half-earth/half-earth/scripts/interaction_modes/InteractionMode.cs

using Godot;
using System;

public class InteractionMode : Node
{
    [Export]
    private NodePath _buildModePath;
    private BuildMode _buildMode;

    [Signal]
    delegate void OnInteractionModeChanged(Mode oldMode, Mode newMode);

    public enum Mode
    {
        SELECT = 0,
        BUILD = 1
    }

    public Mode Current { get; private set; }

    public void _on_Build_Mode_OnModeEntered()
    {
        ChangeMode(Mode.BUILD);
    }
    public void _on_Build_Mode_OnModeExited()
    {
        ChangeMode(Mode.SELECT);
    }

    private void ChangeMode(Mode mode)
    {
        if (Current == mode)
            return;

        var old = Current;
        Current = mode;

        EmitSignal(nameof(OnInteractionModeChanged), old, mode);
    }

    public override void _Ready()
    {
        base._Ready();

        _buildMode = GetNode<BuildMode>(_buildModePath);

        Reset();
    }

    public void Reset()
    {
        Current = Mode.SELECT;
    }
}