Recadio stays out of the way, living at the menu bar. When your select one input or output inside Recadio, it remembers your choice forever. From that point on, it'll keep them selected whenever you have the same inputs and outputs connected to your Mac.
Recadio doesn't know if it's you or your Mac changing the preferred device. If there is already a preference set in Recadio, it will revert any change made from outside Recadio, including the ones you make. To prevent this behavior you can either quit Recadio temporarily, or change the device from Recadio's interface.
Every Mac comes with a program called Gatekeeper, with the only purpose of making it harder to install things that don't come from the official App Store. This helps prevent malicious programs from being installed by accident, but also makes it harder to install Recadio. You can follow Apple's own instructions to install software downloaded from outside of the App Store here.
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