Web-500 — Add User Presence

[wisc-gmv]

A real time context that can be used to subscribe to user Presence in in Track-API.

Example usage:

const end = await customer.realTime().presence()
    .on('update', onUpdate)
    .on('delete', onDelete)
    .start();

// Unsubscribe
end();

[chrishighet]

Looks good to me