Initialize Nape once in nape demos

[Geokureli]

Continued from: HaxeFlixel/flixel-addons#465 (comment)

The FlxNape demo is broken because of this. Each of its demos is a different state, and each state calls FlxNapeSpace.init(). By switching the demo you're on a couple of times, there's going to be multiple instances of FlxNapeSpace updating the Nape space, causing physics to speed up. You can't even see all of the demos as they're intended to be, because when you switch to the second one, the physics are being updated twice instead of once and so on.

We potentially need to change the following demos:
https://haxeflixel.com/demos/FlxNape/
https://haxeflixel.com/demos/FlxNapeTerrain/
https://haxeflixel.com/demos/FlxNapeTilemap/

[ACrazyTown]

Just tried to take a crack at these fixes myself, and I think there's yet another issue with FlxNapeSpace that I have not noticed in my initial pull request.

We can't call FlxNapeSpace.init() only once because the actual Nape space gets cleared and nulled when the state switches. There's no way to avoid this and the Space is only (re)created when FlxNapeSpace.init() is called.

[Geokureli]

I'm pretty unfamiliar with nape. In layman's terms, what does this event do, and what do you think should change?

Side note: but isn't FlxNape somewhat unusable in it's current state? is it safe to say that no one is likely using this, given that it doesn't seem to work even with various workarounds?

[ACrazyTown]

To be honest, I don't know much about Nape either, but I think that it's currently one of the only options for realistic physics available. Also the integration with Flixel is nice.

I'm pretty unfamiliar with nape. In layman's terms, what does this event do, and what do you think should change?

It just seems to clear the physics world when the state changes. For reference, Echo does pretty much the same thing. The difference is that FlxNapeSpace also nulls the reference to the actual physics space. I assume you're expected to just call FlxNapeSpace.init() again to recreate it, and that's where we loop back to my original issue. It adds another instance of the plugin without checking if it exists already.

Side note: but isn't FlxNape somewhat unusable in it's current state? is it safe to say that no one is likely using this, given that it doesn't seem to work even with various workarounds?

Are you referring to FlxNapeSpace (the plugin) and not FlxNape (the demo)? If so, I would assume that most people using Nape with Flixel are also using the provided helpers. A GitHub search for "FlxNapeSpace" shows 200 results in code: https://github.com/search?q=FlxNapeSpace&type=code

Otherwise, I think the demo is cool and a place to learn. It certainly helped me whilst figuring out how to use Nape.