It seems that at the moment it is not possible to provide a callback to Relay.createContainer which will be called when the initial and all following data is fetched.
In my example I need to know a value of the initially fetched data outside of the render function of my wrapped component.
It should be possible to supply a callback to the Relay.createContainer function that gets called whenever data was fetched and before the wrapped component gets updated.
If this is already possible can you please give an example?
This can be accomplished with standard React lifecycle callbacks: componentWillMount is called the first time a component mounts and before it renders, and componentWillReceiveProps is called thereafter whenever new props (including Relay data) are passed to the component.