Cannot get didEnd(_contact: SKPhysicsContact) to work properly in swift 4

Question

 func didBegin(_ contact: SKPhysicsContact) {
    var firstBody: SKPhysicsBody
    var secondBody: SKPhysicsBody

    if(contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask)
    {
        firstBody = contact.bodyA
        secondBody = contact.bodyB
    }
    else
    {
        firstBody = contact.bodyB
        secondBody = contact.bodyA
    }
if (firstBody.categoryBitMask & Constants().playerCategoryBitMask != 0)
    {

        if(secondBody.categoryBitMask & Constants().borderCategoryBitMask == 4)
        {       touchingWall = true
                print("Touching the wall ");
        }
    }
 }

didBegin is working great!

However didEnd not sure how to do it?

func didEnd(_ contact: SKPhysicsContact) {
    var firstBody: SKPhysicsBody
    var secondBody: SKPhysicsBody

    if(contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask)
    {
        firstBody = contact.bodyA
        secondBody = contact.bodyB
    }
    else
    {
        firstBody = contact.bodyB
        secondBody = contact.bodyA
    }
    if (firstBody.categoryBitMask & Constants().borderCategoryBitMask != 0 )
    {

        if(secondBody.categoryBitMask & Constants().playerCategoryBitMask != 0 )
        {
            touchingWall = false
            print("Not Touching the wall ");
        }
    }
}

I also have

let playerCategoryBitMask:UInt32 =  1
let borderCategoryBitMask:UInt32 = 4

score 8 · Accepted Answer · edited Feb 16 '18 at 04:44

This is because you´re using a method called the bitwise AND operator (&).

The bitwise AND operator (&) combines the bits of two numbers. It returns a new number whose bits are set to 1 only if the bits were equal to 1 in both input numbers:

let eightBits1: UInt8 = 0b00000001
let eightBits2: UInt8  = 0b00000001
let lastBit = eightBits1 & eightBits2  // equals 0b00000001

Combining the bits only the last bit 1 would return 1 all the rest would return zero.

A simpler explanation:

I declare two variables:

let x = 1
let y = 1

Here both x and y has the value 1, when you use bitwise AND operator the result will be also 1 and when checking if the result it is not equal to zero it would be true (any result that it is not equal to zero would return true).

let eightBits1: UInt8 = 0b00000001 // 1
let eightBits2: UInt8  = 0b00000001 // 1
let lastBit = eightBits1 & eightBits2  // equals 0b00000001 // 2

The result would always be the same as x (which is equal to y) in this case 1.

if (x & y) != 0 {
    print("Same")
} else {
    print("Not same")
}

In this case:

let x = 1
let y = 2

let eightBits1: UInt8 = 0b00000001  // 1
let eightBits2: UInt8  = 0b00000010 // 2
let noBits = eightBits1 & eightBits2  // equals 0 -> 0b00000000

You get false and not same will be printed out since the result of the bitwise operator it IS equal to zero

Basically, if you use the Bitwise AND operator using two identical numbers the result will always be the same number.

To your issue:
In your didBegin, you´re comparing:

if (firstBody.categoryBitMask & playerCategoryBitMask) != 0

Here your firstBody.categoryBitMask is 1 and playerCategoryBitMask is also 1 hence true and you enter the if-statement.

In your didEnd you´re comparing:

if (firstBody.categoryBitMask & Constants().borderCategoryBitMask) != 0

Here your firstBody.categoryBitMask is 1 and borderCategoryBitMask is 4 hence the result is zero and you don´t enter the if-statement because 0 it is equal to zero.

Now that you know this, you can modify your code and make it work.

Thank you Rashwan! So basically if I have the player stop touching the wall, what should the bitmask combination be? 001 & 100 which are 1 & 4 in decimal? — Marin, Feb 14 '18 at 23:26
@Marin, np tried to be as specific as possible hence all the edits. The combinations 001 & 100 (1 and 4) will result in 000. To be able to match the combinations needs to be aligned (same bit/position). — Rashwan L, Feb 14 '18 at 23:33

Cannot get didEnd(_contact: SKPhysicsContact) to work properly in swift 4

1 Answers1