO;
physicsPoop.dynamic = YES;
return self;
}
#end

Answer 1

I think your problem is related to this line -
poop.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(poopSize_x,poopSize_y)];
You initialise your categoryBitMask to 2 and collisionBitMask to 0 in the init of your node, but this line will reset the physicsBody for your poop and assign the default collisionBitMask and categoryBitMask values of 0xFFFFFFFF

Answer 2

set collisionButMask equal to 0 for no collision or equal to the categoryBitMask of the sprite you would like the balls to collide with.