启动和停止活动指示器swift3

I am learning Swift and now working on my home task. One of my buttons calls a function where I reload a collection view and it takes time. I am trying to implement an activity indicator to show the process. The problem that if I start the indicator, load the collection view, and then stop the indicator within the button action, the indicator doesn't appear. What would be the correct way to implement the indicator in this case? Here is the piece of the code:I am learning Swift and now working on my home