Fixed the existing instance being deleted in Awake

[nnra6864]

Replaced else with else if (instance != this) as a condition for the instance to be deleted in Awake.
This fixes a really weird bug where if singleton was used in the editor and then again in play mode, it would delete the original instance and create a new one.
Closes #21

[nnra6864]

I would highly appreciate somebody looking into this a bit deeper before merging.
From my testing, it didn't introduce any issues, however, there could be edge cases where this is unwanted behavior.

[hasanbayatme]

I am guessing it is due to the marshaling that Unity does for its objects, and MonoBehaviour nonetheless is a UnityEngine.Object, therefore it is not null in C#'s context as it is still living as a wrapper for the underlying C++ object until it is garbage collected, but it is not this either, so adding this check shouldn't theoretically make a difference, I am wondering whether in your case the issue might have been resolved using another solution or possibly another trial or fix? was it resolved in conjunction with #23 PR?

Unity has a custom implementation for for == equality check that does this native checking for the objects.

That == is already happening for instance == null, I assume there is an interoperability issue going on, most possibly when switching from Edit to Play mode.

[hasanbayatme]

This could in turn be related to how ExecuteAlways is handled internally as well, either way I'm merging it in.

[nnra6864]

am wondering whether in your case the issue might have been resolved using another solution or possibly another trial or fix?

I am positive it's the PR I made.
As soon as I use just else, the issue comes back.

was it resolved in conjunction with #23 PR?

I initially tried utilizing the execution order to fix the issue I was experiencing, that didn't help tho.
I just deleted the execution order attribute and left the else if alone and the issue stays fixed, I don't think execution order has anything to do with the fix in my case.

I tried to debug this before I came up with the solution, but I am sadly not skilled enough to figure it out.
Best I can do is help you setup a consistent way of reproducing this issue on your end, if you are up to of course.

[hasanbayatme]

Interesting, thank you for the elaboration.

Best I can do is help you setup a consistent way of reproducing this issue on your end

Sure, I'm interested to see how this occurs and whether this is an issue related to specific Unity versions or not.

[nnra6864]

Ok, so here is sort of a step by step guide:

1. Have a MonoSingleton, lets call it Singleton, that holds an object, lets use Player as an example:

        [SerializeField] private Transform _player;
        public static Transform Player => Instance._player ??= GameObject.FindWithTag("Player")?.transform;

2. Have a script with [ExecuteAlways] that would use this object in Update:

        private static Transform Player => Singleton.Player;
        private void Update() => transform.position = Player.position;

3. Make sure that Project Settings -> Editor -> When entering Play Mode is set to Do not reload Domain or Scene

4. Enter the playmode

Some of the code above might seem weird, I tried to make it as close as possible to what I have on my end. Some of it may also not be functional, I mostly wrote it on github so no way to test.

Unity 6000.3.0a5
Linux(in case that's in any way relevant)