Would a new lifecycle method be apt/possible: beforeAttachedToRootStore

[justin-hackin]

I think it would be useful to be able to have a beforeAttachedToRootStore in order to initialize model properties based on a context applied to the model before it is attached to the tree. Having a node show up in the tree without its property initialized forces me to an existence check at the beginning of a very long chain of function calls which makes my code hard to reason about.

In my use case, I've created a generic prop factory function that takes a type ref and a path relative to the root store and by default sets the reference to be the first item in the array of items specified by the path. I can't perform the initialization of the property onInit because the path to the items to be reference is resolved relative the root store.

[justin-hackin]

To give some context into my use case I made a code snippet to demonstrate my experiments with the new context apply feature here: https://codesandbox.io/s/poc-referencedropdownprop-wp0mp?file=/src/index.ts . Aside from this little hiccup, I'm pretty happy with this solution. I'm creating a whole collection of prop factories wired to material UI components that make creating a data structure + control panel a breeze. The usefulness of generics and contexts are really wowing me 🤯🤩 Feedback welcome, I might make the concept into a package eventually.