popup to add transports to Mirror's networkmanager

NetworkTransportDrawer.cs

[CustomPropertyDrawer(typeof(Transport))]
public class NetworkTransportDrawer: PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        var fieldRect = position;
        var buttonRect = position;

        fieldRect.width -= 50;
        buttonRect.xMin = fieldRect.xMax;
        buttonRect.width = 50;

        EditorGUI.PropertyField(fieldRect, property, label);

        var options = FindAllTransports();
        var strings = options.Select(x => x.Name).Prepend("Add").ToArray();
        var index = EditorGUI.Popup(buttonRect, 0, strings);
        Debug.Log($"NetworkTransportDrawer {index}");

        if (index > 0)
        {
            var target = (MonoBehaviour)property.serializedObject.targetObject;
            // -1 because 0 is "Add"
            var transport = (Transport)Undo.AddComponent(target.gameObject, options[index-1]);

            property.objectReferenceValue = transport;
            property.serializedObject.ApplyModifiedProperties();
        }
    }

    static Type[] Transports;
    private Type[] FindAllTransports()
    {
        if (Transports != null)
            return Transports;

        var transportType = typeof(Transport);
        Transports = AppDomain.CurrentDomain.GetAssemblies()
            .SelectMany(assembly => assembly.GetTypes())
            .Where(type => transportType.IsAssignableFrom(type) && !type.IsInterface && !type.IsAbstract)
            .ToArray();

        return Transports;
    }
}

Todo.md

• Add TransportPickerAttribute attribute so popup doesn't apply to all Transport fields, just the one with attribute
• if Transport is already on gameobject, set property.objectReferenceValue to that one instead of adding new component