There is a misconception that people with ADHD struggle to focus on anything. As ever with ADHD, it is not as simple as that.
People with ADHD find it very hard to focus on the uninteresting or boring. When the subject of focus is interesting or stimulating, there is no problem and we can “hyperfocus” deeply. In hyper-focus we filter out all distractions and focus intensely for exceptionally long periods of time – whether studying, playing, creating, thinking or dreaming. This is why an ADHD child is perfectly able to hyper-focus and play computer games for hours! Hyper-focus is great when used responsibly!
From Wikipedia: “Hyperfocus” – an intense form of mental concentration or visualization that focuses consciousness on a narrow subject, or beyond objective reality and onto subjective mental planes, daydreams, concepts, fiction, the imagination, and other objects of the mind.