Change position of a SKSpriteNode that has a physicsBody

Question

I can\'t change the position of a SKSpriteNode by changing

self.position = newPosition;

It doesn\'t work anymore if it has a physicsBody.

Answer 1

Seems similar to SKSPriteNode position changes to 0,0 for no reason

As stated in answer and comments of this question, it seems you must either set the position before setting the physicsBody and/or set the physicsBody after adding the node to your scene.

I've had this problem in several scenarios and I haven't found out what exactly causes it to fail sometimes.

Answer 2

Yes, you can!

I'm not sure what exactly you're doing and where exactly you run this code and what the observed effect is that you meant by "can't change position" but changing a node's position always works, whether the node has a physicsBody or not, and whether the physicsBody is dynamic or static. I verified this in a simple test project with dynamic set to both YES and NO, with both circle and edge loop body types, using both position property and move actions.

You can change the node's position either by setting the position property directly or by using a move action - both variants work. If you use a 0 duration for the move action, it effectively becomes the same as setting the position property.

So whatever problem you're observing, it's not because you can't generally change a node's position once it has a physicsBody. That's absolutely not the case.

I'm guessing you may have run into one of the following problems:

• node is already running a different move action, overriding your position changes
• you were looking at the wrong node (use logs to verify actual position of the node in question)
• you change position elsewhere, for example setting the node's position every frame thus invalidating any other position change
• if it's not one of the above, then possibly something else I couldn't think of ...

Answer 3

Yes you can, but it has its price.

I think you have to make a decision: Either let the position being calculated by the PhysicsEngine OR set the position on your behalf.

Look, for an object in a physics world, there is no magical movement from 'outside', there is only forces, collisions, drifts, ... and that will lead to any position. You can not manipulate the position without putting forces on some related nodes.

And in the moment you try BOTH, having a physicsBody (e.g. on your player), but also try to move them around by setting position manually or running actions for moving, YOU LEAVE the world of physics. Your player will be MOVED through walls and so on, against all physics rules.

So, to want an object being manipulated by the physics engine on the one hand and also to want "positioning" by code is kind a contradiction.

There are - of course ways - to combine both ways (e.g. the mentioned 'workaround' by temporarily removing the physicsBody), but this has also its price and has to be sequentially. You can not have both ways at the very same time.

Answer 4

I had the same problem. Apparently you cannot explicitly set the position of a Sprite node once it has a PhysicsBody. I solved it by temporarily removing the sprite node's PhysicsBody.

CGFloat yPosition = 400.f;
SKPhysicsBody* goldfishPhysicsBody = _goldfish.physicsBody;
_goldfish.physicsBody = nil;
_goldfish.position = CGPointMake(_goldfish.position.x, yPosition);
_goldfish.physicsBody = goldfishPhysicsBody;

Answer 5

I think the problem you are having is that in order for you to control a phsyics body, you need to set it to dynamic.

self.myStar.physicsBody.dynamic = NO;

If the dynamic property is set to YES, then the physics engine is in control of it's movement.

As noted in the comments, setting dynamic to NO shouldn't restrict movement via SKAction or position property. However, if it is set to YES, it's possible that something in the physics engine is affecting your attempt to move the node.

If you want the movement to not pop, then you need to set a duration higher than zero to your SKAction or it will pop as you have described. Setting duration to 0.0 is the same as just changing the position.

For example :

[self runAction:[SKAction moveTo:newPosition duration:1.0]];

will move the node to the new position over 1 second.

Answer 6

I ran into a similar problem. When using SKAction, even with duration set to 0.0 I got strange behaviours especially when two SKActions had been triggered at the same time.

I tried setting position directly but as mentioned by others this doesn't work when using the SKPhysicsContactDelegate.

However for me it worked to remove the node from its parent, I then set the new position, and other things I want to change, and then I add the node again to its former parent.

It's not ideal but in some cases it might help.

As an example with the SKPhysicsContactDelegate method didBegin:

func didBegin(_ contact: SKPhysicsContact) {

    guard let node = contact.bodyB.node else { return }

    node.removeFromParent()
    node.position = CGPoint(x: 10, y: 10)
    addChild(node)
}