Automatically creates and applies a full mask to all clips in a Humanoid rig so that all parts of the animation are maintained.

AutoMaskHumanoidRigsProcessor.cs

using UnityEngine;
using UnityEditor;
using System.Linq;

namespace Invertex.Unity.Editor.Importing
{
    /// <summary>
    /// This script will automatically generate an AvatarMask that will enable a Humanoid animation to animate all parts of your asset.
    /// It will assign it to every clip in a new asset or every existing clip on an asset.
    /// USAGE: Place in an "Editor" folder somewhere in your assets.
    /// Replace the values of 'restrictToPathsContaining' with the paths you want to restrict this script to (or make it just a single "" to do project wide, but not advised.)
    /// </summary>
    public class AutoMaskHumanoidRigsProcessor : AssetPostprocessor
    {
        //Change this to the path you want to restrict this script to.
        //Ideally the name of the folder that has your custom animations
        private static readonly string[] restrictToPathsContaining = { "_Game/Characters/", "/SomeOtherLocation/" };
        private static bool forceHumanoid = true; //Force new models to import as Humanoid

        void OnPostprocessModel(GameObject g)
        {
            ModelImporter modelImporter = (assetImporter as ModelImporter);

            if (modelImporter == null
                || !restrictToPathsContaining.Any((restrictPath) => assetPath.Contains(restrictPath)) 
                || !modelImporter.importAnimation) { return; }

            if(modelImporter.animationType != ModelImporterAnimationType.Human)
            {
                if(modelImporter.importSettingsMissing && forceHumanoid) { modelImporter.animationType = ModelImporterAnimationType.Human; }
                else { return; }
            }

            ModelImporterClipAnimation[] clipAnimations = modelImporter.clipAnimations;
            //If clipAnimations is 0, no changes to animations have been made yet, import the default clips
            if(clipAnimations.Length == 0) { clipAnimations = modelImporter.defaultClipAnimations; }

            //Create a single Mask for all the Humanoid animations to reference instead of wasting space with a new mask for each
            var mask = new AvatarMask() { name = $"{g.name}_Mask_All"};
            mask.AddTransformPath(g.transform);
            //Store it in the asset so we don't lose reference to it
            context.AddObjectToAsset(mask.GetHashCode().ToString(), mask);
          
            for (int i = 0; i < clipAnimations.Length; i++)
            {
                var clip = clipAnimations[i];
                clip.maskType = ClipAnimationMaskType.CopyFromOther;
                clip.maskSource = mask;
            }
       
            modelImporter.clipAnimations = clipAnimations; //In case we're working on the default array, assign it back
        }
    }
}