SpriteKit sprite flickers on collision - swift

Question

I have this onscreen joystick which moves sprite around by changing it's position and it works well. I've made some walls for the sprite to collide with (using Physics). And it's not going further as expected but if I keep going into a wall sprite seems to flicker (change it's position for a millisecond and going right back)
override func update(deltaTime seconds: TimeInterval) {
cNode.position = CGPoint(x: playerCurrentPosition.x + xChange,
y: playerCurrentPosition.y + yChange)
}
Did some more debugging and it seems for one update sprite's position is incorrect:
pos Y: -19.168764114379883
pos Y: -19.164892196655273
pos Y: -19.16112518310547
pos Y: -29.928884506225586. <---
pos Y: -19.15281105041504
pos Y: -19.148771286010742
Update:
Okay, I've tried moving with move By like so :
print("[", xChange,", ",yChange,"]", cNode.position.y)
cNode.run(SKAction.moveBy(x: xChange, y: yChange, duration: 1/60))
And I got the same flicker on Y position:
xChange, yChange, cNode.position.y
[ -0.08623046134167665 , 0.4925081801724735 ] -19.026506423950195
[ -0.08623046134167665 , 0.4925081801724735 ] -19.026336669921875
[ -0.08623046134167665 , 0.4925081801724735 ] -19.02557373046875
[ -0.08623046134167665 , 0.4925081801724735 ] -29.92888641357422
[ -0.08623046134167665 , 0.4925081801724735 ] -29.928884506225586
[ -0.08623046134167665 , 0.4925081801724735 ] -29.928884506225586
[ -0.08623046134167665 , 0.4925081801724735 ] -29.928884506225586
[ -0.08623046134167665 , 0.4925081801724735 ] -19.02621841430664
[ -0.08623046134167665 , 0.4925081801724735 ] -19.025728225708008
[ -0.08623046134167665 , 0.4925081801724735 ] -19.02647590637207
[ -0.08623046134167665 , 0.4925081801724735 ] -19.02564811706543
[ -0.08623046134167665 , 0.4925081801724735 ] -19.02585792541504
[ -0.08623046134167665 , 0.4925081801724735 ] -19.02660369873047
[ -0.08623046134167665 , 0.4925081801724735 ] -19.025630950927734
[ -0.08623046134167665 , 0.4925081801724735 ] -19.026046752929688
[ -0.08623046134167665 , 0.4925081801724735 ] -19.026309967041016
[ -0.08623046134167665 , 0.4925081801724735 ] -19.025686264038086
Update 2:
I've made a recording of the issue: Here
Update 3:
It seems my outer green rect had some collision masks, I removed those and it got better, but still there's some bouncing, have a look: Here
(red box is pinned to the green one)

Answer 1

The problem is almost certainly the fact that you are using code - triggered by the joystick - to set the sprite's position directly yet the sprite and the wall cannot overlap due to their physics bodies 'collisionBitMask` settings.
So the sprite is up against the wall and you use the joystick to move the sprite towards the wall. You update the sprite's position which places it overlapping the wall node. The SK engine never saw the 2 nodes begin to overlap but suddenly sees that these nodes overlap massively when they shouldn't and attempts to move them via an impulse so that they don't overlap anymore, causing the sprite to 'jump' to a new position where it doesn't overlap and so it appears to flicker.
You could avoid this my using an SKAction to move the sprite in response to the joystick input which I think will let the SK engine trap the collision between the wall and the spite as soon as they start to overlap and you could use contact detection to then stop the movement.