Useful items if you have problems with invading cats or a cat on an expensive or restricted diet. The cat flap is much cheaper than other suppliers and the feeder is an alternative if you don't want to change your cat-flap or have other pets.
These devices read the microchip you already have between your cat's shoulder blades and unlock if the cat has been authorised. You can allow multiple cats access. The flap only restricts entry, but has the usual manual latch for controlling access further.
We have a cat on an expensive veterinary diet and a rogue neighbourhood cat that loves to sneak in and clean our cat's plate. So I'd been keeping an eye on these types of product for some time.
The feeder works on the same principal but uncovers a bowl. It gets very positive reviews. An added benefit is that it is mostly airtight when closed and so keeps wet food fresher for longer.
I was going for the flap, but decided the feeder was easier since I wouldn't need to remodel the cat entrance in our porch wall. We might also have needed extension tunnels with would add to the cost of the flap and the amount of diy required. The feeder is also useful for restricting other cats and dogs from the expensive cat food.