I've used web sources in my dashboards and I've still had to run the data load to bring down the data. the act of opening an App will not trigger this for good reasons.
1. you want to control the time you load data to ensure quality and consistency
2. if you loaded each time a user opened the app you would A. leave the user waiting for the load script to run before they could use it. B. it would run each time a user opened the app so would overload the server with load requests leading to poor performance.