I have some conditions for a deal to move from one stage to the next based on the status of related list items and some related Creator records. Right now I don't know a simple way to validate the statuses of these related items before allowing the transition. I can run a function to validate after the transition has happened, but then the blueprint stage has already changed and the next transitions are available. I also can't reverse the blueprint stage automatically if the custom conditions aren't met.

I was thinking a good solution would be to allow a custom function in the "During" phase that returns a string. The function would be triggered when all other "During" components of the transition are complete and the transition button is pressed. The function could perform any sort of validation and if all custom conditions are met, the response is an empty string, and the transition occurs. Otherwise the developer can construct and return a string that would show in a popup window to let the user know what conditions need to be met before the transition can go through.