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So I'm developing a game using SpriteKit. And to make the game more interesting I decided to use UIDynamicAnimator to add some physics to the UIKit elements within my game. Everything works great except there is one major problem. The moment I alloc/init a UIDynamicAnimator like so: 

animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];

The physics bodies in my game get completely offset. Even if I don't specify a reference view the physics still get offset.

What's strange is that i'm creating the UIDynamicAnimator in a completely separate ViewController so it's not like it's sharing the same reference view or something like that.

Update: Even the act of simply creating a UIDynamicItemBehavior like so:

UIDynamicBehavior* a = [[UIDynamicBehavior alloc] init];

messes up my scene's physics.

I'm assuming this is an internal bug with the physics engine that SpriteKit and UIDynamics share.

Anyone have any suggestions?

Epic Byte
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  • In the WWDC 2013 video [Getting Started with UIKit Dynamics](https://developer.apple.com/wwdc/videos/?include=206#206) Apple discusses when one might use one and when one might use the other. Clearly Apple views them as alternatives (which use the same physics engine behind the scenes, see [Simulating Physics](https://developer.apple.com/library/ios/documentation/GraphicsAnimation/Conceptual/SpriteKit_PG/Physics/Physics.html)), not as two technologies you'd use at the same time. – Rob Feb 09 '14 at 11:53
  • DUDE I HAVE THE SAME EXACT BUG – michaelsnowden Aug 22 '14 at 05:46
  • this bug is still around in 2022 HOW LMAO – bze12 Oct 29 '22 at 03:06

2 Answers2

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Ok I found a pretty hacky solution. All you need to do is use the UIDynamics framework in some way before presenting your SKScene. For example, simply running this one line of code before presenting your scene will remove the weird physics bug caused by simultaneously using SpriteKit and UIDynamics at the same time:

UIDynamicBehavior* a = [[UIDynamicBehavior alloc] init];

I'm assuming internally this prepares the physics engine in such a way that sprite kit physics will behave normally. Unfortunately I do not trust this solution because it might not work one day and my entire game would suffer because of it.

So as an alternative to the UIDynamics framework I've decided to just use UIView Animation to replicate some of the interesting physics effects that I got with the UIDynamics Framework.

EDIT FOR IOS 8 According to doctordoder, this bug still exists in iOS 8, and this workaround still works.

Epic Byte
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0

Note that in iOS 9: the @Epic Byte's solution still do the half workaround.

It starts the physics engine earlier (before we initialize SpriteKit game etc.) but unfortunately it mixes it up.

And allocation and init the UIDynamicAnimator during the SpriteKit game will cause the physicsWorld and it's properties like gravity act differently.

Also it seems like the physicsWorld is removed for a short time (switching engines?) during UIDynamicAnimator initialization.

Conclusion:

  • Alloc and init UIDynamicAnimator asap and handle your physics world differently e.g. call

    [UIDynamicAnimator new]; in delegate's didFinishLaunchingWithOptions

or

  • Do not mix the UIKit Dynamics and SpriteKit
Jakub Truhlář
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