When property observers get called [Swift]

struct Notification {
    let postImageURL: String?
    let postID: String?
    var userIsFollowed = false
}

Say we have a Notification struct.

var notifications = [Notification]() {
    didSet { tableView.reloadData() }
}

Using this struct, I made an array that has a property observer.

The property observer is called in all three of the following situations.

self.notifications = notifications

notifications[index] = Notification()​

notifications[index].userIsFollowd = isFollowed

1. It's called when you change the variable that has the property observer itself.

2. It's called when you change a single element of the array that has the property observer.

3. It's called when you change a property of that element.

It seems like usually only case 1 is explained, so I wrote this up.

Also, it's off topic, but it's good to remember that a property observer is called on the thread that changes the property.