How to lazy watch a provider

[ZhZhZhen]

I have a providerA. I hope to provide a null value when providerB does not exist. When providerB exists, I want to watch B.
I found that there is a ref.exists(provider) method to determine whether the provider exists or not. But how can I automatically trigger the watch in A when B is created?

[rrousselGit]

It's a challenging problem. There is no easy way to do this today.