shmodot/scripts/OrbitCamera.cs

using Godot;
using System;

public class OrbitCamera : Spatial
{
    [Export]
    private float _lookSensitivity = 200f;
    [Export]
    private float _zoomSensitivity = 10f;
    [Export]
    private Vector2 _fovRange = new Vector2(10, 60);

    [Export]
    private NodePath _cameraPath;
    private Camera _camera = null;
    private Camera Camera
    {
        get
        {
            if (_camera == null)
            {
                _camera = GetNode<Camera>(_cameraPath);
            }
            return _camera;
        }
    }

    private Vector2 _rotation;
    private bool _canRotate = false;

    public override void _Input(InputEvent e)
    {
        if (e is InputEventMouseMotion mouseMotion)
        {
            HandleMouseMovement(mouseMotion);
        }
        if (e is InputEventMouseButton mouseButton)
        {
            HandleMouseButton(mouseButton);
        }
    }

    public override void _Process(float delta)
    {
        Rotation = Vector3.Zero;
        var sensitivity = -1f / _lookSensitivity;

        Rotate(Vector3.Right, _rotation.y * sensitivity);
        Rotate(Vector3.Up, _rotation.x * sensitivity);
    }

    // left click to drag
    private void HandleMouseButton(InputEventMouseButton mouseButton)
    {
        switch ((ButtonList)mouseButton.ButtonIndex)
        {
            case ButtonList.Left:
                _canRotate = mouseButton.Pressed;
                break;
            case ButtonList.WheelUp:
                Zoom(-1);
                break;
            case ButtonList.WheelDown:
                Zoom(1);
                break;
        }
    }

    private void Zoom(int dir)
    {
        if (dir != 1 && dir != -1)
            throw new ArgumentException();

        var fov = Camera.Fov;
        var zoom = _zoomSensitivity / Camera.Fov;

        fov += (float)dir * zoom;
        fov = Mathf.Clamp(fov, _fovRange.x, _fovRange.y);

        Camera.Fov = fov;
    }

    private void HandleMouseMovement(InputEventMouseMotion mouseMotion)
    {
        if (!_canRotate) return;

        var delta = mouseMotion.Relative;
        _rotation += delta;
    }
}