There are several reasons why contact and collision are not detected in your Swift project. Below is a checklist I've made when setting up collision on Swift:
1. On Swift 3.0, function didBeginContact is changed to:
optional func didBegin(_ contact: SKPhysicsContact) { ... }
2. Have you implemented the SKPhysicsContactDelegate protocol?
class yourClass: SKScene, SKPhysicsContactDelegate { ... }
2. Have you implemented the Contact Delegate?
self.physicsWorld.contactDelegate = self
3. Have you defined the necessary Category Bit Masks?
let firstCategory: UInt32 = 0x1 << 0
let secondCategory: UInt32 = 0x1 << 1
4. Have you assigned the correct Category Masks to your Sprite Nodes?
firstNode.physicsBody?.categoryBitMask = firstCategory
5. Have you configured the correct Contact and Collision Masks to your Sprite Nodes?
firstNode.physicsBody?.contactTestBitMask = secondCategory
firstNode.physicsBody?.collisionBitMask = secondCategory
6. Remember that one of the colliding Sprite Nodes must be dynamic
yourNode.physicsBody?.isDynamic = true
