SpriteKit中的多个物理实体

我有一个40x40的矩形节点，我想检测底部触及平台的时间 .

我试过这个

let feet = SKPhysicsBody(rectangleOfSize: CGSize(width: hero.frame.size.width, height: 1), center: CGPoint(x: 0, y: -(hero.frame.size.height/2 - 0.5)))

然后设置 categoryBitMask ， collisionBitMask ， contactTestBitMask 并将其添加到英雄

hero.physicsBody = SKPhysicsBody(bodies: [feet])

但在 didBeginContact 中 println() 不打印 .

我需要一个用于矩形底部的物体和一个用于顶部的物体，因为如果英雄从下方撞击一个平台，碰撞应该将他推倒 .

Update

这是我如何设置位掩码

let heroFeetCategory: UInt32 = 1 << 0
let edgeCategory: UInt32 = 1 << 1
let groundCategory: UInt32 = 1 << 2

let feet = SKPhysicsBody(rectangleOfSize: CGSize(width: hero.frame.size.width, height: 10), center: CGPoint(x: 0, y: -(hero.frame.size.height/2 - 5)))
feet.collisionBitMask = edgeCategory | groundCategory
feet.contactTestBitMask = groundCategory

feet.categoryBitMask = heroFeetCategory


hero.physicsBody = SKPhysicsBody(bodies: [feet])
hero.physicsBody?.usesPreciseCollisionDetection = true
hero.physicsBody?.velocity = CGVectorMake(0, 0)

hero.physicsBody?.restitution = 0.0
hero.physicsBody?.friction = 0.0
hero.physicsBody?.angularDamping = 0.0
hero.physicsBody?.linearDamping = 1.0

hero.physicsBody?.allowsRotation = false
hero.physicsBody?.mass = 0.0641777738928795

world.addChild(hero)

并为地面

let ground = SKSpriteNode(color: UIColor.whiteColor(), size: CGSizeMake(38, 38))
ground.name = "groundName"
ground.position = CGPoint(x: 0, y: -(self.frame.size.height/2 - ground.frame.size.height/2))

ground.physicsBody = SKPhysicsBody(rectangleOfSize: ground.size)
ground.physicsBody?.collisionBitMask = edgeCategory | heroFeetCategory
ground.physicsBody?.contactTestBitMask = heroFeetCategory
ground.physicsBody?.categoryBitMask = groundCategory
world.addChild(ground)

我如何检测他们是否触摸

func didBeginContact(contact: SKPhysicsContact) {

    var notTheHero: SKPhysicsBody!;

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

    println(notTheHero.node) // print "heroName"
    if (notTheHero.categoryBitMask == groundCategory) {
        println("touch began?"); // is never called
    }
}