Calculate distance correctly for animations using byValue

[noahsark769]

Hey! Awesome library, thanks for building it. I noticed that there's a bug with animations that use byValue, e.g.:

SKAction.scale(by:duration:delay:usingSpringWithDamping:initialSpringVelocity:)

The distance isn't calculated correctly, so the behavior uses byValue as the distance.

For example, say there's a node with a current scale of 1.0 - if we tried to scale it by 1.2, SpriteKit-Spring will currently scale it to 1.0 + 1.2 instead of 1.0 * 1.2, which in this case would more than double the scale instead of scaling appropariately. I put up an example app at https://github.com/noahsark769/NGSpriteKitSpringTest to demonstrate this.

The fix is to calculate the initialDistance based on multiplying it by initialValue.

Let me know if I need to do anything else to fix this bug! Thanks!

[noahsark769]

@ataugeron bump! Would be awesome to get this out and pushed as a new version of the cocoapod 👍

[ataugeron]

In addition to the inline comments, I merged #16 so we should make sure this CL is compatible.

[ataugeron]

Let's not mix code changes with version bumps in the same PR. I'll take care of bumping the version and updating the pod once all PRs are merged.

[ataugeron]

Thanks for catching this issue. However I don't think this is the correct fix. It appears the behavior of the "by" param in SKAction is inconsistent:
SKAction.scaleX(by: 0.5, y: 0.5, duration:0) // will divide xScale and yScale by 2
SKAction.moveBy(x: 0.5, y: 0.5, duration: 0) // will add 0.5 to position.x and position.y

For this reason I don't think we should change the behavior of the animate(keyPath:...) functions, but just adjust the behavior of the scaleBy(...) functions above to compute the correct byValue to give to animate(keyPath:...).

Does this make sense?

[noahsark769]

Wow you're right, great catch! I agree, since scale is the only multiplicative SKAction we should handle it as a special case in the scaleBy functions.