Is there any English equivalent to the idiom "cat always dreams of meat"?

We have an idiom in Urdu that could be translated as, "cat always dreams of meat". The meaning is something like, "one always sees what one desires".

Let me try to put it in a context. Suppose,

A group of friends was planning to visit one of their acquaintance to condole for her fathers death. Another of their friend approaching towards them, while catching part of the discussion, and exclaimed, "Hey, I'd love to join you guys, her mom makes fantastic croissants" — having no knowledge of her father's death.

Now here we can use this, "cat always dreams of meat".

We could say, in your example, "Hassim must have food on the brain!" This means that his current thoughts are of food and affecting his other actions, not necessarily that he has a one-track mind for food all the time. (One-track mind can be used with for, as I just did, but usually it is simply stated that someone "has a one-track mind" and the focus is left implicit. Beware, the phrase is especially used to accuse people of thinking only of sex, but can be used to imply any topic.)