Collision detection in Sprite Kit

Question

I'm using collision detection in Sprite Kit. It is working and preventing my sprites from crossing paths. However, I'm not getting notifications in didBeginContact: and I don't seem to have any control over how the physics engine responds when a collision occurs.

I have various cars (SKSpriteNodes) moving around following paths using the SKAction followPath:asOffset:orientToPath:duration:

Previously, if two cars crossed paths they would both just continue as normal with one driving over the other. To implement collision detection I have made the following changes...

Added this to my @interface:

<SKPhysicsContactDelegate>

Added this to my @implementation:

static const uint32_t carCategory = 0x1 << 0;

Added this in my init method:

self.physicsWorld.contactDelegate = self;
self.physicsWorld.gravity = CGVectorMake(0.0, 0.0);

I create my blue car:

- (void)addBlueCar
{
    _blueCar = [SKSpriteNode spriteNodeWithImageNamed:@"Blue Car.png"];
    _blueCar.position = CGPointMake(818.0, -50.0);
    _blueCar.name = @"car";
    [self addChild:_blueCar];

    CGSize contactSize = CGSizeMake(_blueCar.size.width - 5.0, _blueCar.size.height - 5.0);
    _blueCar.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:contactSize];
    _blueCar.physicsBody.categoryBitMask = carCategory;
    _blueCar.physicsBody.collisionBitMask = carCategory;
    _blueCar.physicsBody.contactTestBitMask = carCategory;
}

And I also create a red car:

- (void)addRedCar
{
    _redCar = [SKSpriteNode spriteNodeWithImageNamed:@"Red Car.png"];
    _redCar = CGPointMake(818.0, -50.0);
    _redCar = @"car";
    [self addChild: _redCar];

    CGSize contactSize = CGSizeMake(_blueCar.size.width - 5.0, _blueCar.size.height - 5.0);
    _redCar.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:contactSize];
    _redCar.physicsBody.categoryBitMask = carCategory;
    _redCar.physicsBody.collisionBitMask = carCategory;
    _redCar.physicsBody.contactTestBitMask = carCategory;
}

There are other cars as well, but I'm just using these two until I figure out what the problem is. I want to be notified of any collision between any two cars. That's why I'm just using a single category, carCategory.

The first problem is that I get no notifications. I have this yet it never logs anything to the console:

- (void)didBeginContact:(SKPhysicsContact *)contact
{
    NSLog(@"Contact");
}

The second problem is that when two cars cross paths they start nudging each other off course. This is ok, but I don't seem to able to manage any aspects of how the collision is handled. The bizarre thing is that nothing changes when I do this to both of my car creation methods:

- (void)addBlueCar
{
    _blueCar = [SKSpriteNode spriteNodeWithImageNamed:@"Blue Car.png"];
    _blueCar.position = CGPointMake(818.0, -50.0);
    _blueCar.name = @"car";
    [self addChild:_blueCar];

    CGSize contactSize = CGSizeMake(_blueCar.size.width - 5.0, _blueCar.size.height - 5.0);
    _blueCar.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:contactSize];
//    _blueCar.physicsBody.categoryBitMask = carCategory;
//    _blueCar.physicsBody.collisionBitMask = carCategory;
//    _blueCar.physicsBody.contactTestBitMask = carCategory;
}

Collision detection still works the same with those three lines commented out for both cars. This just doesn't seem right to me. Collision detection only ceases when I also comment out this line:

    _blueCar.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:contactSize];

So my main question is: why is the contactTest not reporting anything? My second question is: why is collision detection happening when I don't assign a categoryBitMask, collisionBitMask or contactTestBitMask to any car?

Answer 1

collisionBitMask's default value is 0xFFFFFFFF (all bits set). Therefore the node will collide with each physicBody on the scene.