This song is in the key of A major. Using the chords A minor, B♭ major 7, F major 7 and G major 7. This means that it is non-diatonic as the chords do not all fit in the key.
It is a mostly instrumental song which takes a lot of example from 90s dance and house music. Calvin Harris uses synths and his knowledge of electronic production to create a mellow song that you can also dance to.