How can I listen for UILabel content changes?

Question

I want to listen the content changes in the UIlabel and prevent it from changing when its length is greater than 7. What should I do? I tried set / get / willset / didset. It seems that it can't meet my needs

I wrote a simple demo ，When I press the button, add a number to display area. I don't want his length to exceed my expectations

In my real project, I'm developing a calculator What I can think of is to judge the length of displayValue, but doing so will make my code wordy

Answer 1

didSet can help you better. add this code in line number 6 in your code.

var displayValue: String = "" {
    didSet {
        if displayValue.count <= 7 {
            customLabel.text = displayValue
        }
    }
}

Then in your action function, you need to just do it.

@IBAction func clickBtn( _ sender: Any) {
    displayValue = displayValue + "0"
}