Aligning second sprite to physics body of first sprite in Spritekit/Swift - swift

Question

I have a sprite with a physics body.
To move this sprite, in the update() function I continually set the velocity of the sprite to 150 units to go right, and -150 to go left. (See attached code.)
I have a second physics body that I'd like to have follow the first sprite. The coordinates of this second physics body are at the bottom of the first one, and 20 points to the right.
I'd like the second physics body to always follow the first, with the offset.
My code mostly works but I've noticed that the distance between the first and second bodies varies slightly, and I want there to be no variation. When moving right, the distance between them is compressed a little. When moving left, the distance between them increases a little. See a video HERE: https://www.youtube.com/watch?v=K9FhIdMwp7k
Here is the code I'm using:
override func update(_ currentTime: TimeInterval) {
switch playerDirection {
case "stopped":
playerSpeed = 0.0
case "right":
playerSpeed = 150.0
case "left":
playerSpeed = -150.0
default:
print("default")
}
// ball is the first sprite and footBall is the second sprite:
ball.physicsBody?.velocity = CGVector(dx: playerSpeed, dy: ball.physicsBody!.velocity.dy)
footBall.position = CGPoint(x: (ball.position.x + 20), y: ball.position.y - (ball.size.height / 2) + 4)
myCam.position = CGPoint(x: round(ball.position.x), y: self.size.height / 2)
...
I've been playing around with using override func didSimulatePhysics() without success, also.
I did a different test that just does away with using velocity to move the player, and instead directly increment the ball.position.x (ball = player) and footBall.position.x and when I do this everything is perfectly aligned. But if I take this route I'll have to change how my game physics work elsewhere in the game, which I'd prefer to ignore.
Thanks for having a look.

Answer 1

Would it meet your requirements if you add the second node as a child of the first? That is, instead of having both at the same level:
scene.addChild(ball)
scene.addChild(footBall)
replace with:
scene.addChild(ball)
ball.addChild(footBall)
and just set the footBall position as the offset you need:
footBall.position = CGPoint(x: 20, y: - (ball.size.height / 2) + 4)
then any time the ball moves the footBall will also move as it is a child node, so you can remove the manual position update code for the footBall.

Answer 2

I used SKPhysicsJoint to get around the problem. Here's my working code (testing at this point, so still kinda sloppy, but it works in principle) :
//add a nice player
ball = SKSpriteNode(texture:spriteArray2[0])
ball.size = CGSize(width: 150, height: 48)
//ball.physicsBody = SKPhysicsBody(circleOfRadius: ball.size.height / 2)
ball.physicsBody = SKPhysicsBody(rectangleOf: CGSize(width: 16, height: 48))
ball.zPosition = 1
ball.physicsBody?.affectedByGravity = true
ball.position = CGPoint(x: 80.0, y: 200.0)
ball.physicsBody?.categoryBitMask = 1
ball.physicsBody?.collisionBitMask = UInt32(2 | 32 | 256) //2 = floors, 32 and 256= doors.
ball.physicsBody?.contactTestBitMask = 0
ball.physicsBody?.isDynamic = true
ball.physicsBody?.restitution = 0.0
ball.physicsBody?.allowsRotation = false
ball.physicsBody?.mass = 60.0
ball.physicsBody?.friction = 0.00
addChild(ball)
//add a nice ball for when jumping
footBall = SKSpriteNode(color: SKColor.blue, size: CGSize(width: 24, height: 16))
footBall.position = ball.position
footBall.name = "footBall"
footBall.zPosition = 1
footBall.physicsBody = SKPhysicsBody(circleOfRadius: 4)
footBall.physicsBody?.affectedByGravity = false
footBall.physicsBody?.categoryBitMask = 16
footBall.physicsBody?.collisionBitMask = 2 // floors
footBall.physicsBody?.contactTestBitMask = UInt32(32 | 64 | 128 | 256) // the doors bear snake
footBall.physicsBody?.isDynamic = true
footBall.physicsBody?.restitution = 0.0
//footBall.physicsBody?.allowsRotation = false
//footBall.physicsBody?.mass = 60.0
//footBall.physicsBody?.friction = 0.00
addChild(footBall)
let myCGPoint = ball.position // sets joint position
let myJoint = SKPhysicsJointFixed.joint(withBodyA: ball.physicsBody!, bodyB: footBall.physicsBody!, anchor: myCGPoint)
scene?.physicsWorld.add(myJoint)
...
then down in the update loop I set velocity for the player as so:
ball.physicsBody?.velocity = CGVector(dx: playerSpeed, dy: ball.physicsBody!.velocity.dy)
In my original code, in my update loop I also adjusted the position of the footBall to match the ball (player) position as needed, but in the revised code, because of the joint being used, by moving the ball the footBall moves along with it without any need to otherwise apply any force or velocity or change the x position of the footBall. The footBall just tags along for the ride, which works.
At first, this new method didn't work correctly, and I discovered through trial and error that the reason why is that "footBall.physicsBody?.allowsRotation = false" cannot be specified as a property on footBall. By uncommenting this line it works fine, however.
The point of this exercise is to use the footBall as a jumping test point for when the player's feet are stretched out during a jump. So the footBall is thus slightly forward of the main player's body. Then I'll dynamically turn on or off this footBall as needed for jumps.
Somewhat related are the following pages on "foot sensor":
http://gamedevwithoutacause.com/?p=1076
http://www.iforce2d.net/b2dtut/jumpability