To achieve the setup you described with Spine Unity, you can consider the following approach to streamline your workflow and avoid cumbersome workarounds:

1. Use Skins for Equipment: Instead of creating attachments on the fly, consider using skins to manage different equipment like helmets and cloaks. Skins allow you to organize attachments and switch them easily at runtime. You can create a skin for each piece of equipment and apply it when needed.

2. SkeletonPartsRenderer and Draw Order: The SkeletonPartsRenderer is useful for separating parts of your skeleton for different rendering needs. However, managing draw order and visibility can become complex. Instead of manually adjusting sorting orders, you can use Spine's built-in draw order functionality to manage which slots are rendered on top. This can be controlled through animations or by setting the draw order programmatically.

3. Visibility Management: For managing visibility, instead of setting alpha to zero, you can use Spine's attachment visibility features. You can set attachments to null to hide them or use a placeholder attachment that is invisible. This avoids issues with submesh instructions being affected by alpha values.

4. Dynamic Attachment Management: If you need to dynamically add or remove attachments, ensure that your code handles the creation and destruction of attachments efficiently. You can cache attachments and reuse them instead of creating new ones each time.

5. Optimize Renderer Usage: If certain parts of your character (like cloaks) require special rendering, consider using separate renderers only when necessary. You can enable or disable these renderers based on the current equipment state to optimize performance.

By organizing your equipment using skins and leveraging Spine's built-in features for draw order and visibility, you can simplify your setup and reduce the need for complex workarounds. This approach should provide a more maintainable and efficient workflow for managing character equipment in Spine Unity.

Keydus Your description is detailed, but we would still prefer to see the actual project to more accurately confirm what you are trying to do and what you are having trouble with, rather than just guessing from the text. So if you don't mind, could you create a minimal Unity project that confirms your problem and email it to us?: contact@esotericsoftware.com
Please include the URL of this forum thread in the email so we know the context. Then we can take a look at what's wrong.

Keydus I have successfully opened your Unity project and our Unity expert Harald will review and advise you later.

@Keydus Thanks for sending the reproduction project. Unfortunately it was not a minimal Unity project which shows only the few relevant lines of code which trigger your issue but whole gameplay classed SpineRendererSeparatorSortingOrder, PlayerEquipService and UpdateEquipmentAttachmentAnimation which contain far too much logic for us to efficiently check.

From the images and the desired outcome I think you are in the following situation:
1) Your skeleton has varying draw order due to front and back view.
2) You want to insert (interleave) two items between certain slots.

What you need to do:

• 1) Prepare the insertion-slots in your skeleton where the separation shall always happen.
• 2) Either:
  • a) Ensure that the number of separated parts is always the same (3 with 2 splits). Then the hat GameObject item shall be at sibling index 2, the cloak at sibling index 4. When changing from front to back exchange the sibling index of the two items, cloak at sibling index 2 and hat at sibling index 4. Keep references to the two inserted GameObjects, then changing sibling index should be trivial.
  • b) If animations dictate your draw and split order randomly, you could set one or multiple events at each animation when the draw order changes so that the split is somewhere else. E.g. raise events string: "hat", int: "2" and an event string: "cloak", int: "4" or raise a packed event string: "parts", int:"2", float: "4" to pack more information into one event if you would like to save resources.

Keydus Unfortunately it was hard to make a very minimal reproducible code as there are multiple things involved here such as creating attachments dynamically.

If multiple things are involved, your code should include multiple lines of code and multiple asset references, not multiple classes of which only parts are used.

We know that it may take some effort to reduce your code down to the required parts. Please note however that you can do that much better than we would be able to, as you understand what is necessary while we do not. Please understand that we don't have the time and resources to step through your code and see which parts are actually relevant.

Keydus Having an animated character with 2 views (F and B) controlled by animations
Having two slots (F and B) for the hat without any attachment

I previously assumed that you have only one shared item-insertion (separation) slot for both the back and front view versions. Things are more complicated if you have two slots for each item, one per view. Is there a reason why you need separate separation-slots for front and back view? You should be able to have a single separation slot and move it around in the draw-order of your animations to where the respective external GameObject item shall be inserted.

Having looked at your Unity project again, I noticed that your inserted item sprites are not actually Sprite GameObjects but skeleton attachments. Why do you want to separate those into split GameObject parts with SkeletonRenderSeparator? SkeletonRenderSeparator is for splitting a skeleton into e.g. 2 parts so that you can interleave other GameObjects in-between them. Could you explain the motivation for using SkeletonRenderSeparator, what problem are you trying to solve with it?

Keydus Wanting to have the possibility to tell a priority in the sorting order using SkeletonRendererSeparator (as stated here : https://fr.esotericsoftware.com/spine-unity-utility-components#SkeletonRenderSeparator)

What do you mean by "priority in the sorting order using SkeletonRendererSeparator"? Do you mean setting Sorting Layer and Order in Layer at SkeletonPartsRenderers? If so, why, what do you intend to use it for?

Ensure that the number of separated parts is always the same (3 with 2 splits)

If I understand correctly, by 3 with 2 splits you mean

   0 is the skeleton
   1 is an attachment (hat or cloak depending on DrawOrder & cie)
   2 is another attachment (hat or cloak depending on DrawOrder & cie)

No, I didn't mean that, that would make no sense. I meant if you have a skeleton with 2 slots where things should be inserted, you split your skeleton twice, resulting in 3 parts renderers:

SkeletonPart1
- split, insert GameObjects for e.g. hat here
SkeletonPart2
- split, insert GameObjects for e.g. cloak here
SkeletonPart3

Keydus In this whole part I don't think I'm completely following what you said about the sibling index. What should I do with them?

This might have a been due to me misunderstanding what you are trying to do, I first assumed that you wanted to insert the hat and cloak as a GameObject item, and not as a skeleton attachment.

Keydus I might have a solution to dynamically change the sorting order of the PartsRenderer on the animation change to apply the one calculated from the visibility (should be rendered first or last)

The question is why do you need to change sorting order of parts renderers, see questions above?
And first of all, why do you need a SkeletonRenderSeparator if all your items are normal attachments?

but still I can't know which attachment/slot is rendered in which PartsRenderer

Your Skeleton project in the Spine Editor defines the draw order, and combined with the chosen separator slots (and visible attachments) the separate parts may vary, omitting empty parts, yes.

Keydus You told me just before that I could maybe have one slot for the hat and just switch the sprite based on the current animation playing.

That's not what I meant. Anyway, let's best continue with the other approach discussed below.

Keydus However, since the cloak is a MeshAttachment, it's not possible to do so? If I have only one slot for my cloak, I'll have a strange movement?

" If I have only one slot for my cloak" is confusing. A MeshAttachment is an attachment, and there is always only a single slot to which an attachment is attached, you never have multiple. Or do you mean that you think you need one slot and with a single MeshAttachment for each cape, as your capes are so radically different that they can't share a mesh? I would argue that most capes are pretty similar but if you think they are completely different, sure, just create different MeshAttachments for each cape-type. You would still want to attach them at the same single Slot and not create a slot per cape-type as they are mutually exclusive and only one can be equipped at any time. Note that "the same slot" can mean "the same two slots" for front and back, F_cape and B_cape if you have one cape slot per facing-direction.

Could you describe why you think the MeshAttachment would have strange movement?

Keydus In fact, that's what I didn't want to do to avoid repeating this step for every character I create, this is cumberosme in term of maintenance. I thought that it might be trivial to set a specific rendering order for a given attachment.

If you only want to change rendering order for single slots, it's trivial, change Skeleton.DrawOrder:
http://esotericsoftware.com/spine-api-reference#Skeleton-drawOrder
Note that animations can also change your draw order. So you might then need to either apply your draw order change again or every frame.

I would not consider this clean design for equipment items since whenever your animation wants to change draw order to e.g. move a hand from in-front to behind your hat, you will end up with a hacky back and forth between adjusting animations and Unity-side order settings, where you can't preview in the Spine Editor how things will look. Thinking of fine-tuning draw order of hair vs specific helmet types, doing that in the Unity project side by tweaking numbers and then adjusting Spine Skeletons again would give me nightmares. That's why I would recommend having template attachments which can be positioned cleanly in the Spine Editor, adjusted in draw order in animations and previewed to ensure everything is perfectly ordered. But it's up to you of course what you prefer and what your use case requires.

Keydus Cloaks must have movements when the character moves (not only following the bone)

You didn't mention that before, but you could either use Physics Constraints set up in Spine to let them move automatically, or use SkeletonUtilityBone with automatic hinge chain physics setup as shown in the example scene SkeletonUtility Platformer HingeChain Physics and in this documentation section:
https://esotericsoftware.com/spine-unity-utility-components#2D-and-3D-Hinge-Chains-for-Physics

Keydus Equipments might be rendered in front or behind everything depending on the configuration of the sprite

You likley don't want to render it in front or behind everything, but at certain points in the draw order depending in the equipment item.

What would be the best way/setup to go?

One of the best solutions would be the one I've described in the previous posting, having template attachments for each equipment-type, and letting skin-bones control visibility of all front or back attachments.

Keydus More information here : I think that the main problem resides in the fact that there is no way to set a specific rendering order for my equipment besides creating a new slot to make it render behind everything, which I don't like since I'll have to repeat it for all characters

Please see what I wrote above regarding changing DrawOrder. Please also be sure to give the Slot documentation section a thorough read:
http://esotericsoftware.com/spine-slots

@Keydus Very glad it helped, thanks for letting us know!