ios - Where is the right place to add an extension? -

I got an extension to surface here Where is the best place to add this extension?

At first I added it to ViewController (I did not use any extensions before), where I needed the functionality

Later I needed it in another view controller and It was pasted again, but found an error that was already announced. But when I tried to use the reshuffle () I could not find any disturbance, but my array did not get worse at all.

So I wonder where is the problem shuffle () do I miss anything while using another ViewController? Any prefix or something else?

Thank you!