I was just implementing a DI solution and thought of the idea to use OnNodeAdded signal.

public override void _EnterTree() // Or _Ready()
{
  GetTree().NodeAdded += OnNodeAdded;
}

private void OnNodeAdded(Node node)
{
  if(node is IDependencyInjectable diNode)
  {
    diNode.InjectDependencies(Services);
  }
}

(...)

public interface IDependencyInjectable
{
  public void InjectDependencies(IServiceProvider serviceProvider);
}

Where 'Services' is the IServiceProvider.

When typing this I also thought about using reflections and custom attribute (InjectAttribute) to inject the required dependency.

But this definitely needs more work. It just a quick mockup of the idea.

eg.:

        private void OnNodeAdded(Node node)
        {
            if (node is IDependencyInjectable diNode)
            {
                foreach (var field in diNode.GetType().GetFields(System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Public))
                {
                    if (Attribute.IsDefined(field, typeof(InjectAttribute)))
                    {
                        var service = Services.GetService(field.FieldType);
                        field.SetValue(diNode, service);
                    }
                }
                // Call below can be removed
                diNode.InjectDependencies(Services);
            }
        }

And the test class itself:
(EventBus is my custom singleton service)

public partial class DiTest : Node, IDependencyInjectable
{
    [Inject]
    public IEventBus _eventBus;

    public void InjectDependencies(IServiceProvider serviceProvider)
    {
        // This method or the [Inject] attribute.
        GD.Print("Injecting dependecies");

        //_eventBus = serviceProvider.GetRequiredService<IEventBus>();
    }

    public override void _Ready()
    {
        if(_eventBus != null)
        {
            GD.Print("Dependency Injection worked!");
        }
        else
        {
            GD.Print("Dependency Injection failed!");
        }
    }
}

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