What is the definition of view pager's position?

I tried it with the number of page and the title of page but they didn’t work.
How should I do this?

See video on youtube or try google search…

The position variable refers to the position of your “tab” in the view page tab line (don’t know if I’m clear here)
For example you have three tabs : 1, 2 and 3 showing different screens (a screen is a vertical arrangement) : Facebook, Twitter and Instagram.
If you are on the Twitter screen the positon will be : 2 because it is the second tab (counting left to right), etc.