Notify objects when changes to properties of other objects. Very similar to willSet and didSet, but they can be defined outside of the type defination.

observation = observe(
            \.objectToObserve.myDate,
            options: [.old, .new]
        ) { object, change in
            print("myDate changed from: \(change.oldValue!), updated to: \(change.newValue!)")
        }

Why? UI rendering (UIKit) works in the main thread.
How? All heavy lifting (time-consuming) should be moved out of main thread, including:

• Disk I/O
• Networking, e.g., API calls
• Large computations
• Use GCD, and/or Operation