Unable to know when scene model is rendered in sceneview

We are facing an issue with rendering a 3D model using Sceneview. This is creating a problem for us in using animations on 3D model.

1. Is there a callback to let us know if 3D model is fully rendered
2. when animation is played, just after 3D model is rendered, sometime animation is played and at other times it is not played as per the design.

We are using below method:

runAction(action, completionHandler: {          <#code#>        }) completing handler does not always invoke.

Kindly help.