cobblemon/docs/form_change_effects.md

Form Change Effects

A forme change done for a temporary form (mega evolution, Cherrim, Wishiwashi) will run an action effect to perform the transition. This can take as long as it needs, as battles will wait until it is complete before letting the turn proceed.

Choosing Which Action Effect To Use

Which action effect is used for which form is something you control. The system will first check to see if the form, defined inside a species JSON, mentions an actionEffectIdentifier. This could be cobblemon:mega_x_transformation, for example. If it has, then it will attempt to load that action effect and error if it does not exist. If it hasn't specified an actionEffectIdentifier, then it will try finding one for cobblemon:form_{form name}, such as cobblemon:form_school. If that also fails, then it will use cobblemon:generic_form, which you can find in resources/data/cobblemon/action_effects/forms/ folder of the mod.

Forms can also define a transition for when you are moving from a temporary form back into a permanent form. Wishiwashi in its School form is temporary, its Solo form is permanent. If you wanted a special effect for when it is reverting, the logic is the similar as above but with _revert at the end of everything. In other words, if you specified actionEffectIdentifier, then it will look to see if there is one with the same name but with _revert on the end. If it doesn't find it, it won't error like the above case, but instead look for cobblemon:form_{form name}_revert, such as cobblemon:form_school_revert, and failing that, cobblemon:generic_form_revert.

Timing The Swap

At some point during the effect, you're going to want the form of the Pokémon to swap over instead of just playing the effects. The swap will occur at whichever point happens first: the "transformation" hold is removed from the action effect context, or the action effect completes. If you make it happen earlier, the action effect will continue playing even though the switch has occurred.

If you review generic_form.json in the action_effects data folder, you'll see that it adds the "transformation" hold first thing, and then removes it at a particular timing. This will make the model change over at that moment. This is identical in execution to the "effects" hold used in move action effects.

Some of the effect JSONs that are in the mod:

generic_form.json

{
  "timeline": [
    {
      "type": "add_holds",
      "holds": ["transformation"]
    },
    {
      "type": "entity_particles",
      "effect": "cobblemon:statup_actor",
      "delay": 0.1,
      "locators": ["root"]
    },
    {
      "type": "pause",
      "pause": 1.0
    },
    {
      "type": "remove_holds",
      "holds": ["transformation"]
    },
    {
      "type": "pause",
      "pause": 2.0
    }
  ]
}

generic_form_revert.json

{
  "timeline": [
    {
      "type": "add_holds",
      "holds": ["transformation"]
    },
    {
      "type": "entity_particles",
      "effect": "cobblemon:statdown_actor",
      "delay": 0.1,
      "locators": ["root"]
    },
    {
      "type": "pause",
      "pause": 1.0
    },
    {
      "type": "remove_holds",
      "holds": ["transformation"]
    },
    {
      "type": "pause",
      "pause": 2.0
    }
  ]
}