Hi,
To those arriving into Barcelona on Dec 21st. 2018, note it's very likely traffic will be in gridlock for a good part of the day, especially from very early in the morning.
The reason being the protest of many Catalans against the Spanish Government. Several marches and demonstrations have been announced, one being particularly pernicious to mobility in the streets of the city that day: thousands of protester will be slow marching in their cars through the streets of Barcelona.
Thus, it's strongly advisable to take the metro (available in both airport terminals T1 and T2) or the train (available only in T2) to go to/from the city to avoid surface traffic, you don't want to get 'stuck'.
Other than traffic mobility, you will be able to enjoy the city as always minus some inconveniences here and there. Note that patriotic demonstrations here in Catalonia tend to be uneventful and peaceful despite the level of animosity against the Spanish government is rather high these days.
Having said that, earlier today the Spanish government has threatened to send Spanish paramilitary police, Guardia Civil, like those sent during the indy referendum of last October 1st and that were portrayed in newscasts around the world for their violent charges, so it's very advisable to stay away from them.
Enjoy your visit!