Execute a method when a variable value changes in Swift

Question

I need to execute a function when a variable value changes.

I have a singleton class containing a shared variable called labelChange. Values of this va

Answer 1

In Swift 4 you can use Key-Value Observation.

label.observe(\.text, changeHandler: { (label, change) in
    // text has changed
})

This is basically it, but there is a little catch. "observe" returns an NSKeyValueObservation object that you need to hold! - when it is deallocated, you’ll receive no more notifications. To avoid that we can assign it to a property which will be retained.

var observer:NSKeyValueObservation?
// then assign the return value of "observe" to it
observer = label.observe(\.text, changeHandler: { (label, change) in
    // text has changed,
})

You can also observe if the the value has changed or has been set for the first time

observer = label.observe(\.text, changeHandler: { (label, change) in
    // just check for the old value in "change" is not Nil
    if let oldValue = change.oldValue {
        print("\(label.text) has changed from \(oldValue) to \(label.text)")
    } else {
        print("\(label.text) is now set")
    }

})

For More Information please consult Apples documentation here