after watching the video about the recent fiasco in Mckinney Texas I was curious about whether any of the teens that the officer pulled over had the legal right to just walk away.

Under what scenario is a police officer allowed to detain you? Do they need a reason to believe that you committed a crime, or can they just detain you without any reason at all?

If you need a state, use Texas, if you need a city, use McKinney. Please make it clear under what scope the laws actually reside.