I say "treat" loudly and my adults pigs are at the bars (indoor pigs) waiting for something. Usually do this and hand-feed, and while eating that, put the rest in their cages. I have to be careful with my sow though, as she has gotten "too excited" in the past and nearly scaled her wall.
Even though outdoor pigs, using a word for a veggie treat (or any treat, even hay tips) as you are approaching, should get them warming up a bunch to you and coming when you call. I have week old pups that are staying put now when I approach the cage and put my hand in, and they'll accept treats from the hand or wait until I drop it below their mouth if unsure, so it's not hard to tame them.