How to loop through an array of objects in swift

Unwrap and downcast the objects to the right type, safely, with if let, before doing the iteration with a simple for in loop.

if let currentUser = currentUser, 
    let photos = currentUser.photos as? [ModelAttachment] 
{
    for object in photos {
        let url = object.url
    }
}

There’s also guard let else instead of if let if you prefer having the result available in scope:

guard let currentUser = currentUser, 
    let photos = currentUser.photos as? [ModelAttachment] else 
{
    // break or return
}
// now 'photos' is available outside the guard
for object in photos {
    let url = object.url
}