third party tools

Have you tried using the Scene Editor in Xcode, specifically the Timeline, where you can add an Animate with Textures action? Pretty darn simple. Just give it a name like "WalkFront". Then you can make a sprite run the animation sequence with a couple lines of code...

let walkAnimation:SKAction = SKAction(named: "WalkFront", duration: 1 )!
player.run( walkAnimation )

Depends what you mean by "creating animation sequences"

Spine is mostly a tool for taking sprites made up of seperate parts and animating them similar to bones on 3d models, only in 2d. There's similar software, but as of yet I don't think any of them support swift. There's also Spriter, Synfig Studio, Dragon bones and many more, they mostly perform a similar job. There's also pixel art sprite tools, and even some tools to take 3d models and turn them into 2d sprite sheets. At a push, I believe you could also do this manually part by part by creating your own animations in Xcode, by animating seperate parts and combining them, but i think this would require more work in the long run, with less useablity.

If you already have sprite sheets and just want to create animations, I think CartoonSmart has already answered your question. (p.s. one of the first videos i watched about Swift was a CartoonSmart one...good stuff!)

It seems like you mean creating animations as in building (skeletal) hierarchies and matching SKActions to time, rather than frame animations. I can't really think of any tool that does this for SpriteKit + Swift. You could always try SpriteKit's own Scene Editor, but it's currently heavily lacking features, so it may be a waste of time until you realize something you need isn't even possible in Scene Editor.

What I've done in the past to for example create a skeletal character is to build it in AfterEffects or Photoshop at the same coordinate scale, and then just manually save the image parts into the asset catalog and build the character in code using coordinates from AE or PS. It's very time-consuming and error-prone, so of course it would be nice to have a program that does it automatically. Furthermore, animating it is another beast altogether. If you're animating in AE, it's super-difficult to match those animation functions to SKActions.

One thing that might help the process is to build the character in Scene Editor and then animate in code by referencing the elements in the .sks file. We're doing that now in one project. It's pretty hellish to create complex animations in code like this, but it's the only way we've found to work. Here's hoping SpriteKit actually has a much-improved Scene Editor in Xcode 9 🙂.