InputActionVariable.cs

using System;
using UnityEngine;
using UnityEngine.InputSystem;
using UnityEngine.Localization.SmartFormat.Core.Extensions;
using UnityEngine.Localization.SmartFormat.PersistentVariables;

namespace InputHints
{
    internal struct InputActionVariable : IVariableValueChanged
    {
        public event Action<IVariable> ValueChanged;
        readonly InputAction _action;
        readonly InputHintsSpriteMapping _source;

        public InputActionVariable(
            InputAction             action,
            InputHintsSpriteMapping source)
        {
            _action = action;
            _source = source;
            ValueChanged = null;
        }

        public override string ToString()
        {
            return $"InputActionVariable({_action.name}, {_action.GetBindingDisplayString()})";
        }

        public object GetSourceValue(ISelectorInfo selector)
        {
            return _source.GetSprite(_action);
        }

        public void OnValueChanged()
        {
            Debug.Log($"F{Time.frameCount} Sending OnValueChanged to {this} Has Listeners: {ValueChanged != null}");
            ValueChanged?.Invoke(this);
        }
    }
}

InputActionVariableGroup.cs

using System.Collections.Generic;
using System.Linq;
using UnityEngine;
using UnityEngine.InputSystem;
using UnityEngine.Localization.SmartFormat.PersistentVariables;
using Utils;

namespace InputHints
{
    public class InputActionVariableGroup : IVariableGroup
    {
        readonly InputActionAsset _actionsAsset;
        readonly InputHintsSpriteMapping _spriteMapping;
        readonly Dictionary<string, InputActionVariable> _cachedVariables;

        public InputActionVariableGroup(InputActionAsset actionsAsset, InputHintsSpriteMapping spriteMapping)
        {
            _actionsAsset = actionsAsset;
            _spriteMapping = spriteMapping;
            _cachedVariables = new Dictionary<string, InputActionVariable>();

            InputHints.UsedDeviceChanged += HandleUsedDeviceChanged;
        }

        #region INTERFACE IMPLEMENTATIONS

        public bool TryGetValue(string key, out IVariable value)
        {
            Debug.Log(
                $"INPUT HINT CACHE: {_cachedVariables.Select(p => $"{p.Key}: {p.Value.ToString()}").StringJoin()}");

            if (_cachedVariables.TryGetValue(key, out var cachedVariable))
            {
                value = cachedVariable;
                return true;
            }

            var action = _actionsAsset.FindAction(key);
            if (action != null)
            {
                value = _cachedVariables[key] = new InputActionVariable(action, _spriteMapping);
                return true;
            }

            /*
             * Try to find action by replacing underscores or dashes with spaces:
             * (In case the action name has spaces in it..)
             * We first try the exact match above in case the action
             * ACTUALLY has a dash or underscore in the name.
             */
            key = key.Replace("-", " ");
            key = key.Replace("_", " ");
            action = _actionsAsset.FindAction(key);
            if (action != null)
            {
                value = _cachedVariables[key] = new InputActionVariable(action, _spriteMapping);
                return true;
            }

            value = default;
            return false;
        }

        #endregion

        void HandleUsedDeviceChanged()
        {
            foreach (var pair in _cachedVariables)
            {
                pair.Value.OnValueChanged();
            }
        }
    }
}

InputHintsLocalizationSource.cs

using System;
using UnityEngine;
using UnityEngine.InputSystem;
using UnityEngine.Localization.SmartFormat.Core.Extensions;
using UnityEngine.Localization.SmartFormat.PersistentVariables;

namespace InputHints
{
    [Serializable]
    public class InputHintsLocalizationSource : ISource
    {
        #region PUBLIC AND SERIALIZED FIELDS

        [SerializeField] string _prefix = "input";
        [SerializeField] InputHintsSpriteMapping _mapping;
        [SerializeField] InputActionAsset _actionsAsset;

        #endregion

        InputActionVariableGroup _variableGroup;

        #region INTERFACE IMPLEMENTATIONS

        public bool TryEvaluateSelector(ISelectorInfo selectorInfo)
        {
            /*
             * INITIALIZE THIS SOURCE, IF THAT FAILS, ALWAYS
             * RETURN FALSE (i.e. we can't use this source)
             * maybe it's bad that there's no warning for this?
             */

            Init();
            if (_variableGroup == null)
            {
                return false;
            }

            /*
             * Do this the same way PersistentVariablesSource handles it:
             * check if the CurrentValue is ALREADY the nested mapping, then drill down and
             * get the value from the given text.
             */
            if (selectorInfo.CurrentValue is InputActionVariableGroup group &&
                group.TryGetValue(selectorInfo.SelectorText, out var variable))
            {
                var cache = selectorInfo.FormatDetails.FormatCache;
                if (cache != null && variable is IVariableValueChanged valueChanged)
                {
                    if (cache.VariableTriggers.Contains(valueChanged) == false)
                    {
                        cache.VariableTriggers.Add(valueChanged);
                    }
                }

                selectorInfo.Result = variable.GetSourceValue(selectorInfo);
                return true;
            }

            /*
             * If we're at the first part of the selector, see if the
             * prefix matches the prefix we set here, then return the mapping as result.
             */
            if (selectorInfo.SelectorOperator == "" &&
                selectorInfo.SelectorText == _prefix &&
                _variableGroup != null)
            {
                selectorInfo.Result = _variableGroup;
                return true;
            }

            return false;
        }

        #endregion

        void Init()
        {
            if (_variableGroup == null && _mapping != null && _actionsAsset != null)
            {
                _variableGroup = new InputActionVariableGroup(_actionsAsset, _mapping);
            }
        }
    }
}