Expression type '@lvalue String?' is ambiguous without more context when += to UILabel.text

问题

I am trying to add the string "7" to a label when someone tabs the button labeled 7.

But when using += "7", it gives me the error "Expression type '@lvalue String?' is ambiguous without more context", when using the operator = "7" it works fine. Why doesn't += "7" work?

class NumberPadController: UIViewController {
    @IBOutlet weak var valueLabel: UILabel!

    /// set value in main vc and return to that.
    @IBAction func doneEntering(_ sender: Any) {
        guard let valueString = valueLabel.text, let valueDouble = Double(valueString), let presentingVC = self.presentingViewController as? ViewController else {
            // FIXME: Show error
            dismiss(animated: true, completion: nil)
            return
        }

        presentingVC.valuePassedFromNumPad = valueDouble
        dismiss(animated: true, completion: nil)
    }

    @IBAction func seven(_ sender: Any) {
        valueLabel.text += "7" // Expression type '@lvalue String?' is ambiguous without more context
        valueLabel.text = "a" // works fine
    }
}

回答1:

The text property is optional. One way to do this safely would be to use append along with optional chaining:

valueLabel.text?.append("7")

or use += with optional chaining:

valueLabel.text? += "7"

If the label is nil, these would safely do nothing. If you'd like the label to be "7" if it was nil, then use @RickyMo's solution.

回答2:

text property is optional. To do it safely :

valueLabel.text =  (valueLabel.text ?? "") + "7"

来源：https://stackoverflow.com/questions/55056650/expression-type-lvalue-string-is-ambiguous-without-more-context-when-to-u